General Cycling Discussion - GPSs: How accurate?

I've been riding my road bike without a computer, because I have a handheld GPS. Supposedly it's accurate to 31 ft. I think that just means where I am on the planet. Are they more accurate than a cycle computer gauged to wheel size, and taking in weight on the wheels. I mean just how accurate are they? When I rode with it and compared it to my computer on the bike it was like off by 2-3 tenths of a mile. Today I looked down and saw my road speed average at 17 mph, and watched as it dropped to 16.9 as I came up on a hill and a stop light, and my top speed came in at 36.1 mph, I was going down hill in the big gears.
Just how accurate are they? Anyone else use one for their rides? I know in the big Tours they've been using GPS fixes on the leaders and the peleton, but are they accurate?

I have a GPS unit on my cell phone that is accurate to 3 feet. When I'm using it in my car the speed pretty much matches the speed on my speedometer.

The GPS is very inaccurate. My GPS indicated that I went 50 MPH on a rather large downhill. It's possible since it was a new bike but highly unlikely. Still. I won't go back to using maps again as the GPS saved my but from getting really lost.

It's unfortunatly these devices are made inaccurate for security purposes.

Last week, on a train heading to an LBS in Philadelphia to get my road bike. I wasn't sure which stop to get off so I put my GPS next to the window and watched the numbers tell me how close I was to the shop. Incredible.

I've been riding my road bike without a computer, because I have a handheld GPS. Supposedly it's accurate to 31 ft. I think that just means where I am on the planet. Are they more accurate than a cycle computer gauged to wheel size, and taking in weight on the wheels. I mean just how accurate are they? When I rode with it and compared it to my computer on the bike it was like off by 2-3 tenths of a mile.

2-3 tenths of a mile error means your bike computer is not calibrated properly.

However, even if the bike computer is calibrated exactly, there will be some difference between it and the GPS. When you ride you swerve slightly all of the time. The bike computer records the distance your front tire actually rolls, so the computer records the swerved distances The GPS isn't sensitive enough to do this. The error of the GPS is about one percent for this reason, I've observed.

I think a GPS unit is a poor substitute for a bike computer for a few reasons. It eats too many batteries, you have to take the thing off of the bike if you stop anywhere, it can lose the signal under trees or among buildings, it takes a minute or two before you ride to fix its location, and, as I've said, it doesn't record your actual distance traveled. Still, they're a cool thing to have in addition to a computer.

the civilian version of GPS is set up to be no more accurate then +/-25 feet. The part of the satellite signal that allows it to be more accurate is scrambled and only military equipment can use it for its full accuracy.
That said, since you are riding on roads on a map and not flying in the air toward a target :), the software on the device can "figure out" where it is since it assumes you are staying on the road.... also, this 25ft tolerance is not cumulative so you may be off by 25 feet after a 50ft ride or after a 50 mile ride...

The difference was at the end of a 25 mile commute to work, and then that night on the way home. As for batteries, I use rechargeables, so I'm not trashing the planet any more than necessary. I do notice when I look at the trails, the plotting of my routes it does show pretty accurately which side of the road I was on, there is that measure of difference. It is a fun tool to have, and sort of fun to look back and see where I've been. Can't wait to get out on a century with it.

Lets say there is a hill that is real steep, and it is one mile long. But because it is real steep and one mile long if it it is seen from outerspace it only looks like .85 of a mile. Would I loose the difference in the distance, the .15 mile becuase I used GPS and it calculates my movement from outerspace rather than by wheel revolutions? Theis is where difference between cyclo computer and gps eminate from, in my humble opinion.

I used GPS in a bike tour in Thailand. It worked fine, it got me unlost on one occasion and it found a hotel in nakhon Sawan on another occasion. But over all as the roads were verywell marked it so it was more of a toy.

Lets say there is a hill that is real steep, and it is one mile long. But because it is real steep and one mile long if it it is seen from outerspace it only looks like .85 of a mile. Would I loose the difference in the distance, the .15 mile becuase I used GPS and it calculates my movement from outerspace rather than by wheel revolutions? Theis is where difference between cyclo computer and gps eminate from, in my humble opinion.

I used GPS in a bike tour in Thailand. It worked fine, it got me unlost on one occasion and it found a hotel in nakhon Sawan on another occasion. But over all as the roads were verywell marked it so it was more of a toy.

a GPS measures locations in 3D (Long/Lat/Alt), so if you travel "up" the GPS can detect the distance you travel. It is possible that the software on your device "assumed" you are traveling in a 2D world and does not take altitude into account. Airplanes use GPS just fine.. :)

Lets say there is a hill that is real steep, and it is one mile long. But because it is real steep and one mile long if it it is seen from outerspace it only looks like .85 of a mile. Would I loose the difference in the distance, the .15 mile becuase I used GPS and it calculates my movement from outerspace rather than by wheel revolutions? Theis is where difference between cyclo computer and gps eminate from, in my humble opinion.

I used GPS in a bike tour in Thailand. It worked fine, it got me unlost on one occasion and it found a hotel in nakhon Sawan on another occasion. But over all as the roads were verywell marked it so it was more of a toy.

I'm not sure I quite understand what you're saying, but unless the programmer who coded a particular GPS unit were cutting corners or something, hills shouldn't really matter. GPS gives three dimensional coordinates, which should make it pretty simple for software to account for distance traveled vertically. Small changes though, like swerves that merriwether mentioned or small hills that don't result in enough change in position to be noted by the receiver could contribute to error.

2/10's of a mile after a 25 mile ride is less than 1% error. That could easily be a calibration error on your bike computer or the swerving effect.

I use a magellian sportrak gps, its WAAS enabled and all. It works good but I just wasn't sure about the hills but the comments I've read make sense.

The thing everyone seems to forget is that GPS is a signals based positioning device. It determines speed and such of course by looking at incremental differences and changes in position. Being signals based means it can drift and is subject to jitter. Over the course of a long period of time this gets "internally corrected" but in shorter segments and realtime you will see discrepencies. Take a GPS receiver, set it on a table and look at the altitude and position readings. You will most likely see them bounce about a bit. This is sometimes due to SA but most often it is due to the unit trying to reconcile inaccuracies due to signal anomalies.

Used to use a GEKO 201, it was pretty much spot on. Lovely unit. Sold it to get a bluetooth jobbie next month...

if yoy happen to drive a car equip with a GPS and if it happen to be a stolen car, the police would be at your rear view mirror in a jippy.

if yoy happen to drive a car equip with a GPS and if it happen to be a stolen car, the police would be at your rear view mirror in a jippy.

Ummm... not exactly. There's a difference between a GPS receiver and one with a receiver tied to a location transmitter. A receiver only allows you to pick up the satelite signals so you can determine your location. How you reconvey that information is up to you and is often done out-of-band by means of some ancillary radio transmitter.

I used GPS in a bike tour in Thailand. It worked fine, it got me unlost on one occasion and it found a hotel in nakhon Sawan on another occasion. But over all as the roads were verywell marked it so it was more of a toy.

This is exactly why I use my GPS. Getting lost with a bicycle is time consuming and exhausting. Even though the GPS is not perfect, it's not made with the type of accuracy that a cycle computer can provide. If you want total accuracy, get a cycle computer.

This summer, I'm exploring our extensive bus transit system. I'll be riding all the bus lines (with my folding bike) until the last stop leaving me hopelessly lost. That is, until I turn on my GPS! I've never did anything like this and there are dozens of bus route that cross this state. I'm really looking forward to getting lost as the prospect of ending up in a town I've never been to excites me to no end!

Ummm... not exactly. There's a difference between a GPS receiver and one with a receiver tied to a location transmitter. A receiver only allows you to pick up the satelite signals so you can determine your location. How you reconvey that information is up to you and is often done out-of-band by means of some ancillary radio transmitter.
Khoun,
thanks for the information, I thought it was like that, anyways thanks, I stand corrected
oscar

I've used the Geko 301 for about 3 months. On a typical ride, the position accuracy is within 16-30 feet, except when the WAAS kicks in, which only occurs in very open territory. Accuracy with WAAS is 10-16 feet. But either mode is quite accurate for my purposes. Accuracy with the barometric altimeter is okay, but accuracy with the GPS corrected altimeter is even better, probably +/- 5 feet.

I use the GPS for plotting routes on mapping software after my rides. I have found a discrepancy between the distance plotted on my cyclo computer, vs. distance on the GPS. The GPS always measures less total mileage by about 2%. My thinking about the difference was this: the GPS uses a track log to plot navigation data. The Garmon manual explains that this is like leaving a "trail of bread crumbs". The track log puts down a bread crumb every few seconds. So, when one rounds a corner with a GPS, there will always be some corner cutting by the track log. In tight switchbacks this becomes more obvious when you look at the route being plotted. Over the course of miles and miles, the corner cutting results in a few miles discrepancy.

Also btw, if anyone would like to see how the map plots look, I can shoot you an email of a recent ride...

I've been riding my road bike without a computer, because I have a handheld GPS. Supposedly it's accurate to 31 ft. I think that just means where I am on the planet. Are they more accurate than a cycle computer gauged to wheel size, and taking in weight on the wheels. I mean just how accurate are they? When I rode with it and compared it to my computer on the bike it was like off by 2-3 tenths of a mile. Today I looked down and saw my road speed average at 17 mph, and watched as it dropped to 16.9 as I came up on a hill and a stop light, and my top speed came in at 36.1 mph, I was going down hill in the big gears.
Just how accurate are they? Anyone else use one for their rides? I know in the big Tours they've been using GPS fixes on the leaders and the peleton, but are they accurate?


Hi Naisme,

I am a Survey Engineer and I use GPS all the time in my work. In a nutshell, a GPS receiver is deisgned to measure the time it takes for a signal to travel from an SV (space vehicle) to your receiver, velocity x time = distance. This distance is called a range, and having several ranges from SVs will give you your position, you need at least 3 for a 2D position and 4 for a 3D position (lat, long and altitude).

The accuracy of a GPS is depended on several things, the most significant being:
1) satellite geometry, the further satellites are spaced apart the better the accuracy (in simple terms).
2) tropospheric delays, the lower part of the atmosphere that experiences the most tempreature, pressure and humidity will cause delays in the signals from the SVs.
3) Multipath, being close to tall buildings, trees, buses, etc will cause the signal to bounce and will not follow a straight path to your receiver hence, signal delay.
4) Errors in the clock on the SVs or in your receiver
5) Noise errors in your receiver or signal noise from the SVs.
6) Quality of receiver and antennea and type of signal it can receive. There are 2 code types, civilian and military.

Selective Availablity was once a soure of error. SA was the intential degradation of SV signals to increase time delay to your receiver. SA has been now switched off.

How accurate are they? Well, tough question to answer. I have a GPS backpack that I use for work that uses a corrected signal and I get 1 metre (3 feet) positional accuracy 95% of the time. But this GPS backpack is a single fregency receiver. Another one we have, which is a dual freqency receiver can measure to 2cm. But dual fregency receivers are VERY expensive and not worth the extra money for the normal user. If you are out hiking and your GPS brings you within 3 metres of your campsite is there a big advantage of knowing that if you had used a dual frquency receiver that you are with 2cm of your campsite?

My home handheld receiver gets about 5 metres 95% of the time but it is not WAAS corrected. Although they claim 10 metres, I have found this to be slightly better.

WAAS corrected receivers are about 3 metres 95% of the time, but CAN be better (or worse) based on the errors I listed above.

Essentially the WAAS receivers will be less...jittery and will allow the signal to be more settled, hence you get a better accuracy.

A bike computer error list is much shorter (provided there are no manufactoring errors throwing the thing off):
1) incorrect calibration for the wheel size
2) tire pressure
3) weight on the bike

To ensure a properly calibrated computer it is best to do a roll out test. Weight and tire pressure, if you keep them the same for the roll out test will be negligable.

Please keep in mind that GPS receivers measure horizontal distances unlike bike computers which will measure slope distances.

What I mean is if you ride up a hill that is 2km from the base to the top but only is 1.5km run (rise over run) your GPS unit will measure 1.5km which is the horizontal distance NOT the slope distance. A bike comouter will measure the number of times your wheel will go round and hence this is a slope distance as you are measuring each uphill and downhill.

In short, your bike computer is going to be more accurate than a GPS receiver. How much more depends on the terrain and the source of errors I listed.

Then again if you go on a 100km bike ride and your computer tells you 100.3km and your GPS says 99.6km. What's the big deal? But that depends on your pickiness on how close you want to be.

Digger

I'm seeing a lot of inaccurate information about GPS in this thread. Correct information is so readily available that nobody should have to put up with bad info. For example, the Department of Defense does indeed have the capability to degrade the GPS signal. That's called Selective Availability (SA). But SA was set at zero by order of President Clinton on May 1, 2000. That means that full system accuracy is available and has been since then. Design accuracy for basic GPS is 100 meters, but in practice it is often much better. The Department of Defense has a supplementary system in place that is called Differential GPS (DGPS). DGPS increases accuracty to 3-5 meters by using land based reference stations. You can't use DGPS, but don't despair. There is also WAAS, which is the same idea but not owned by the Defense Department. You can buy any number of WAAS enabled handheld GPS units. WAAS accuracy is under 3 meters. So life is good.

The GPS system itself only provides position, which may include altitude if you pay for it in your unit. All functions that result from position-based calculations are dependent on the quality of the unit you buy. If your model gives erroneous speed readouts, don't blame GPS. It's your unit. Same thing with total distance. That's calculated, but it should be pretty darn good.

This stuff isn't rocket science, but it is science - which means it doesn't have to be mysterious.

I'm seeing a lot of inaccurate information about GPS in this thread.

Once again, everyone seems to think that GPS is perfectly reliable. we need to descriminate between accuracy and precision. Remember that precision is related to resolution and determines how close two or more measurements are to one another. Accuracy is how close to the "true" value each measurement can be made. While GPS may be more accurate than a wheel-sensor based bike computer, it is probably not as precise. The main reasoning for this is the CEP due to the fact that GPS uses a signal's based mechnism for deriving position. This signal can degrade and vary either through artificial (such as SA) or natural (sunspots, atmospheric abarations, etc) means. This produces a certain amount of jitter. This is known as DOP (Degradation of Precision). There have been several attempts to reduce jitter but none have been able to totally eliminate it. dGPS uses a second GPS receiver that's datalinked to the first and provides for range correction. WAAS uses some ground reference stations which broadcast up their reference signals (which include range-error correction) to a couple of geostationary satelites (one over east coast and another over the west coast) that then rebroadcast down to WAAS/SD equipped receivers so they can correct the primary GPS signals. This was initially developed by Ratheon (who termed it "Satelite Differential) in conjunction with the FAA to allow for precision approaches when it was found that standard dGPS wasn't cutting it. Here is a plot of a GPS ground receiver (in a stationary position). Note that this is for a non-moving receiver and that even with WAAS/SD (Satelite Differential), the accuracy can vary by +/- 3m which even if consistant to a second-order can still produce a high degree of imprecision over say a century ride.

This signal can degrade and vary either through artificial (such as SA) or natural (sunspots, atmospheric abarations, etc) means. This produces a certain amount of jitter. This is known as DOP (Degradation of Precision).

DOP is actually Dilution of Precision not Degragation. There are many kinds of DOP but the most qualtifiable DOP is GDOP under which you have HDOP, VDOP, PDOP and TDOP. DOP is not a jittery signal but rather the magnitude of error in a position.

V- vertical (height)
H- horizontal (lat and long)
T - Time
P - positional DOP can have the largest effect on positional accuarcy as it is based on 3 dimensions (lat long and height) i.e. the larger the PDOP the greater the effect on accuracy. The best value of PDOP to have is 1. This is NOT 1 meter or 1 foot, but is rather a unitless number to give and indication of positional accuracy.

Lets please not go any further with this conversation as we will get into sigma value accuracy and statistics and confuse everyone on this thread.

Naisme, GPS is accurate BUT depends on what you are doing and just how picky you want to be. Because of the uncertainty of all the errors I have listed for GPS your bike computer is going to give you a better estimate of distance than your GPS. No, you probably won't notice this over, say 500 metres if you compare the two, but will be more evident with an increasing distance.

Personally, if I did not have a bike computer, but rather a GPS I woudl be more than happy with its accuracy on just how far I rode tha day. But because of weight, battery power required, complexity, multipath probelm in the city I would choose a bike computer.

In short, a bike computer is more accurate (yes, if calibrated properly) than a GPS receiver.

Digger

Digger, that is an excellent and thorough summary.

However, isn't the source of atmospheric error primarily ionoshperic, not tropospheric? It's also directly related to satellite geometry, as you mentioned. The lower the bird is on the horizon, the greater the ionospheric crossection the signal must cut.

Good to see another surveyor on the board...khuon, what do you use GPS for?

khuon, what do you use GPS for?

Flight navigation, automotive navigation, geocaching, time-synchronisation for networking equipment among other things. I'm also generally a self-proclaimed technogeek.

Digger, that is an excellent and thorough summary.

However, isn't the source of atmospheric error primarily ionoshperic, not tropospheric? It's also directly related to satellite geometry, as you mentioned. The lower the bird is on the horizon, the greater the ionospheric crossection the signal must cut.

Good to see another surveyor on the board...khuon, what do you use GPS for?

The troposphere is the lower portion (8 to 13 km up) of the atmosphere that experiences changes in temperature, pressure, and humidity (i.e. weather changes).

The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air.

Gastrocnemius, yes, you are correct the ionosphere is much more difficult to model than the troposphere and hence a larger source of error. I did not include the ionosphere in my list of possible GPS errors. Thank you for pointing that out.

Both the troposphere and ionosphere cause signal delays. If a SV is low on the horizion the signal has to travel through more of both hence more signal delay. Much of the troposphere error can be removed using differential where as the ionosphere, one can only remove about half of the possible error, and even then, you can still have up to a 10 meter error.

Khuon, I am also a geocacher! I plan to create my own geocache sometime this fall. For now I am scouting a location and hope to make it challenging. Have you a cache of your own?

Digger

Khuon, I am also a geocacher! I plan to create my own geocache sometime this fall. For now I am scouting a location and hope to make it challenging. Have you a cache of your own?


I've been meaning to create my own cache. This area is fairly rich in caches and everytime I see a spot I think would make for a good cache, I look on my GPS (I store a lot of cache locations using EasyGPS) and there's already a cache nearby. There's some woods behind my house which I do a lot of MTBing in so I was thinking of locating a cache there. The problem is that it's very dense coverage and the minute you enter it, you get total loss of signal. Maybe I'll try and locate it at the very border.

I've been meaning to create my own cache. This area is fairly rich in caches and everytime I see a spot I think would make for a good cache, I look on my GPS (I store a lot of cache locations using EasyGPS) and there's already a cache nearby. There's some woods behind my house which I do a lot of MTBing in so I was thinking of locating a cache there. The problem is that it's very dense coverage and the minute you enter it, you get total loss of signal. Maybe I'll try and locate it at the very border.


Hmm, my area has about, ohhh, 10 or so in a 20km radius. Not sure what I can advise you to do other than instead of a 'normal' cache where you just walk to it on your GPS receiver, make it more difficult than that. Such as walking to a point but then having to use a compass and pace off a distance to find it.

One cache in my area told a pirate story about how a ship went aground back in the early 1700s and all lives where lost expect for one. The survivour hid the 'loot' nearby where the ship went down. The last line of the story went like this "The captains hook will point the way, 130 paces more or less it lay"

So, you follow your position down to this freakin' huge boulder along the shore, the waves are crashin in and you say to yourself how can a cache be here? The ocean would wash it away. So you climb up on this boulder only to discover a rusty hook embedded into the rock pointing off into the woods. Ahhhhhhh! The Captains Hook!

So you line yourself up with the hook, count off about 130 paces, crash through the woods and just about when your ready to give up in frustration and take a seat BAM! There it is under some bushes. You have to crawl on your belly under VERY dense bushes up inside this 10 foot clearing. There's the treasure! Skull and bones and treasure chest!

It was the absolute best cache I ever visited.

Maybe something along those lines. Make it more difficult and fun. I plan to do some sort of 'Bigfoot' story, where a camper was snatched up by bigfoot and give a position to a location where you find some of the campers bones with a clue to another location. Haven't worked out the details yet, but with the last clue you will have to turn your GPS off as no position will be given (UTM that is).

Digger