GPS elevation query

Quite a few years back I worked for an american company making military radios these all singing all dancing devices had gps fitted, and they had to be tested before being shipped.

It's amazing how we managed to stay on our feet whilst working in a building that rose and fell by over 60 meters every single test

I believe Strava now has the best, most accurate and comprehensive elevation data on the internet, as they have been effectively crowdsourcing the data from everyone with a barometric garmin who logs their rides with them. This means they end up with a very good average from all the people who ride the roads in different days/pressures etc.

I used my new Garmin Touring for a ride the other day and it showed 1082 m elevation. When I loaded it onto Garmin Connect it changed to 819 m. I then noticed that there waa a switch at the bottom of the page with 'elevation enabled'. When I disabled this it went back to 1082. There is an explanation near the button that explains that the Touring that doesnt have an altimeter takes elevation from GPS, but Garmin Connect will compare the map with actual surveyed heights and correct the elevation aa this is supposed to be more accurate.

Like many of the above, I've compared readings. If you look at the raw reading from a GPS device as a plot it usually looks jagged - like the kayak example above I've used a forerunner on a flat run which shows lots of ups and downs. When I used sporttracks to record the data, I used an elevation correction function in the software, which smooths the data, giving a much lower number. The websites apply a similar function to the recorded data. When I've run the comparisons, I've found the order to be:
Raw
Garmin with barometric
Ride with gps
Sporttracks smoothed
Bikeroutetoaster (sadly defunct)
Map my run/ride

With the sort of variance described above, mapmyrun being consistently 20-30% lower than bikeroutetoaster. I've never used strava, so I don't know where it sits. I guess that this means that mapmyrun has the most aggressive smoothing algorithm.

So for comparing routes, just use the same one, for demonstrating awesomeness use one of the top ones!

Heh heh - quite agree ScottChegg.

Got no problems with whatever climbing it turns out to be (the whole route I'm looking at doing according to Strava is 198 miles 13,500ft) - just really surprised that two different sets of mapping data can produce such a wide variance of elevation over the same route, they're not even close.

This is my take on elevation data:

Plan a route on said route planner
Take the route planners elevation date as a guide
get out on bike and enjoy ride
if there is more climbing than expected suck it in and MTFU!!

IIRC, and I may well not, most mapping sites use the free elevation data produced by NASA. The data they release for free for the "rest of the world" has a measurement interval of just under 100m (its a round number of arc seconds or some such unit which comes out about 100m on the earth's surface). This interval will filter out many undulations that you will have to go over and so make routes look smoother. In the US they release much higher precision data I believe.

Having had a quick google I think this is the stuff: SRTM on Wikipedia

Please note I may have just imagined all that.

I have Google Tracks on an Android Smartphone, which uses GPS and the elevation for the same ride can vary day by day..I assume MapMyRide uses the same system

I got a bottom of the range Garmin as a Xmas pressie and have noticed the elevation figures on the maps you create in Garmin Direct are more consistent ie they all give the same height for points on my ride. But the calculated figures for total Ascent / Descent vary each day

the map data is likely to be the most accurate overall. I don't think the accuracy of consumer GPS units is such that the elevation can be trusted, they are mainly aimed at pinpointing your position on a 2d map. I once took my garmin out kayaking and over a 3 hour session it mapped my elevation at +-60m above / below sea level which was not entirely accurate, which while some of the time was spent submerged, it wasn't that deep.

Oops, I probably mean measurement interval, not sampling interval.

Note that there is no correct elevation gain number. It will depend inherently on the sampling interval, and you can make those ridiculously large, as well as ridiculously small.

It's a bit like the "How long is the coastline of Britain" question...

Been playing around with the Strava Route planner and comparing it to MapMyRide tonight for a ride I'm planning next year

One section of it (Pontypridd to Llandovery - 50 miles) came out as 4200ft on Strava and 2550ft on MMR. Identical routes.

How can they be this far apart? Not like the route is over roads that are hardly used either

As an experiment, I've just looked at a ride of 1km from my house on Map My Rides - it gives the final 200m or so as 3 per cent gradient, which is way out - I have no idea how much it actually is, but if it hit over 20 per cent I wouldn't be surprised.

[Edit: Checked on OS map - it's between 14 and 20 per cent. Contours vs distance suggests closer to 20]

Thing is, it's preceded by a shorter but equally sharp downhill section. So as first poster above said, there's clearly some sort of error there - in this case I reckon it probably takes somewhere around the start of that dip and the top of the climb and misses out the whole bit in between.

Over a long ride, things like that are going to seriously add up.

ps I don't ride that road often

pps My rides are always more than 1km - just thought I'd use that distance on a road I know well to see how MMR compares with the actual terrain

You're quite clearly overthinking this; it's the biggest number, obviously. Unless someone can produce NASA data to the contrary.

Id go with the biggest number.

I use a Garmin and elevation figures vary even on the same routes but ridden on different days. How ever you measure it you 'only' climb what you climb and not really knowing just adds to the adventure.

Three different methods of calculating elevation:

1. GPS derived.
2. Barometric (I believe this is what Garmins use).
3. Map data.

All have their plus and minus points, and which is more accurate depends on the exact situation and the quality of the data.

Yes.

I get a lot of variation between my Garmin, bikemap.net and bikeroutetoaster. Like you I don't know what to trust.

I suspect it's to do with the sampling frequency used to calculate heights. For example, if the maps are only sampling elevation every 100m then they may miss small undulations that the Garmin picks up, if it's calculating more frequently.

However, that's speculation on my behalf. I'd like to know if anyone has a better insight into what's happening.