# Why Barometric?



## Johnny Chicken Bones (Jul 13, 2005)

I had an old Garmin Forerunner unit, lost on a trail somewhere. 
Didn't like it too much, would have thought I didn't need it. Am now surprised to see I miss it. 

It had the usual features but two that I specifically miss. 
-Sunset. 
-An altimeter that worked off the GPS, not the barometric pressure. 

Some reading/searching leads me to see that many current bike computer/GPS units are using the barometric pressure to account for altitude changes/readings. 

Any idea why? 
I'd have thought using Barometric pressure was less accurate. Plus heck, it's accessing satellites anyway how much harder is it to provide me w/ the 'tude? 

-JCBs


----------



## Sidewalk (May 18, 2015)

GPS network only works reliably on two dimensions, not great at the third (but not bad, really). Baro is more accurate.

Only thing I have learned from using baro is to leave auto pause on when stopped or changes in pressure from weather will screw up the readings. I gained an extra 2000 one day from that!


----------



## Harold (Dec 23, 2003)

Yes, barometric altimeter is actually better than GPS elevation. Down on the ground with a handheld receiver that's moving all the time, 3D GPS location calculation isn't that great. At times, the accuracy might be reasonable, but at other times you might not have a 3D fix at all. The error is that variable.

With a barometric altimeter, there are a few advantages. For one, a properly calibrated altimeter is MUCH more accurate. Also, the sensitivity of altitude measurements is higher, so you're more likely to capture much smaller changes in elevation on your ride, which can add up.

Yes, there are negatives to it. I despise auto pause for other reasons, so I just deal with the fact that if I stop for lunch or if I need a serious break after a really long climb, that my elevation is likely to drift from changing barometric pressure. If it's bad, I'll apply elevation corrections to my ride after I upload it. Usually it's not too bad. It's relatively uncommon that the weather makes a huge difference to the elevation on a ride.

Frankly, there's no "good" way to quickly measure elevation. Every method is fraught with errors and limitations. Those errors apply to every spot elevation measurement, so if you're trying to obtain a climbing total for your whole ride, those errors get summed up for every measurement over the course of the ride. No elevation measurement should be taken as gospel. It's a lot like trying to measure calorie consumption on your GPS. Except calorie consumption is even worse. It's incredibly inaccurate.


----------



## J.B. Weld (Aug 13, 2012)

Harold said:


> Frankly, there's no "good" way to quickly measure elevation. Every method is fraught with errors and limitations.


Maybe so but I've used barometric and GPS for elevation and both have been very consistent which is the most important thing for me. They do give different numbers from each other, the GPS seems to cheat me a little but it doesn't matter to me whether I climbed 100,000 ft in a year or 105,000 as long as it's reasonably consistent from day to day and year to year.


----------



## Harold (Dec 23, 2003)

J.B. Weld said:


> Maybe so but I've used barometric and GPS for elevation and both have been very consistent which is the most important thing for me. They do give different numbers from each other, the GPS seems to cheat me a little but it doesn't matter to me whether I climbed 100,000 ft in a year or 105,000 as long as I get consistency from day to day and year to year.


Yes, they both tend to be relatively consistent if you use the same method all the time. If you're talking purely consistency, GPS based elevation is probably a bit more consistent (even though it loses a bit on the accuracy side) because you don't have the errors introduced from weather conditions on the ground (though space weather definitely plays a role).

The reason I made that statement about it in the first place is because too many people get caught up in accuracy accuracy accuracy and they set themselves up for disappointment when the device does not meet their expectations. I'd have to say that unmet unrealistic expectations are a bigger problem with bike computers than they are with most other products in the cycling industry. And it comes back to the fact that most folks really don't have a clue how they work, so they don't grasp the limitations of the devices.


----------



## Johnny Chicken Bones (Jul 13, 2005)

Well done MTBR and you folks. Thank you. 
In the early 90s I used to use an Avocet Veritech or some such wrist worn altimeter. 
It told time well but that was about all. It was based in barometric pressure. 

Looks like the new versions are better. 
Thanks. 
Now, Sigma or Garmin? Any hard strong thoughts on those? 

It's fall in some places. Go ride.
-JCBs


----------



## Harold (Dec 23, 2003)

Johnny Chicken Bones said:


> Well done MTBR and you folks. Thank you.
> In the early 90s I used to use an Avocet Veritech or some such wrist worn altimeter.
> It told time well but that was about all. It was based in barometric pressure.
> 
> ...


Barometric altimeters need to be calibrated fairly frequently to avoid being totally screwed over by changing weather.

A number of years ago, Garmin started adding some auto-calibration capabilities to its fitness models to help with that.

What specific models are you asking about? The two brands overlap in their models/pricing. I would NOT buy the lowest priced Garmins at all. I don't really know much about Sigma's GPS bike computers, but their more basic cyclocomputers work well for what they are. My personal preference is for a GPS computer with a wheel sensor. An accurate GPS will always short you on distance due to aliasing, and the wheel sensor overrides the GPS-calculated distance. The GPS positions are the same, but the wheel sensor captures the whole curve, instead of shortcutting it.

If your desired price is less than the cost of a Garmin Edge 520, I'd be looking at the Lezyne models. Once you get to about the cost of the Edge 520, the competitors to Garmin change. Wahoo being one of the more established.


----------



## Johnny Chicken Bones (Jul 13, 2005)

There are rides where we carry bikes for up to an hour. A wheel sensor means the device shuts down on occasion (I like auto pause). 
But worse it means that I can't move the device between multiple bikes w/o more debris. 

I'm considering the Sigma Rox 7.0 and the Garmin Edge 520. 
When you say I should avoid the low end Garmin, are you saying the small little square units? The Edge 25 or 20? 

I still haven't seen one that lists the sunset/sunrise times. 
And haven't found one that has a run time over 12 hours. 
But, meh... It'll work out eh?


----------



## Harold (Dec 23, 2003)

Johnny Chicken Bones said:


> There are rides where we carry bikes for up to an hour. A wheel sensor means the device shuts down on occasion (I like auto pause).
> But worse it means that I can't move the device between multiple bikes w/o more debris.
> 
> I'm considering the Sigma Rox 7.0 and the Garmin Edge 520.
> ...


Auto pause sucks for mtb. Learn to do without it. What your Garmin will report on the screen will never agree with what any analysis software will report (they will usually calculate stopped/moving time on their own, anyway). Still, I doubt that hiking with the bike will result in activating auto pause any more than it would without the wheel sensor. I know my slow @ss climbing would cause auto pause to turn itself on and off many times on a climb I'm actually riding. It's not worth the hassle. I'm definitely of the "record everything, sort it out later" mindset.

The device can work just fine with or without a wheel sensor. Just because you have one on one bike doesn't mean you HAVE to have one on every bike. If the sensor is there, Garmin will use it. If not, then it will work fine anyway. Modern Garmins don't use Bike profiles like they used to (my Forerunner 310xt used works that way). Instead, it uses "Ride" profiles which only affect data that displays during the ride. Sensors do not get attached to a specific profile. Some do get extra data attached (like wheel circumference for wheel sensors).

Yes, limited battery life bothers me for modern GPS bike computers, too. The Edge 520 has been okay for me for now, at least. I've gone up to 10hrs with it and had extra juice. You CAN use the Edges, at least, with USB power supplies (some of which can be powered with AA batteries or even hub generators) to extend battery life. Garmin finally started making one of their own recently, that attaches really cleanly to the new Edge 1020 (which is too damn big, IMO). It's too bad it requires an adapter to work with the Edge 520, because the mount setup for the 1020 is slick.

FYI, I just looked at my Edge 520's available data fields. Both sunrise and sunset are selectable fields for the customizable data pages. It is disappointing that Garmin doesn't supply a comprehensive list of all of the available fields for each model. It would be useful for folks like you who have a specific field you'd like to display, but can only get that info from people who have the device in hand and can check.

And yeah, the models I absolutely cannot recommend are the Edge 20 and Edge 25. They are super limited with data fields, are not very customizable, and their recording interval is garbage. IMO, MTB should have 1sec recording or better.


----------



## Johnny Chicken Bones (Jul 13, 2005)

You can't make me quit auto stop. Never. You can pry it from my cold dead etc....
But you are correct. I've had it quit on climbs as well. Definitely during hike-a-bike. 

I run a dyno on two of the bikes but am not yet powering anything but lights. 
But, a 26 hour sit down session will require some sort of power. I'll bring a small USB battery to boost it off of. 
Very good to know I still can use the unit during it being powered. 

I appreciate your thoughts, thanks. 
Go pedal. 

-JCBs


----------



## notso (Jan 22, 2015)

The Lenzene super has a very long battery life (approx. 24hrs). I've had a ton of problems with the Bluetooth connectivity on my unit and some accuracy (drift) issues, but the battery seems to last forever.


----------



## MSU Alum (Aug 8, 2009)

Just a little perspective on barometric. It can be more accurate than GPS, but more variable as well.
For example, in an aviation application, you sometimes have to make significant corrections to barometric readings for pressure AND temperature. Elevations (altitudes) can sometimes be off by 20% or more, once you drop below a certain temperature and off by less above a certain temperature, as it's based on a standard day. So, if the pressure changes, or the temperature changes, it can affect a baro reading. For the most part, GPS is close enough and is not temperamental for riding. They both work, but GPS is just easier.


----------



## Harold (Dec 23, 2003)

MSU Alum said:


> Just a little perspective on barometric. It can be more accurate than GPS, but more variable as well.
> For example, in an aviation application, you sometimes have to make significant corrections to barometric readings for pressure AND temperature. Elevations (altitudes) can sometimes be off by 20% or more, once you drop below a certain temperature and off by less above a certain temperature, as it's based on a standard day. So, if the pressure changes, or the temperature changes, it can affect a baro reading. For the most part, GPS is close enough and is not temperamental for riding. They both work, but GPS is just easier.


It's worth noting that for riding purposes, most people don't even know what straight GPS elevation readings are for their rides because they don't look at their total climbing numbers until after they finish and load their rides to Strava or whatever. Spots where GPS reception is weak (like down in deep valleys or on steep northern facing slopes) frequently lack a GPS elevation at all, or if you do, it's pretty inaccurate. If you ride in places with steep elevation changes, you'll have that crop up periodically over the course of a ride. Every upload service I know automatically scraps GPS elevations from your file and overwrites them with elevations calculated from a digital elevation model based on your position. Strava will do it to your GPS that has a barometric altimeter for otherwise flaky data. I've been there, too. I had a Bryton Rider 310 that got that treatment from Strava every damn time, even though the device had a barometric altimeter. It was because the device would oddly insert duplicate timestamps occasionally, which triggered Strava's automatic reprocessing of EVERYTHING.

When it comes to temperature fluctuations, that's why most handheld electronic devices also have an onboard thermometer. Garmins had it for a long time without offering temp as a viewable field, because it used the thermometer only for correcting altimeter data based on temp fluctuations. Folks wanted temp as a viewable field, so Garmin made it one later on. But the temp sensor has been present for a long time on devices with the barometric altimeter. There was a back door to get to a system screen that showed the info, which is how users learned of it.


----------



## Johnny Chicken Bones (Jul 13, 2005)

Harold,
I was going to make some comment about the "GPS Geek" at the bottom of your reply but then realized that you really have some impressive knowledge base. 
Thanks. 

I guess from what I've learned none of the devices I've used over the years are quite as I believed! 
I'll be passing on backdoor hacks and going w/ a Sigma Rox 7. 
It'll have plenty of features for me and likely ample I don't need. 

-JCBs


----------



## Harold (Dec 23, 2003)

Johnny Chicken Bones said:


> Harold,
> I was going to make some comment about the "GPS Geek" at the bottom of your reply but then realized that you really have some impressive knowledge base.
> Thanks.


Welcome. I've been around a bit. Got introduced to GPS tech before selective availability got turned off by Clinton in 2000. That scrambled civilian GPS signals to intentionally make them less accurate. Using civilian GPS was a bloody nightmare back then. Have used a whole bunch of devices since, ranging from common consumer grade stuff to $10,000 survey grade hardware. Also have a Master's level minor in spatial science.


----------



## MSU Alum (Aug 8, 2009)

Harold said:


> It's worth noting that for riding purposes, most people don't even know what straight GPS elevation readings are for their rides because they don't look at their total climbing numbers until after they finish and load their rides to Strava or whatever. Spots where GPS reception is weak (like down in deep valleys or on steep northern facing slopes) frequently lack a GPS elevation at all, or if you do, it's pretty inaccurate. If you ride in places with steep elevation changes, you'll have that crop up periodically over the course of a ride. Every upload service I know automatically scraps GPS elevations from your file and overwrites them with elevations calculated from a digital elevation model based on your position. Strava will do it to your GPS that has a barometric altimeter for otherwise flaky data. I've been there, too. I had a Bryton Rider 310 that got that treatment from Strava every damn time, even though the device had a barometric altimeter. It was because the device would oddly insert duplicate timestamps occasionally, which triggered Strava's automatic reprocessing of EVERYTHING.
> 
> When it comes to temperature fluctuations, that's why most handheld electronic devices also have an onboard thermometer. Garmins had it for a long time without offering temp as a viewable field, because it used the thermometer only for correcting altimeter data based on temp fluctuations. Folks wanted temp as a viewable field, so Garmin made it one later on. But the temp sensor has been present for a long time on devices with the barometric altimeter. There was a back door to get to a system screen that showed the info, which is how users learned of it.


I know it sounds counter-intuitive, but the temperature at the elevation you are at isn't useful for determining your current altitude above a point (such as sea level, or a point in between). ISA deviation and temperature at the point from which you are measuring have to be included in the calculation.

If you are in, for example, a 767 the airplane has all sorts of data that is incorporated with (and including) the outside air temperature (including current ISA deviation) to determine its altitude. But, for example, if the temperature at the station (airport) that you reference is -10 degrees C and you indicate 2000 feet above the station, you are actually 10%, or 200 feet, lower than your altimeter shows. And that's if you have entered the known Baro pressure at that station.

Naturally, it's of no consequence mountain biking. You really only want to know changes in elevation, you're not going to fly into a peak! And fluctuations, as you mentioned, can be accounted for. There's nothing critical about knowing your exact current altitude...it's just interesting.

ICAO did an extensive study of commercial airports around the world to determine which ones have temperature/terrain considerations that could result in CFIT and found about 50 in the U.S. where that altimetry information has to be compensated for. 
Under those circumstances, GPS determined elevation is actually more accurate. Due to a slew of restrictions and satellite info, it's use isn't authorized for altitudes in aviation, but it's unaffected by temperature, ISA deviation and barometric pressure.

I only get "into the weeds" on this, because I think you'd be interested.
When we dispatched, at the airlines, into airports (or listed alternates) that had RNAV only (GPS based) approaches, Dispatch ran a predictive program called "PRAIM" (Predictive Receiver Autonomous Integrity Monitoring) to ensure satellite coverage and signal integrity would be sufficient. If you were airborne and you were forced to go somewhere else due to unpredicted weather or airport issues, the airplanes all had an internal program that ran those calculations called "RAIM" (P for predicted being a non-issue once in the air) that gave you the info you needed to assure signal quality. It wasn't unusual, near SLC and LAS in particular to get GPS failures due to the Air Force GPS jamming that occasionally occurred.


----------



## Harold (Dec 23, 2003)

MSU Alum said:


> I know it sounds counter-intuitive, but the temperature at the elevation you are at isn't useful for determining your current altitude above a point (such as sea level, or a point in between). ISA deviation and temperature at the point from which you are measuring have to be included in the calculation.
> 
> If you are in, for example, a 767 the airplane has all sorts of data that is incorporated with (and including) the outside air temperature (including current ISA deviation) to determine its altitude. But, for example, if the temperature at the station (airport) that you reference is -10 degrees C and you indicate 2000 feet above the station, you are actually 10%, or 200 feet, lower than your altimeter shows. And that's if you have entered the known Baro pressure at that station.
> 
> ...


Hm, interesting stuff. I'm less knowledgeable about the barometric altimeter stuff, so your experience regarding its use in aviation is new to me. Still, it all makes sense. I do know enough to know that measuring elevation (it's elevation when you're on the ground, and altitude when you're above it ;-) ) is a very inexact process. Yeah, consumer hardware is good enough for getting pretty close to correct for a single elevation measurement. But unfortunately that's not how cyclists tend to use elevation. Cyclists like to know how much elevation change their whole ride had, and that process tends to compound the errors involved. Most people don't realize exactly how much error is involved because there's no way to DIRECTLY measure exactly how big that climb is.

There's been the occasional person in this forum over the years (it's been a LONG time since I saw a comment like this, actually) who has said that since GPS altitude is good enough for aviation purposes, it's good enough for them. Sounds to me like that's not exactly the whole story on aviation use of elevation/altitude. Another issue worth pointing out WRT the aviation use of GPS is that airplanes are typically above the major culprits of GPS interference for ground-based receivers. Like terrain that obscures significant parts of the horizon and low elevation satellites, or trees/buildings that reflect GPS signals, introducing multipath errors.

Do aviation receivers make use of ground-based GPS augmentation systems? I've used some pro-level gear in the past that included a separate receiver for GBAS that provided real-time differential corrections. I've also done differential corrections with GBAS in post-processing. Seems like if it did do real-time differential corrections, it would be another factor affecting the accuracy of the use of GPS in aviation. That's just something you're not going to get in a consumer grade fitness GPS. Probably ever.


----------



## MSU Alum (Aug 8, 2009)

Harold said:


> Do aviation receivers make use of ground-based GPS augmentation systems? I've used some pro-level gear in the past that included a separate receiver for GBAS that provided real-time differential corrections. I've also done differential corrections with GBAS in post-processing. Seems like if it did do real-time differential corrections, it would be another factor affecting the accuracy of the use of GPS in aviation.


Yes they do (or can, they don't always). 
There are two types of corrective signals available as you've noted. One is GBAS and is available for certain types of approaches known as GLS approaches. The other is WAAS enabling which allows for other types of RNAV types of approaches without the need for temperature corrections.

I had all that info somewhere, once, but it seems to have disappeared. Just thought you'd be interested.


----------



## Harold (Dec 23, 2003)

MSU Alum said:


> Yes they do (or can, they don't always).
> There are two types of corrective signals available as you've noted. One is GBAS and is available for certain types of approaches known as GLS approaches. The other is WAAS enabling which allows for other types of RNAV types of approaches without the need for temperature corrections.
> 
> I had all that info somewhere, once, but it seems to have disappeared. Just thought you'd be interested.


Yeah, WAAS used to be a big thing with consumer gear. I don't think my Edge 520 has it. There's no mention of it in the literature, and I haven't seen an enable/disable option anywhere on the device.


----------



## MSU Alum (Aug 8, 2009)

Harold said:


> Yeah, WAAS used to be a big thing with consumer gear. I don't think my Edge 520 has it. There's no mention of it in the literature, and I haven't seen an enable/disable option anywhere on the device.


The whole thing is pretty fascinating. You have to love a system that actually has to make corrections for the fact that time passes at different rates on satellites compared to on the ground!


----------



## Harold (Dec 23, 2003)

MSU Alum said:


> The whole thing is pretty fascinating. You have to love a system that actually has to make corrections for the fact that time passes at different rates on satellites compared to on the ground!


Practical applications in special relativity!


----------



## Jayem (Jul 16, 2005)

MSU Alum said:


> Yes they do (or can, they don't always).
> There are two types of corrective signals available as you've noted. One is GBAS and is available for certain types of approaches known as GLS approaches. The other is WAAS enabling which allows for other types of RNAV types of approaches without the need for temperature corrections.
> 
> I had all that info somewhere, once, but it seems to have disappeared. Just thought you'd be interested.


WAAS solves the altitude precision issue of gps, allowing gps based instrument approaches with vertical guidance as accurate as traditional radio based glide paths, in fact, the newest RNP AR approaches include radius to fix 3D paths that weave through valleys and in between mountains. The aircraft displays the RNP and actual error during the approach, if your error exceeds RNP, you have to do a missed approach. There is still error from the atmosphere, but it's pretty insignificant. These GPS approaches have to allow an aircraft to come down and fly a small vertical path inclined about 3 degrees from the surface of the earth and they have to be able to tell the aircraft they are 200' from the surface so they can make their decision to go around or land. Where the infrastructure supports it, the GPS approaches using WAAS get airplanes down to 200', just like traditional land-based approaches, in other cases they usually go to 250', still pretty low. That's not the accuracy, that's how low the airplane can go without seeing the runway, which means it needs to be highly accurate. Needless to say, the actual performance is highly accurate for altitude. WAAS is simply a stationary satellite (relative to a position on the earth, so its rotating in orbit as fast as the earth turns), which makes it a fixed distance in altitude to that location, this makes the ranging to determine altitude incredibly simple, as compared to the pseudo-ranging algorithms used by the regular GPS constellation. These days I think there are at least 3 WAAS sats giving coverage over the US from AK to FL. Why this isn't used in consumer electronics, I don't know. It's been around now for more than 10 years. This is also known as SBAS, sat-based argumentation. The WAAS sat also acts as an additional "regular" GPS sat for your receiver, so it also boosts signal integrity and reliability compared to the relatively low angle/orbit GPS sats flying around at thousands of mph.

The traditional GPS sats do not work on anywhere near the same principle. Often times for simplicity, people are told that the GPS unit "triangulates" your position, but to do this in the most basic sense, the sats would need to know where they are in space, and moving around at thousands of mph, they actually don't know this, for the purpose of your receiver figuring out where it is. What they do know is the time, because each sat has an atomic clock. If you get a bunch of sats that have broadcast a signal that is time stamped, you can compare those signals to when they were received, seeing more or less delay due to more or less distance covered. This sets up a bunch of equations with a bunch of unknowns. By doing differential equation math, the receiver calculates a distance to the sats that "makes sense" based on when all of the signals were received, relative to when they were sent (using speed of light, etc.). This is why the process is called "pseudo-ranging", because the receiver isn't directly triangulating position. This is why you need more satellites than what seems intuitive to get a 3D position.

The "why barometric" may be due to not having world-wide coverage with the US WAAS system. If you sell a receiver that is based on GLONASS, GPS, and the Yurp system that will work all over the world with baro, maybe that makes more sense, but I'd also have to think that putting the additional circuits in for WAAS would be a pretty small deal at this point too.


----------



## JackWare (Aug 8, 2016)

Wow!
It's the 1st time I've ventured into this section of the forum, and have just spent 15 minutes learning from you guys.
The amount of time and effort you have all put it is incredible and inspiring.
Thank you.


----------



## Johnny Chicken Bones (Jul 13, 2005)

This really has turned into quite the wonderful thread. 
I now have the Sigma ROX 11. (Went w/ the 11 for it's Bluetooth feature)

Haven't landed my 747 yet, in fact, haven't taken off. I'm still trying to jump start it off the VW. 

The Sigma will do the job. 
Remember the commen about how I'll never stop using auto stop? 
Yea well.... I just linked to my iPhone and up loaded to the cloud and then etc etc into a Sigma program that is WAY WAY too detailed for my needs. 
If I was using a heart rate monitor and various meters on the bike, well there'd be more info available to me but it wouldn't change my day much. 
What did was looking at the map and seeing that we had 56 stops on a ride. 
56! It was a busy say sure, lots of locals on the trials but Jesus. That's too much. I won't stop stopping though I just might turn the auto stop off. 

Well done on all the info. 
Keep it coming, and thanks. 

-JCBs


----------



## woodway (Dec 27, 2005)

Interesting explanation of how GPS receivers calculate altitude. Simple enough that even I could understand it:

https://www.unavco.org/education/resources/tutorials-and-handouts/tutorials/elevation-and-geoid.html


----------

