# Accuracy of GPS vs. Cyclocomputer



## billee (Jul 31, 2004)

I just installed a Specialized Speed Zone Sport Classic cyclocomputer. After calibrating it using the tape measure method, I tested it against a Magellan Sport Track Pro GPS.

When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.

The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.


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## Eric_H (Jul 1, 2004)

The GPS is probably off for the reasons you have stated, but the cyclocomputer is off as well. When you are making sharp turn, you are traveling on the sides of the tires which is a smaller diameter than the one inputted to the computer. Having a smaller diameter than was inputted will lead to longer distances and faster speeds.



billee said:


> I just installed a Specialized Speed Zone Sport Classic cyclocomputer. After calibrating it using the tape measure method, I tested it against a Magellan Sport Track Pro GPS.
> 
> When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.
> 
> The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.


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## NappyT (Jun 3, 2004)

*Your correct...*

You will also notice a milage difference on climbs too.


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## HTail (Jan 29, 2004)

*GPS more accurate*

An engineer friend of mine that's familiar with the operation of GPS has the potential to be more accurate if it's implemented correctly. Utilizing satellite triangulation is the most accurate because elevation can be included in the calculations. Typical altimeters use barametric pressure which can vary and cause error depending on the weather.


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## EBasil (Jan 30, 2004)

Yeah, in a straight line, like driving down the freeway, your GPS is more accurate than even your car's speedo and odo.

The reason for the "shortening" on a twisty mtb ride is that the GPS "draws" straight lines from each "one second sampling point", rather than that curve you actually rode. If you take a switchback, for example, your tires might roll 3 or 4 revolutions going around the curve, but the GPS might hit samples at the entrance and exit of the curve, which might be only 3 feet apart when it "draws" the straight line between the points, cutting off the curve.

The altimeter on your GPS is very accurate...often much better than a barometric unit.


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## Piki (Aug 18, 2021)

I purchased lately a cyclocomputer (sunding wired) and I set it up correctly I thought. Seperately I use my phone's "sportractive app" to calculate for elevations etc. The GPS of "sportractive app" calculated a lot more distance than the "sunding" measured. I mean the app calculated 10 km while sunding measured only 6-7 km. At the beginning I thought that sunding was defective. But after a few days I found out that the problem was due to increased distance between the sensor and the magnet, so the sensor wasn't recording all the passes of the magnet, and subseqently the measured distance was much shorter than the distance measured by the app. I solved it by repositioning the magnet closer to the sensor. Next day the measurements were: app 18 km, sunding 18.2 km. I report it because others may experience similar problems. So I recommend careful implementation of the magnet-sensor to avoid mismatches that could drive you crazy.


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## Jwiffle (Jan 26, 2004)

First time I've ever heard anyone say the gps altimeter is more accurate than a barometric, yet twice in this one thread. 

Yet, the higher end gps units rely on a barometric altimeter instead of gps for elevation. 

My experience says that the barometric altimeter is much more accurate, unless you get caught in a storm. The changing pressures can cause erratic readings before and after the storm. But 99% of rides (since I probably get caught in storms less than 1%of my rides), the barometric is more accurate.


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## Jayem (Jul 16, 2005)

Jwiffle said:


> First time I've ever heard anyone say the gps altimeter is more accurate than a barometric, yet twice in this one thread.
> 
> Yet, the higher end gps units rely on a barometric altimeter instead of gps for elevation.
> 
> My experience says that the barometric altimeter is much more accurate, unless you get caught in a storm. The changing pressures can cause erratic readings before and after the storm. But 99% of rides (since I probably get caught in storms less than 1%of my rides), the barometric is more accurate.


WAAS is able to provide excellent vertical accuracy. So much so that it's used to generate vertical glidepaths for aircraft down to the runway with the same precision as a fixed Instrument Landing System (ILS). That's the lowest minimums that aircraft usually fly, known as a precision approach. There are lower minimums, but they require exotic equipment and pilot authorizations, such as what is necessary to land "Cat III" in zero-zero conditions.

The problem with baro is you need to calibrate it at the start of your ride, every time you use it. Changes day to day, just hour to hour, and from location changes, etc., will throw it off significantly. It will measure changes in altitude very well, but it needs fairly constant updating (an altimeter source) to do so.

I wouldn't say either is necessarily better, but WAAS is being integrated into more and more units, so the usefulness of baro is probably going to be declining. Dedicated garmin units also tend to read multiple sat constellations, like glonas, etc. Now, if we are talking about using a phone...they tend to often just take the path of least resistance, using cell-tower triangulation for position, etc...


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## Harold (Dec 23, 2003)

Jayem said:


> WAAS is able to provide excellent vertical accuracy. So much so that it's used to generate vertical glidepaths for aircraft down to the runway with the same precision as a fixed Instrument Landing System (ILS).
> 
> The problem with baro is you need to calibrate it at the start of your ride, every time you use it. Changes day to day, just hour to hour, and from location changes, etc., will throw it off significantly. It will measure changes in altitude very well, but it needs fairly constant updating (an altimeter source) to do so.
> 
> I wouldn't say either is necessarily better, but WAAS is being integrated into more and more units, so the usefulness of baro is probably going to be declining.


Doubtful. The higher vertical accuracy of GPS in aeronautical applications is kindof inherent to aeronautical use. The receiver is above the ground with minimal obstructions like the terrain itself.

It's DEPENDENT on the overall GPS accuracy, which is extremely variable for consumer grade gear in ground-based uses. Barometric altimeters are relatively cheap and a fairly known entity (the calibration thing has always been true, even when barometers were analog pieces of mountaineering gear). Garmin has had on-device auto calibrations for awhile. Not sure how they're doing it on the newest stuff, but when it was introduced, it was triggered by saved waypoints. So you could save a waypoint at trailheads and other points along your route and when you would pass by that location, the computer would auto calibrate your elevation to that of your saved waypoint. Wasn't perfect by any stretch, but improved consistency of an elevation profile. My handheld GPS allows me to manually calibrate the altimeter, which is going to be best practice, but is pretty onerous for a fitness device.


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## Jayem (Jul 16, 2005)

Harold said:


> Doubtful. The higher vertical accuracy of GPS in aeronautical applications is kindof inherent to aeronautical use. The receiver is above the ground with minimal obstructions like the terrain itself.
> 
> It's DEPENDENT on the overall GPS accuracy, which is extremely variable for consumer grade gear in ground-based uses. Barometric altimeters are relatively cheap and a fairly known entity (the calibration thing has always been true, even when barometers were analog pieces of mountaineering gear). Garmin has had on-device auto calibrations for awhile. Not sure how they're doing it on the newest stuff, but when it was introduced, it was triggered by saved waypoints. So you could save a waypoint at trailheads and other points along your route and when you would pass by that location, the computer would auto calibrate your elevation to that of your saved waypoint. Wasn't perfect by any stretch, but improved consistency of an elevation profile. My handheld GPS allows me to manually calibrate the altimeter, which is going to be best practice, but is pretty onerous for a fitness device.


While there could be reasons to attribute GPS accuracy to, what you are saying is not true.

The vertical accuracy of GPS in aircraft is not good without WAAS. That's what is necessary to make it work for these applications. The height of an aircraft at 5,000 is nothing compared to the satellite height and the angles generated. In other words, you aren't seeing drastically different angles. The WAAS approaches are close to the ground, we are talking within 200 feet. The vertical accuracy of GPS is not used much at higher altitudes, it's all real close to the ground stuff, for approaches.

Regular GPS without WAAS is not capable of vertical glidepaths or decision heights. It's inaccurate for vertical accuracy for the same reason it's inaccurate on the ground. I'd have to go back and read up again on why, I seem to recall it's something to do with the angles are generally pretty low for GPS, since they move pretty fast, you are more likely to be seeing the sats at lower angles, rather than directly overhead (high angle). That could be totally wrong though. 

Yes, manual setting of the altitude at a known location is generally the best way to do it if you are using baro. It's not the preferred option with aircraft, for a few reasons, but it's an option there too.

WAAS is not worldwide. It's in more locations now, but it's not everywhere.

Interesting fact, GPS availability is actually better up here at northern latitudes. We "see" the sats for longer before they pass behind the earth, due to our location and their orbits, which means we see more at once. Makes sense when you realize the system was for launching missiles over the pole.


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## Harold (Dec 23, 2003)

Jayem said:


> While there could be reasons to attribute GPS accuracy to, what you are saying is not true.
> 
> The vertical accuracy of GPS in aircraft is not good without WAAS. That's what is necessary to make it work for these applications. The height of an aircraft at 5,000 is nothing compared to the satellite height and the angles generated. In other words, you aren't seeing drastically different angles. The WAAS approaches are close to the ground, we are talking within 200 feet. The vertical accuracy of GPS is not used much at higher altitudes, it's all real close to the ground stuff, for approaches.
> 
> ...


WAAS is still a satellite-based system (at least, the corrections are transmitted to receivers via satellite signal), and it works in conjunction with GPS, over a signal that's compatible with GPS receivers. You could say it's part of GPS. A part that monitors the system and provides for important corrections. Other regions have their own similar types of systems.

It's been available in consumer grade GPS receivers for a very long time. It provides a relatively modest boost in accuracy in consumer grade GPS gear. Every handheld I've used for the past 20yrs has used it, and indicated WAAS-capable satellites with a "D" on the screen showing satellite locations and signal strength.

Aviation gear has a whole lot more going on with it than consumer grade handhelds and fitness receivers. Those devices are never going to be able to reap the same benefits of WAAS that aviation gear does. 

I don't think I've ever seen WAAS listed in the specs for fitness receivers, so it's really not an on-topic conversation point here. I assume that means they're not WAAS-enabled, and I'd be curious to learn why the capability is not included in fitness receivers.

This is what Garmin has to say about it, which is slightly more relevant here than the FAA stuff that's aviation-focused.

Garmin | What is WAAS?


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## Jayem (Jul 16, 2005)

Harold said:


> This is what Garmin has to say about it, which is slightly more relevant here than the FAA stuff that's aviation-focused.
> 
> Garmin | What is WAAS?


I'd still argue that. WAAS was instituted to fix the altitude issue mainly. There's some good "how" on that page, but the main issue with GPS moving forward in the early 2000s was the altitude issue. SA, Selective Availability was a potential issue, but GPS units were starting to overcome it even if SA was turned on. Differential GPS and multiple networks like Galileo makes WAAS sound redundant, but that's not what it's for, it's to significantly boost the vertical accuracy, which is pretty crappy to begin with. Being a stationary satellite relative to the ground, the height is a known value, distortion in the ionosphere can be accounted for based on the speed of light, etc. A touted side-benefit is that it functions as "an additional satellite", which is true, but the main reason for it is to boost vertical accuracy, since that was the weak point of GPS up until then. 

But most units are at least starting to use multiple sat networks now. Most of my altitude error is due to me being lazy and not calibrating the baro on my units with baro first.


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## NordieBoy (Sep 26, 2004)

(nostalgia) it was so cool when SA was turned off (/nostalgia)
Didn't help much with geocaching at the time though.

With barometric altitude here, I can gain 700m and lose 850m in a loop back to home.
Going over a sunny summit into a cooler misty valley with a 10°c temperature drop can play havoc.
I've spent a day in the hills in the sun @20° and come back through a river valley near home @5°.
It's a shock to the system that's for sure.


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