GPS - accuracy of altitude figures

nasahapley

I was going to ask this on another thread about route-mapping websites, but didn't want to hijack it.

I've been thinking of getting a GPS unit for the bike. At present I use a route mapping website (Bikely), to get an idea of how much climbing a route involves. I know the altitude figures I get from Bikely won't be mega-accurate, but I was wondering: are GPS units that much better? I ask because the total climb function is pretty much the only thing I'd want to know from the GPS that my computer can't do already, but a few of my fell-running mates say that they don't really trust their GPS to give a very accurate figure (I don't know if bike-specific ones are generally better in this respect).

I suppose the follow up question is: are there any makes/models of GPS that are known for being more accurate than others?

Cheers,
N

GeorgeShaw

AFAIK, altitude on a GPS can be done in two ways: by GPS or by a pressure-measuring altimeter. Altitude by GPS is pretty inaccurate. Altitude by pressure is a lot better, but you're still affected by pressure changes on a long ride. It also measures better if you give it a few minutes to calibrate properly. But still a pressure altimeter is better than a GPS altimeter so, for example, the Garmin 305 is better than the 205.

After that, you're at the mercy of the software you put the data into. I've downloaded to several different packages, and they all give different results. The reason for this is, I think, that they all smooth the data in different ways. For example, if you are going along a relatively flat road, you might be rising and falling 5 metres, or there might be a measurement error of the same. Will the software record all such changes, or will it ignore them?

jibberjim

GeorgeShaw wrote:

AFAIK, altitude on a GPS can be done in two ways: by GPS or by a pressure-measuring altimeter. Altitude by GPS is pretty inaccurate. Altitude by pressure is a lot better, but you're still affected by pressure changes on a long ride. It also measures better if you give it a few minutes to calibrate properly. But still a pressure altimeter is better than a GPS altimeter so, for example, the Garmin 305 is better than the 205.

The 305 doesn't have a pressure altimeter. The 705 does.

The 705 often gets it wrong too when the pressure is changing on a ride.

I use Sport Tracks - with the elavation correction plugin, which corrects pretty well, the data for the UK is pretty good.

myopic

I use a Garmin eTrex Summit which does it by GPS. Most of the readings I get are spot on with OS maps, though occasionally I get readings I know can't be right - eg 5 meters below sea level! I think there's as much variability as there is with location - the more satellites you're tuned into at the time the better the accuracy, so it varies. Overall the profiles for rides are as accurate as you need them to be.

GeorgeShaw

jibberjim wrote:

The 305 doesn't have a pressure altimeter. The 705 does.

From the Garmin 305 specs page: https://buy.garmin.com/shop/shop.do?cID=160&pID=331#specsTab

Barometric altimeter: yes

schweiz

The Garmin Edge 305 does have a barometer to measure air pressure. The purpose of the barometer on the 305 or any other GPS device is to smooth out the errors in the WGS84 model that is used by the GPS system. In this model the earth is mapped as a simple spheroid, so errors are not uncommon.

For a GPS receiver to display height, it needs to be receiving a signal from at least 4 satellites. The greater number of satellites and the relative position of these satellites to the receiver (i.e. some are overhead, some are on the horizon) all affect the accuracy of the unit both in plan and altitude.

The Edge 305 altitude measurement works by calibrating the GPS height with the local air pressure when the unit is switched on. The change in altitude can then be calculated by the both the GPS and the barometer, giving a hybrid solution. As you ascend, the air pressure drops 1 millibar for ever 10 metres of ascent, and vice versa. Assuming air pressure stays constant then this is an accurate system on its own. However as weather fronts move across, the local air pressure changes (I often get +/-10m altitude change on a 1 hour turbo trainer session) so it cannot be used alone, hence why the hybrid solution provides a better altitude figure.

Sometimes (normally on my commute) I don’t wait for a 3D fix before I start and the 305 is not sufficiently calibrated and my start altitude can be 100m in error however at the end of the commute, the finish altitude is out by only 20m and the profile is smooth and shows no big steps in altitude change.

Another interesting point with the 305 is that it will record all the little ups and downs caused by GPS errors and you can end up with a total ascent/descent for the day that is too much (up to 10%) but then the tool that I use for downloading data to (Ascent – Mac only) filters out these blips and I get a reasonable altitude figure for a days ride. When I compare the profile to a Topo Map it looks accurate too.

That being said, I was climbing the Julierpass in south eastern Switzerland last summer and at one point my altitude was dropping even though I was definitely going up!

ColinJ

I use the cheapest Garmin Etrex which derives its altitude reading from the satellite data. It's usually pretty accurate (within 5 metres or so of the value shown on an OS map) as long as it has a good lock on the satellites. It only needs 'sight' of 3 satellites to derive the ground position, but it needs 4 to derive altitude. Steep narrow valleys can catch it out but usually it is reliable. I've compared the tracklog profile with the profile taken from the OS map route and it is hard to separate them. For my purposes - that's near enough.

GeorgeShaw

... then the tool that I use for downloading data to (Ascent – Mac only) filters out these blips ...

That was my secondary point. Different software will give different values (try it, create a route in BikeRouteToaster or wherever and import it into different software).

You can get too obsessive about this - after all what is "real" altitude gained? With all the little bumps or without? What is a little bump? OK, it's easier when you have real mountains like the Alps, you know for sure how much the climb of the Galibier is, as the only way is up. But in the UK, the values you can get vary widely.

Personally I treat my 305 as a recording device, and import it into the same software each time, so I can compare rides. This gives me a guide to how well I've performed. But whether the data is accurate, or what "accurate" means, well who knows.

schweiz

That was my secondary point. Different software will give different values (try it, create a route in BikeRouteToaster or wherever and import it into different software).

Agreed, Garmin Training Center and Ascent give completely different values for altitude and I think Bike Route Toaster is always on the high side. I'm not sure how accurate the Google Maps height database is.

You can get too obsessive about this - after all what is "real" altitude gained? With all the little bumps or without? What is a little bump? OK, it's easier when you have real mountains like the Alps, you know for sure how much the climb of the Galibier is, as the only way is up. But in the UK, the values you can get vary widely.

By bumps I mean +/- 2m when you are definitely descending or ascending. 100 of those in a days riding and your metres climbed/descended is 5% out on a 4000m day. Lose signal quality and you easily double that although I accept your point that on a rolling country road that is 'flat' you'll have loads of these. To count them or not? I don't know!

Personally I treat my 305 as a recording device, and import it into the same software each time, so I can compare rides. This gives me a guide to how well I've performed. But whether the data is accurate, or what "accurate" means, well who knows.

Agreed. The altitude and the even the distance are not 100% accurate but for me it's an easy way of maintaining a training diary.

synchronicity

My old Polar S710 calculated the altitude by measuring the pressure. The good thing was that it was temperature calibrated. So when you climb up a mountain for example, it is usually colder the further you go up and it takes that into account along the way. Sometimes there is a temperature inversion in Tenerife when Saharan dust is present in the atmosphere... again, it seems to take that into account too. I found the readings were pretty accurate.

The GPSes that I've seen which attempt to triangulate altitude didn't do a very good job...

nasahapley

Thanks for all the info chaps - some really useful stuff there. Good points about how the sampling error may be greater than a lot of the small rises and bumps you'll encounter on a typical hilly ride in Britain - even though I try to seek out big climbs in the Dales, I expect the minor undulations will still make up a very significant chunk of the 'real' total climb (they all count as far as I'm concerned!) Bit of a pain that gps will work best for long alpine climbs, where you can accurately tot up the total climb in a couple of minutes with an IGN map! I think for now I may as well stick with an online route-mapper - everyone likes a new gizmo, but I guess being able to roughly compare the different routes I do is more important than knowing I've got an inch-perfect total climb figure.

(Unless anyone's selling a gps at a knock-down price...)

Martin Van Nostrand

jibberjim wrote:

The 305 doesn't have a pressure altimeter. The 705 does.

.

The 305 certainly does have the same pressure altimeter as the 705.