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Post details Post   (Contract pics)
simon3
2009-04-02 14:55:29
Re Checking or calculating altitude and ascen
My short answer would be to find height gain: use the contours with a bit of guesstimating for where the start or end of a climb is between them.

You may point out that this does not include minor ups and downs that don't appear in the contours or even the height gain and loss going over peat hags etc. You may ask why not use the GPS figure for accumulated height gain.

This then brings us to the long answer. As far as I can see there is no right way of doing this. If you use a GPS it constantly calculates height around about every 2 seconds. These height estimations have an inherent error in them (around 1.4 times as great as horizontal error) which is constantly changing. As far as I can see these random errors make it look like your accumulated height gain is rising so the figure given by a GPS always seems to be more (as in around 30% more) than that given by looking at contours. The GPS may well be recording extra height gain resulting from rough ground, but the signal is probably lost in the noise from random error. Like a coastline or any fractal, the complexity and the height gain expands as you increase the detail.

Surely this can be fixed by a GPS with a pressure based height sensor? Not in my experience. Minor variations of the almost omnipresent wind as you move around cause fluctuations just as bad as the GPS radio based system.

There are various ways of mitigating readings from a GPS tracklog. GPSU software has a filter which smoothes small variations for example. However the only "Modern Navigation" tool in Ireland that seems to give a figure consistently similar to what you would expect from reading contours off the map is OSI Trailmaster which presumably is calculating the height gain by using its in-built DEM (digital elevation model) which tells it how high every point is.

That's what I tend to use now when it is available.
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