Elevation data ??

GavH

When it comes to elevation data, what is the most accurate source?

Yesterday, after downloading my 30 mile ride data to SportTracks, it showed the ride consisted of 2278 ft of ascent. That was in stark contrast to Bikeroutetoaster which I used to plot the route as it predicted only 1645 ft. I've now just plotted the exact same route on Bikehike only to find it suggests 2664 ft!!

What is the most accurate of the three? I have the latest firmware on my Garmin 705 and use the elevation correction plugin for ST so I'm assuming the 2278 ft shown on ST is the more likely to be close to the mark.

Pross

I'd say the Garmin as it's recording actual data and has the barometric back up as a correction I think? I don't use either of the sites you mention but assume they just use OS data or similar to measure between contours.

johncp

Pross wrote:

I'd say the Garmin as it's recording actual data and has the barometric back up as a correction I think? I don't use either of the sites you mention but assume they just use OS data or similar to measure between contours.

I did a one hour turbo session in the garage yesterday - apparently I dropped 66 feet from start to finish according to my garmin 305 with barometer :roll:

Pross

Try raising the front of your turbo up a bit Maybe the effort you were using raise the air temperature and pressure

GavH

Johncp wrote:

Pross wrote:

I'd say the Garmin as it's recording actual data and has the barometric back up as a correction I think? I don't use either of the sites you mention but assume they just use OS data or similar to measure between contours.

I did a one hour turbo session in the garage yesterday - apparently I dropped 66 feet from start to finish according to my garmin 305 with barometer :roll:

That's not unusual. I've just done a quick half hour on the turbo in the garage and apparantly finished somewhere near the roof, having climbed 25ft according to the Garmin. I've tried establishing exactly how the Garmin calculates altitude and suspect it is largely down to barometric pressure (as well as the calculation which 'tell' the device how far it is from each satellite?) which is affected by temperature, hence as the garage heats up (or cools down if you start opening doors or whatever) maybe this impacts on the device?

Yesterday, it was almost spot on, telling me that I finished 1 ft lower than I started nearly 2 hours before.

Stuck on a Giant

I get weird elevation data indoors, and it often takes a while to settle down outside - I seem to live at somewhere between 30 and 70m elevation.

bompington

If you heat up the air in the garage, it should raise the air pressure, and so a barometer would give you a lower altitude.
I think it's called "inaccuracy" and all systems will have it in some form. I've found that my nokia 5800, which is generally easily accurate enough to be a useful tool for navigation whether by foot, bike or car, can sometimes wobble by up to 100 horizontal metres, and its altitude data is so up and down that the route profile after I've walked the dog sometimes looks like the Aiguille du Midi.
I don't know the exact figures but I believe basic GPS systems are only accurate to between 1 and 10 metres, depending on various things - which means that you should get a fairly accurate reading of your current altitude if that's what you want, but you're likely to get a very inaccurate figure indeed if you want to calculate total height gained on a typical british undulating road.

GavH

bompington wrote:

If you heat up the air in the garage, it should raise the air pressure, and so a barometer would give you a lower altitude.

Hmmm, fairly sure that as the air temperature increases, so the air pressure decreases. Given that air pressure decreases with an increase in altitude, so a barometric altimeter will 'sense' the decreasing air pressure as an increase in altitude. Hence my barometer increased 25ft as the garage got warmer.

Anyway, back to the topic, does anyone know which is more accurate, Bikehike, Bikeroutetoaster or whatever the Garmin says? Without anyway to calibrate the start altutude, as I would do with my Suunto Core before I go hill walking or climbing say, how does the 705 know it's start point? Is that lifted from the mapping or by trilateration with the satellites?