# GPS Elevation Data - Baro vs Non-Baro



## Ultra Magnus (Jan 13, 2004)

Instead of adding on to the old thread about GPS stuff, I thought I'd add a new one. I used to ride wiht a Garmin Forerunner 201. I was happy, but the elevation information was less than 100% accurate, to say the least. The Edge 305 with the barometric altimiter provides much cleaner information, but still off a bit. I compared the same course, my commute home, from the Forerunner to the Edge. See for yourself.

The first one is from the Forerunner, the bottom one is from the Edge.

BM


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## KevinB (Oct 5, 2004)

It'd be interesting to compare plots (for both units) from several rides to see which unit shows more consistent results.


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## Speedub.Nate (Dec 31, 2003)

Interesting comparision -- but that's the reality of it.

Now pretend you're the algorithm that's been written to decode that data and provide total elevation gain (for instance).

Those relatively large spikes from the GPS-only data are pretty hard to filter and can provide much more room for error than the barometric data. 

Either way, both have weaknesses. But barometric using GPS as an intermittant correction seems like the way to go.


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## jake7 (Jul 25, 2006)

I've seen barometric units be off by 600 ft. over the course of a 2.5 hr ride due to atmospheric weather fronts. Using my forerunner 201, I've lost signal in the trees on wooded singletrack. Obviously, both drawbacks. I hear that Garmin's new 305 GPS cycling-specific model has SiRFstar III technology which has supposedly eliminated the loss-of-signal issue, but I haven't seen it in action. Review available at:
http://www.performancebike.com/shop/profile.cfm?SKU=22257&subcategory_ID=4110


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## Speedub.Nate (Dec 31, 2003)

In addition to the SiRF antenna, which appears to be phenominal, the Edge automatically calibrates the barometric altimeter to the GPS. I don't know how often it does this (presumably only when stopped and with a certain signal), but it's certainly a good pairing of the two technologies.


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## Harold (Dec 23, 2003)

I think a lot of folks (those who lean towards the gadget-geek persuasion) tend to expect too much from altimeters. Determining altitude is an imperfect science, and it probably always will be. Each type of altimeter makes certain assumptions about the planet and has certain inherent sources of error. The earth is not a perfect sphere...and even then, it's shape is not immutable. The earthquake that caused the huge tsunami made a calculable change in the shape of the earth. For the barometer models, air pressure varies over time and by location. In addition to the inherent error of all altimeters, you have to deal with the error inherent in topographical maps. Topos cannot show every detail of the surface of the earth.

Altimeters are a good quickie reference and for providing general information regarding altitude change over the course of a ride, but I'd never expect stellar precision or accuracy out of them.


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## Bikinfoolferlife (Feb 3, 2004)

What was the total elevation gain/loss computed by each device? I assume by cleaner you mean there actually aren't as many small changes in elevation and is better reflected by your new toy?


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## billee (Jul 31, 2004)

Not a problem in central Florida. The highest elevation in the area is the speed bump down the street from my house.


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## esf_mtb (Apr 12, 2006)

Speedub.Nate said:


> Interesting comparision -- but that's the reality of it.
> 
> Now pretend you're the algorithm that's been written to decode that data and provide total elevation gain (for instance).


I tried pretending to be the algorithm. Couldn't do it.


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## Ultra Magnus (Jan 13, 2004)

I'll have to check my training center software at home tonight, and post tonight or tomorrow the total gain loss and degree of error between differnt rides using the edge.

I did however, just today, realize something. I've noticed that when I start out in the mornings my elevation reads low. Anywhere between 2850 and 2920 ft, when in the afternoons it always says my house is at 2960. When I ride to work, I normally just turn on the unit, and once it locks on to some satelites, shoot off to work. But today, I watched it for a while. It seems that it takes a lot longer to calculate or home in on the elevation via. gps than east/west position. So, I assume it remembers my baro pressure from the afternoon ride home, and when I turn it on the pressure may be a touch higher (due to 30-40F cooler temps in the morning?) so it assumes I've dropped in altitude. As I watched, it slowly continued adjusting the elevation until it homed in on 2960ft. This took about 10mins (I didn't stare at it for 10 solid mintues, just left it on while I got ready for work)... 

With the edge though, when I compare runs in the training center software the total elevatin gain/loss can vary a bit (I'll compare tonight and post tonight or tomorrow) and the starting & ending elevation can vary, but the overall shape ofthe elevation profile is highly consistent. I imagine if I gave it 10 or 15mins to lock in on the altitude, the +/- variation would greatly improve.

As for as reception goes, withe the forerunner I'd lose satelite signals when riding in the open sometimes (no trees to speak of in the desert). I think this was due to either my body or my top tube getting in the way (before I had the handlebar mount, I'd strap it to my downtube). The 305, like I've stated before, maintians a satelite lock in my one story wood framed home, in the middle of the house, not even near a window.

Perfect, no. Huge improvement? YES!

BM


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## ribaldmanikin (Mar 9, 2006)

What was the signal accuracy like on both days? I know that with my Explorist, I'm going to get far different elevation data when I have 3 metre accuracy vs. 9 metre accuracy. 

Also, the newer unit likely has a software update. When I updated the software on my Explorist, the elevation data for the same places changed dramatically.


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## Ultra Magnus (Jan 13, 2004)

Normally, it reads +/-19ft, but I've seen it as good as +/-14 ft, and I think it's referring to lat/long accuracy. I don't think I tells what altitude accurace it's getting.

BM


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## ohpossum (Dec 19, 2003)

bmadau said:


> It seems that it takes a lot longer to calculate or home in on the elevation via. gps than east/west position. So, I assume it remembers my baro pressure from the afternoon ride home, and when I turn it on the pressure may be a touch higher (due to 30-40F cooler temps in the morning?) so it assumes I've dropped in altitude. As I watched, it slowly continued adjusting the elevation until it homed in on 2960ft. This took about 10mins (I didn't stare at it for 10 solid mintues, just left it on while I got ready for work)...


I'd bet the delay in getting good elevation data comes from the time to scan and acquire the extra satellite needed for 3D positioning. The navigation message containing satellite information takes 12 minutes to transmit to the receiver. Instead of waiting for all the info, most receivers display as much information as possible while they scan and acquire new satellites. I don't have any experience with the Edge device you have, so it may be remembering your previous altitude info, but the process of determining altitude via GPS is always gonna take some time.

op


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## Speedub.Nate (Dec 31, 2003)

I was actually half-serious when I wrote that. It's a tough equation to balance without a bunch or error spikes thrown in.

I renumbered bmadau's second plot to serve as an example.

*According to the new plot*, let's say you begin your ride at 2600'. The initial descent takes you down to 600', but during the descent you have approximately 400' of climbing.

Descent = 2000'; Climb = 400', Net = 1600' descent; *NET GAIN = 400'*.

After the big descent, you have a flat-ish rund with a 300' lung buster thrown in for good measure. Then you ascend to 1800', and lose 300' as the ride tapers to the end.

Descent = 400'; Climb = 1200; Net = 600' descent; *NET GAIN = 1600'*.

A reasonable person might say, "Yeah, that's accurate enough". But a rider who wants to know why that ride was so damn tough compared to his other 1600' rides would argue, "I want all those nooks & crannies counted! There's a lot of elevation gain I WORKED FOR that is unaccounted! There's maybe 300 or 400 extra feet of vertical lost in that translation!"

So ok, measure everything!










*But now suppose* that instead of representing 3000', the renumbered plot equals just 30' (somebody mentioned Florida?)!

Suddenly those nooks & crannies represent 1 or 2 or 5 feet of hidden gain. Should everything get counted? A reasonable person would say, "Yeah, you started at 26', you descended to 6', and hopped a ledge and climbed an 18' hill. 12' of gain seems reasonable."

But some riders might argue, "But I gained a few feet of elevation over the course of that initial descent, and I want credit for it!"

So: measure it, or no?

*Now for the extreme example*: Instead of 30 feet, the plot represents 30 inches (or 2 1/2 feet). And imagine it goes on and on and on and on, for miles and miles, never getting above 30 inches of gain.

The reasonable person might say, "Give me ZERO elevation gain... that's basically a FLAT TRAIL!"

But another rider might just as easily say, "If I climbed it, I want credit for it!" and would happily rack up a few hundred feel of Gain on a relatively flat trail.

So all I'm saying is that it can be kind of tough to calculate useful/meaningful elevation numbers, and the problem is only made worse by inaccurate spikes in the recording (ala the relatively rough GPS plot compared to the somewhat smooth Baro plot).

If you're writing the algorithm, where do you draw the line? What data do you exclude? What constitutes a vertical foot of gain? Ahhh, the great things in life to ponder.

(If I recall correctly, CatEye's old AT-100 discarded changes of less that 5', and only recorded gain if passed the 20' range -- any gain less than this was discarded. I don't know what my VDO MC1.0 does. I've seen user claims on other forums that the Garmin Edge GAIN figures can be highly inaccurate on the high side -- sometimes double what is normally accepted. Along the lines of what NateHawk wrote, the only "near-absolute" in elevation data is a surveyed peak; almost everything else is a best guess or approximation.)


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## ribaldmanikin (Mar 9, 2006)

Without a built in altimeter, the GPSr uses the coordinates based on tracking mutliple satellites to track altitude.


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## Ultra Magnus (Jan 13, 2004)

I 100% agree with. I run into that on my lunctime road rides I was doing. I only had the chance to ride it once with the 305. It recorded an elevation gain of like 1000'. the only real climb was the dam hill (goes up to a local small dam), that's pretty tough (for me) 7.5% average, ~10%max, for about 1/2mile (~160ft elevation gain), but the mile up to it gradually increases from 1% to 3% utill you get to the base. For me, it's a tough climb (after watching the tour, I feel like a total wuss for saying that, it only takes 3.5mins to cimb)... But, in town all our streets have rolling dips on them. They go down at about 5%, up again at about 5%, some maybe only dip down 10' or so. Usually your momentum from pedalling on the downside can carry you up the other side with little extra effort. I'm sure it's adding all them up. I wonder what method the UCI uses to calculate total elevation gain. How steep or long or high does a rise in elevation need to be before it's classified as a climb. Personally, I just go over the ride elevation detail, and measure the climbs I know are climbs, and just add them together. That's one more thing for me to do tonight as a comparison... My estimate of climbing vs. the Training Center software's calcuation of elevation gain.

If I climb at 1% for 1000', I've only risen 10'. That's nothing. If I climb at 1% for 100miles, that a mile of elevation gain. That's something, but at 1% how hard would it be? I'd think not very. How much slower would your time be vs. if it were flat?

BM


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## Jwind (Mar 1, 2006)

Interesting, except it means next to nothing without a "control"...


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## Speedub.Nate (Dec 31, 2003)

ribaldmanikin said:


> Without a built in altimeter, the GPSr uses the coordinates based on tracking mutliple satellites to track altitude.


Part of what separates GPS elevation data from Lat/Long measurements is perspective. The satellites' perspectives, actually.

Not exactly an "apples to apples" comparison, but look at the first photo below (it's on an air traffic control tower). If I gave you some nearby reference points, you could pretty easily give me an accurate lat / long of the building.










But even if I gave you a precise elevation of the sidewalk around the building, you'd have a tough time giving me an accurate elevation of the roof of the building.

From the side-view of the same building in the second photo, however, you wouldn't have such a difficult time estimating an accurate height!










Again, this isn't apples to apples! But GPS satellites for the most part look down on their receivers (especially the ground-based receivers). Side-view of the receivers is pretty limited.

What this means is that your handheld GPS receiver is getting signals from a "blanket" of satellites that are roughly in a plane parallel with the ground (excuse this over-simplification if you believe in a round Earth).

From a lat/long perspective, the satellites "surround" the receiver. It's basic triangulation, and it's pretty easy to take the differences in time/position signals sent by the satellites and calculate a position on the ground.

But from an elevation perspective, and without a whole assortment of side-view satellites to choose from, there isn't as much differentiation to calculate this third dimension. Weaker side-view signals are more error prone, and stronger overhead signals are roughly from the same perspective. Poor triangulation is the result.

It's still enough to get you an elevation that's in the ballpark, but definitely with less accuracy than the lat/long affords.


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## Ultra Magnus (Jan 13, 2004)

Yes- Let me illustrate

In the super stupid simple example below, lets look at this triangle.

Let's pretend the earth is flat and the satelite is 1000 units away (feet, miles, km, don't matter), and is at a 1 degree inclination from me. To find it's linear distance from me along the ground I do cos1*1000 and I get 999.847. To find it's alititude from me, I do sin1*1000 and I get 17.452. Now lets assume there's a +/-.05 degree variance. Cos1.05*1000=999.832, only off by .015. Sin1.05*1000=18.325, off by a lot, about 5%, where the distance along the ground is only off by .000015%.

Yes, this a stupid simple example and I'm know there's a lot more to it than I can even comprehend.


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## Nathan Cloud (Jul 18, 2005)

Speedub.Nate said:


> I was actually half-serious when I wrote that. It's a tough equation to balance without a bunch or error spikes thrown in.
> 
> I renumbered bmadau's second plot to serve as an example.
> 
> ...


Not entirely off topic, but your thoughts on calculating "stuff" are of great interest to me - I am in the middle (well, I have it mostly working) of writing a class to parse fancy statistics out of uploaded gpx files for my website.

Here is an example of where I am at: http://www.crankfire.com/work/mtbrstats.php

I have been using the statistics topofusion calculates as a basis of comparison and we are pretty close - it's the average climbing/descending grades that are killing me. Which is where I would like some thoughts:

I am processing the gpx track point by point, determining slope between each point set and then calculating an overall weighted average (weighed by distance of each point set). I am doing so for both ascents and descents. During this I consider "flat" between 0 and 0.75 degrees 'rise', but I am intrigued by Cateye's "discarded changes of less that 5".

So... yeah. Sorry to derail, ignore me if need be. I am curious as to what approach others might take.

Thanks.


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## dave54 (Jul 1, 2003)

As a rule of thumb, the vertical margin of error is 3x the horizontal. Some published sources say 2x, but I have found 3x is more likely, especially if you are in terrain and canopy with a degraded signal.

On nearly every recreation grade GPS, the displayed 'margin of error' is a joke anyway and should be used with a healthy dose of skepticism. One wag (a GPS instructor) half-seriously suggested Garmin just uses a random number generator to determine their accuracy figure.

Baro types are notoriously inaccurate also.


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## Speedub.Nate (Dec 31, 2003)

dave54 said:


> Baro types are notoriously inaccurate also.


On regular rides, both barometer altimeters I've used have been very consistant. As long as they are set to a known elevation at the beginning or a ride, and provided the current conditions aren't so out-of-whack that the pressure gradiant is close to standard, I find they offer data reliable enough that I can navigate a topo map. 2500' to 3500' peaks are usually within 50 to 100 feet of where they're supposed to be! 

Of course, the same can be said for GPS altimetry, since we're usually stationary when pulling the map out of the pack and giving it a looking over.

But I've never seen a baro altimeter "spike" when ploting ride data as GPS-only units so often do.

I think as long as every user knows of the imprecision of determining elevation, they would look at their data and their devices in a more forgiving light. Seems many expect absolute precision which simply isn't realistic.


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## dave54 (Jul 1, 2003)

Speedub.Nate said:


> ..., I find they offer data reliable enough that I can navigate a topo map. 2500' to 3500' peaks are usually within 50 to 100 feet of where they're supposed to be! ...


From the National Map Accuracy Standards:

"2. Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale. "

Here's the total standard. Even USGS topo maps may contain errors. It is best to know the limits of all the tools you are using.

http://rockyweb.cr.usgs.gov/nmpstds/nmas647.html

Funny, I have never needed an altimeter to find my position on a topo map. Most of the time I don't need a compass either.


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## Ultra Magnus (Jan 13, 2004)

I don't use the thing for navigation. It's got rudimentary navigation features but it's wholly inadequate for navigation, especially considering it's built in li-ion batteries with only a 13hr life, or so. It's a training gadget, just for fun. It's fun to see an elevation graph of my ride, see where I went how fast, and what my hr was at that point of the ride. All fun stuff. If you want to navigate, get the 60cs or something cool, with replaceable AA bateries and a bring a bunch of spares...

It's probably illegal, but using one with the "courses" feature would be awesome for an ITT. 

BM


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## upstateSC-rider (Dec 25, 2003)

bmadau said:


> Yes- Let me illustrate
> 
> In the super stupid simple example below, lets look at this triangle.
> 
> Yes, this a stupid simple example and I'm know there's a lot more to it than I can even comprehend.


NASA called, they want their technical drawing back.  
I use a Polar 725 and I've been mostly happy with it. The only problem I've had is what's been mentioned before, big weather changes during a ride. Actually it's not really a problem for me because I don't have anything to compare it to.

Lou.


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## Destroy (Jan 12, 2004)

http://timgiles.free.fr/garminalt.htm

Interesting chart in link above...^^^^^


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## ToddM (Feb 3, 2004)

If you can calibrate the baro to a known USGS landmark etc. and you don't get a pressure front come through I've seen my 76csx give accurate readings. I've also seen it be way off, with both the gps and baro readings, and I've calibrated both in the morning and seen significant difference between them at the end of the day.

You really cannot expect perfect alt. readings from either satellite or baro readings with a gps. First of all the elevation standards have error built into them, even DEM and NED datasets have some error just due to what method they use to calculate their values. 

Second I've seen some places where the gps elevation is very accurate and consistent and I've seen some places where it's near useless. In general the people I know who rely on elevation readings would rather have a baro reading than a gps reading, but they also know they need to calibrate a baro to a accurate benchmark daily if not more frequently and understand that if a weather front is approaching things go cluster. If you want to see this for yourself calibrate your baro elevation in your house to a # and then see how it varies over time. It could easily be a couple hundred feet off the next morning. If you really want to have some fun, calibrate the baro in a car going down the highway then open the window at 75mph.

While I can understand (okay I can't but I'll play nice) why someone would want every inch of elevation recorded on their trail, it's not going to happen. You have to realize that the accuracy and resolution of the sensors in consumer units have limits. This is why many gps units limit the resolution of your altitude. For example a suunto depending on model might only show you 10' elevation intervals, the 76csx records 1' intervals when reporting elevations. However in the specs for the 76csx it clearly says that it's accuracy is expected to be 10' if properly calibrated. That's probably +/- 10' so you could be off 20' either way, and that's expected error. 

The bottom line is altimeters are good for trends, but if you think you are going to map precise elevations to the sub foot you better be willing to spend $20k and carry 40lbs of gps gear on your back and stay at the trail head to let it warm up for 12 hours before you start, and even then it's probably unrealistic.


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## BigLarry (Jul 30, 2004)

*I use only barometric, no GPS calibration*

I've found the GPS calibration tends to add repeated artificial drift up and down into the altitude. The barometric alone does get drift in the 10's of feet from pressure changes in the atmosphere over hours of riding. But the constant GPS calibration can give errors up to 100' over fractions of an hour again and again, leading to altitude noise and excessive climb numbers.

So I just calibrate the altimeter best I can using a topo map, paper or the topo map on my GPS. I will resort to using my GPS elevation in worst case, but the GPS altitude always tends to be off way more than 100', as I later find when looking at the track on a map. But no matter what the initial calibration, the total altitude gain is still very accurate  by using barometric only.

I went through a lot of old tracks and recorded the actual drift from my barometric altimeter on a lot of rides (without using GPS calibration) from the start to end at the same point. Here's my results:

Total altitude drift for 2.5 hour ride (in feet):
-20, 20, 8, 35, 30, 75, 38, 35, 13, -28, 15, -20

Total altitude drift for 5 to 8 hour ride (in feet):
60, 8, 10, 60, 15, 85, 45, 30, 80, -60, -10, 40

This barometric drift is very slow, linear and cumulative, by comparison with GPS calibration that goes up and down all the time. So along the ride the error will even be smaller than the numbers above when using barometric alone.


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## MikeDee (Nov 17, 2004)

billee said:


> Not a problem in central Florida. The highest elevation in the area is the speed bump down the street from my house.


So, any concerns about global warming?


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## B R H (Jan 13, 2004)

Could you explain your "drift" measurements in more detail?


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## BigLarry (Jul 30, 2004)

*Barometric Drift*



B R H said:


> Could you explain your "drift" measurements in more detail?


I measure drift by looking at the barometric altitude reported by the GPS at the beginning and end of the ride while standing at exactly the same spot (often my car or garage) at the same altitude. Even if the altitude calibration is slightly off, the altitude should come back to the exact same number. So I measure the difference from beginning to end of the ride as how much the barometric altimeter has drifted over the ride. I can get these numbers from my tracks.

When I carry two Garmin barometric altimeters (eTrex Vista and 60CSx), they track to each other amazingly well, to within a foot or two, along the entire route, and show the same drift at the end. Also, I get more drift when a weather front is coming in, or the temperature changes dramatically from a cold morning to a hot afternoon. So I'm convinced that most of the drift is normal air pressure changes that occur slowly over time. That is, going from low to high air pressure region. It doesn't take much air pressure change to show a difference in altitude.

Similarly, airports adjust the pressure readings every hour that they report to pilots for calibrating their altimeters on landing, that tend to be accurate to a few feet when calibrated from the air pressue given out over the radio.


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## MikeDee (Nov 17, 2004)

On a recent road ride, I measured 990 ft. cumulative vertical gain on my Avocet 50 cycle computer. My eTrex Vista CX got ~1750 ft., as I recall. I think my GPS is way off, in spite of having the barometric sensor.


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## BigLarry (Jul 30, 2004)

*Net Climb Errrors*



MikeDee said:


> On a recent road ride, I measured 990 ft. cumulative vertical gain on my Avocet 50 cycle computer. My eTrex Vista CX got ~1750 ft., as I recall. I think my GPS is way off, in spite of having the barometric sensor.


I've been through climb accuracy issues with several people and units. Anyone is lucky to get any two altimeters to agree on total climb within 10%, and +/-20% is more typical error. I've done a number of accurate tests to try and figure out what's going on, and can probably adjust to get within 5% of reality, and only that with a lot of adjustments. Most people just tend to find an altimeter and procedure they trust, and use that one. That appraoch is as good as any, since no altimeter is perfect, and climb is really relative to other rides you do.

I carry up to three barometric altimeters (eTrex Vista, 60CSx, and Cateye 100) and only see the error you report with Automatic Calibration turned on (more below). I have calibrated all three barometric altimeters on a long monotonic climb (no dips) around 1000' change, and referenced to two separate topographical maps. I also compare them with Motion Based climb data. There's variation of climb data even for the units. Like sometimes the ratio of climb between two GPS will be identical, sometimes up to 15% different between the same two GPS. But in general, here's what I find:

*Errors in total climb reported by unit:*
Topographical Data 0% (by definition, it's the reference)
Motion Based +2% (using the same altitude data as the 60CSx!)
eTrex Vista -3%
GPSMap60CSx -14%
Cateye 100 -1%

Incidentally, the error is in the new 'x' unit software that's calculating climb, not the altimeter, as the Vista and 60CSx report the same altitude all the way up the hill within one or two feet, but a 10% (100') difference in climb total! The 60CSx must have very bad software as the altimeter shows 1000' difference, but the climb is only 900' - a mathematical impossibility even with dips. Also, Motion Based uses the same barometric altimeter data and gets a fairly accurate summary.

I have seen reports from others wtih a VistaCx showing results similar to my 60CSx, where the GPS data field reports a 15% lower climb than actual.

As for your excess climb, I've seen similar excess climb numbers like that when the Automatic calibration is turned on. With automatic calibration, the altitude can go up and down 100s of feet with natural noise of the 3D GPS signal. Turn off the Automatic Calibration of the barometric altimeter, and I'll bet you get better agreement, maybe even 10-15% lower than actual instead of 2X actual. Similarly, a bike altimeter with only 5' resolution can miss a lot of small climbs and round down on little hills, and under-report the climb in certain terrain.


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## B R H (Jan 13, 2004)

I have no idea how my GPS calculates climbing data since Garmin doesn't provide that information. I don't think they even make it clear whether the stats are computed from barometric data (when available) or GPS data or both! Obviously Big Larry's data shows they don't do it the same for each GPS.

I use TopoFusion to filter the raw GPS data from my eTrex Vista C & compute climbing stats. I see ~5% variance max in climbing elevation on my "standard" routes when filtering to 1 meter for hikes & 3 meters for cycling. I think I might start using 2 meters for road rides & 3 meters for mountain though.

I haven't taken the time to try to do any tests for absolute accuracy since I don't think it's really all that important. Consistency is more important to me than even 10% error in absolute accuracy. I may do some testing out of curiousity now though. Maybe I can even learn more about the firmware in the GPS along the way.

I don't understand why Garmin just doesn't publish their algorithm or give the user more control over the filtering. It could very well be the case that their algorithm gets adjusted in firmware based on all sorts of user settings, some expected, some unexpected! Only the GPS has access to the valuable signal quality data, so Garmin should be able to do the best job filtering the data. Assuming they've got a good algorithm to calculate the statistics, they still need to explain or, better yet, give the user control over, the filter settings!

PS. It's important to let the GPS aquire as many satellites as possible before starting your ride. Turn the GPS on while you are getting ready & reset the stats right before you actually start your ride. You should also get better data with WAAS enabled assuming it is available where you ride.


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## slocaus (Jul 21, 2005)

Here are some references to help everyone understand the differences in height with GPS vs Barometric measurements, and the absolute difficulty of measuring altitude by any method. These get more technical as the list progresses. 

Altitude Accuracy (link)

Mean Sea Level, GPS, and the Geoid (link) Note the second illustration and its caption.

GPS Orthometric Height (link) Many technical references at the bottom of this page.


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## MikeDee (Nov 17, 2004)

BigLarry said:


> As for your excess climb, I've seen similar excess climb numbers like that when the Automatic calibration is turned on. With automatic calibration, the altitude can go up and down 100s of feet with natural noise of the 3D GPS signal. Turn off the Automatic Calibration of the barometric altimeter, and I'll bet you get better agreement, maybe even 10-15% lower than actual instead of 2X actual. Similarly, a bike altimeter with only 5' resolution can miss a lot of small climbs and round down on little hills, and under-report the climb in certain terrain.


I checked again, and MotionBased was showing 1785 ft. climbed. The altitude screen in the GPS shows 1363 ft. which can probably be lowered since it included some spurious altitude gain in the track before and after the ride. The first altitude point in the track was at -154' (I live at ~135'). So, if I filter these things out, it's getting closer to agreement with my cyclecomputer.

I also set the track log from auto to distance, so (maybe) it will only record to the track when I'm actually moving.


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## B R H (Jan 13, 2004)

You'll get better results if you collect more data points. If you have it (Record Method) set to Distance, you also need to check what the Distance value (Interval) is. This probably isn't how you want to set it up.

Try setting the Record Method to Auto & the Interval to Most Often. This will give you enough detail for pretty much any ride for at least 8 hours.

The filtering you really want to do is more complicated than just trimming the track data. I don't know what MotionBased does for filtering, but TopoFusion gives you the control you need to get good results.

What do you expect the number to be? You need to find some reasonable value you feel is accurate to begin with & then you should be able to set up your GPS to give you numbers along those lines. There really is no correct answer! Seriously!


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## richwolf (Dec 8, 2004)

I have a cateye AT100 and I think it is very accurate vis a vis climbing elevation gain over the course of a ride. It is probably within a few percent. Except for pressure changes it holds its "home" altitude level very well usually within 20 to 50 feet. I thought the old Avocet 50 was in the same ballpark.

I ordered a Garmin 60csx and am interested in comparing the two but since I do have the cateye I am not too worried about it. The GPS for me is more usefull for mapping than altitude gained during a ride.

If you download a GPS to Topo software and use the elevation profile on them I find that you are usually 150 percent of the altitude gained. I think the main reason is that the track crosses topo lines on the software that does not occur in reality. I have done enough downloading and mapping with both Delorme topo USA and NG Topo! to realize that neither one gives accurate elevation gain readings. Both are way to optimistic.

I do think people are way to picky on these measuring instruments. To me the Cateye is the bomb. I believe it to be very accurate and you just have to deal with weather fronts.


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## B R H (Jan 13, 2004)

You guys are missing the point. If you care about absolute accuracy, meaning the correct answer, you won't get it with any GPS or barometric altimeter because there really is no such thing as a correct answer.

If you "feel" your bike computer is accurate, that's fine & you should be able to configure your GPS to give basically the same results - certainly within much better than 50% unless one of them is broken! You will actually get more consistent data with the GPS data if you use software to calculate the climbing elevation using the raw track data because it is influenced very little by weather changes - much less than a barometric altimeter!

Also, not all "topo software" works the same. Some calculates elevation gain from it's own elevation database & doesn't even download elevation data from your GPS! If you want to get consistent results, you need to use software that downloads the elevation data from the active track log & filter it. Try TopoFusion. You can download it for free (the maps have a watermark on them until you register). You will understand these things a lot better after playing around with something like TopoFusion.


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## richwolf (Dec 8, 2004)

B R H said:


> You guys are missing the point. If you care about absolute accuracy, meaning the correct answer, you won't get it with any GPS or barometric altimeter because there really is no such thing as a correct answer.
> 
> If you "feel" your bike computer is accurate, that's fine & you should be able to configure your GPS to give basically the same results - certainly within much better than 50% unless one of them is broken! You will actually get more consistent data with the GPS data if you use software to calculate the climbing elevation using the raw track data because it is influenced very little by weather changes - much less than a barometric altimeter!
> 
> Also, not all "topo software" works the same. Some calculates elevation gain from it's own elevation database & doesn't even download elevation data from your GPS! If you want to get consistent results, you need to use software that downloads the elevation data from the active track log & filter it. Try TopoFusion. You can download it for free (the maps have a watermark on them until you register). You will understand these things a lot better after playing around with something like TopoFusion.


The problem with downloading a track to either Topo! or topo USA is that a track is made of track points and there is a limit to the number of points meaning it is not totally smooth. Also no GPS is going to download a 100 percent accurate path since it may be off by over 10 to 20 feet (side to side). In doing so it keeps crossing topo lines which leads to over optimistic elevation gain calculations.

What you get from these software elevation profiles is the general up and down trend. Again I don't really need the software to give me an accurate climb reading since my Cateye does it.

I do know that these programs do not download the GPS elevation tracks.


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## MikeDee (Nov 17, 2004)

B R H said:


> You'll get better results if you collect more data points. If you have it (Record Method) set to Distance, you also need to check what the Distance value (Interval) is. This probably isn't how you want to set it up.
> 
> Try setting the Record Method to Auto & the Interval to Most Often. This will give you enough detail for pretty much any ride for at least 8 hours.
> 
> ...


I expect the altitude gain to be in closer agreement with my Avocet 50. But there is a correct answer. That is what the topography actually is; which I admit, most times is an unknown. But, for example, I know what the elevation at the top of Mt. Diablo is, and I can find out what the elevation is some point near the bottom; such as the entrance gate. There are no dips in the climb, so the altitude gain is a known value to be compared against. I was hoping that the Vista Cx was more accurate than my Avocet 50. So far, it is not. I will try Topo Fusion and see how that works.

I think the problem is that the Altitude Screen on the GPS that shows ***. ft. climbed has garbage data in the beginning and end. As someone mention previously, the unit needs to be on for a while to establish its correct altitude. I mentioned previously that it started my ride at ~-150 ft. which was clearly wrong. Also, I don't want the unit to record data when I'm not actually riding. I believe you can manually start and stop a track recording. I may have to try that too.


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## Ultra Magnus (Jan 13, 2004)

I'm asking out of my ignorance, but who says topo data is is the standard +/-0ft? I hit unkown areas with a compass, topo map, and my Edge 305, and my topo maps depending on the area have not been updated in a couple decades, and who says what the accuraccy of the survey was when they did it? I don't even really know how they make topo maps in the first place.

My maps have contour intervals of 20' and 40' and say I climb up and down 30' dips in the middle of two countour lines (on a map with 40' intervals). My topo map calculation of elevation gain I'd show 0' but the gps would add 30' each time.

I take my elevation data with a grain of salt. I mean, even HR readings can be off a lot (like when it's reading 250bps in the first 2 minutes of a ride). It's just for personal reference.

BM


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## richwolf (Dec 8, 2004)

bmadau said:


> I'm asking out of my ignorance, but who says topo data is is the standard +/-0ft? I hit unkown areas with a compass, topo map, and my Edge 305, and my topo maps depending on the area have not been updated in a couple decades, and who says what the accuraccy of the survey was when they did it? I don't even really know how they make topo maps in the first place.
> 
> My maps have contour intervals of 20' and 40' and say I climb up and down 30' dips in the middle of two countour lines (on a map with 40' intervals). My topo map calculation of elevation gain I'd show 0' but the gps would add 30' each time.
> 
> ...


This link might answer some questions.

http://erg.usgs.gov/isb/pubs/booklets/topo/topo.html


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## MikeDee (Nov 17, 2004)

bmadau said:


> My maps have contour intervals of 20' and 40' and say I climb up and down 30' dips in the middle of two countour lines (on a map with 40' intervals). My topo map calculation of elevation gain I'd show 0' but the gps would add 30' each time.
> BM


Yes but keep in mind that 30' reading is + or - . All that tolerance adds up to an erroneous cumulative reading; often a huge amount. Barometric altimeters are more accurate to measure cumulative altitude gain. But without some kind of filtering, they too are subject to adding up "noise."


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## Ultra Magnus (Jan 13, 2004)

richwolf said:


> This link might answer some questions.
> 
> http://erg.usgs.gov/isb/pubs/booklets/topo/topo.html


That's a handy link.

_"The National Map Accuracy Standards were developed to ensure that Federal Government maps meet the high expectations and requirements of such users. Originally issued in 1941, the National Map Accuracy Standards apply to all Federal agencies that produce maps. These standards require horizontal and vertical map precision. For example, at least 90 percent of horizontal points tested on a 7.5 minute, 1:24,000-scale map must be accurate to within one-fiftieth of an inch on the map (40 feet on the ground). Vertical testing requires that at least 90 percent of the elevations tested must be accurate to within one-half the map's contour interval. For example, on a map with a contour interval of 10 feet, tested points must be within 5 feet of the actual elevation. These and other standards of accuracy and content ensure consistency in both the detail and the appearance of maps. They also ensure compatibility among USGS maps made at different times."_

So horizontal disatance are within 40' on the ground (so that would be +/-20ft?) and vertical measurements are half the countrour interval (on a 40ft interval that would be +/-10ft, if I understand it right?) 90% of the time... How does that compare the the absolute accuracy of a GPS unit???

BM


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## ElHombre (Apr 19, 2004)

Another bit of data: I was curious how well the (barometric) altimeters of my Edge 305 and Vista would compare against each other. I use the Vista with 'Auto-calibration' off because of the 15 min. recalibration noise issue as described by Larry.

Turns out they track each other pretty well:










I hadn't used the Vista in a while so its altimeter was initially off by quite a bit - I rode to the summit of a local hill to manually calibrate it. The Edge, being automatically recalibrated periodically (by a GPS signal more accurate than the Vista's) was reading the right altitude within 5 feet. 
After the manual calibration of the Vista, both devices are showing pretty much identical data (the graph shows the raw data) - wouldn't surprise me if Garmin uses the same pressure sensor chip in these two units. I repeated the same section of trail a couple of times, meteorologic drift was pretty small... I posted some more about this here.


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## BigLarry (Jul 30, 2004)

*Same accurate altitude tracking with Vista versus 60CSx*



ElHombre said:


> Another bit of data: I was curious how well the (barometric) altimeters of my Edge 305 and Vista would compare against each other. I use the Vista with 'Auto-calibration' off because of the 15 min. recalibration noise issue as described by Larry.
> 
> Turns out they track each other pretty well...


I've done similar comparison with my older Vista versus the newer 60CSx (that also has the SiRFIII chip like the Edge). Just using the altimeter with the auto-calibration turned off, I find the two GPS units keep their barometric altitude within just 1 to 3' of each other, all the way through the ride. So the 10's of feet drift on a long ride is likely just weather changes, not instrument drift.

In contrast, the altitude gain shown by the 60CSx is 5-15% lower than the altitude gain shown by the Vista on various rides. In calibrations using a climb up a uniform hill, where the gain is not in doubt, I find the Vista is very accurate, but the 60CSx is 15% lower than the altitude gain, a mathematical imposibility in any algorithm on a uniform climb. I've seen many cases where the new instruments are just plain wrong on their altitude gain calculations.


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## Speedub.Nate (Dec 31, 2003)

BigLarry said:


> ...the 60CSx is 15% lower than the altitude gain, a mathematical imposibility in any algorithm on a uniform climb.


Hrumpf... Imagine if the Garmin engineers agreed among themselves, "no how we write the alogrithm, we're an average 15% too high... so let's just shave 15% off the top of all elevation gain calculations!" Call it a night, turn off the lights, lock the door and go drink a beer.

That 15% loss is strange, considering you can't have negative noise (can you?) -- and the starting and ending elevations are presumably being recorded correctly.


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## BigLarry (Jul 30, 2004)

Speedub.Nate said:


> Hrumpf... Imagine if the Garmin engineers agreed among themselves, "no how we write the alogrithm, we're an average 15% too high... so let's just shave 15% off the top of all elevation gain calculations!" Call it a night, turn off the lights, lock the door and go drink a beer.
> 
> That 15% loss is strange, considering you can't have negative noise (can you?) -- and the starting and ending elevations are presumably being recorded correctly.


Funny!  But that's exactly what I was thinking - the Garmin software hacks gave up on getting a proper algorithm for altitude gain. So they noted for standard use they were seeing noise add in about 15% too much on average, and just subtracted it out! So it indicates too low in general on steep hills without a lot of up and down making noise.


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## Speedub.Nate (Dec 31, 2003)

BigLarry said:


> Funny!  But that's exactly what I was thinking - the Garmin software hacks gave up on getting a proper algorithm for altitude gain. So they noted for standard use they were seeing noise add in about 15% too much on average, and just subtracted it out! So it indicates too low in general on steep hills without a lot of up and down making noise.


Hmmm... but on second thought, noise is noise, so even on level ground you'd expect to see the same rate of error -- unless they managed to get that filtered correctly.

Ah well, somebody inside their walls knows the answer to that little puzzler.


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## 3034 (Apr 12, 2006)

bmadau said:


> That's a handy link.
> 
> _"The National Map Accuracy Standards were developed to ensure that Federal Government maps meet the high expectations and requirements of such users. Originally issued in 1941, the National Map Accuracy Standards apply to all Federal agencies that produce maps. These standards require horizontal and vertical map precision. For example, at least 90 percent of horizontal points tested on a 7.5 minute, 1:24,000-scale map must be accurate to within one-fiftieth of an inch on the map (40 feet on the ground). Vertical testing requires that at least 90 percent of the elevations tested must be accurate to within one-half the map's contour interval. For example, on a map with a contour interval of 10 feet, tested points must be within 5 feet of the actual elevation. These and other standards of accuracy and content ensure consistency in both the detail and the appearance of maps. They also ensure compatibility among USGS maps made at different times."_
> 
> ...


It doesn't, the above standards were written for ground surveys and aerial photogramatry surveys, NOT GPS. TOPO maps are approximate models and the contours are interpolated from spot elevations, without the standards the interpolated contour lines would not create a usable model, such as where to cut the logging road etc.
not show the various ground details


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