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Post details Post   (Contract pics)
simon3
2012-04-19 13:44:54
Re Automatic Calculation of Ascent
Context of comment:At least two people have queried the ascent / descent figures created by the track system. I have also noted a number of anomalies in results with test gps tracks from various sources including ones I made myself.
Message:Anomalies include a) usually an apparently high estimate for height gain b) a different figure for ascent and descent. Captain V's apparently lower estimate is a further exception.

The current algorithm used to determine the height gain or loss is extremely simple: it looks at the height recorded for each trackpoint (of which there can be typically between 500 and 5000 for a track). It then adds a positive increase in height between heights reported to the total ascent and adds a decrease to the total descent.

There are several ways of calculating height gain and each have pros and cons. The "spreadsheet" way that I used years ago was to use a spreadsheet accumulating height gain/loss between waypoints. This has the advantage of being self consistent, but the disadvantage of not recording minor ascents/descents between waypoints.

Then there is the way it is done by the GPS units. However two GPS units going over the same route do not always seem to produce the same figures and are often different by 10% as well as being often 10 to 15% higher than the "spreadsheet" method.

The most consistent method I have seen is that employed by Trailmaster which uses an accurate DEM (digital elevation model) of the land, however this DEM is not at present public and the public versions that are available would not necessarily improve the situation enough to be worthwhile using.

The reason for choosing the current method is that it is transparent. No smoothing or calculation other than addition or subtraction is done, however I would consider a different approach if necessary.

(Note: for speed estimation as used for estimating the users speed and colouring the track an elaborate smoothing algorithm IS used which seems to work well.)
Additional points:So I would be happy to make changes however based a. on evidence and b. on a clear argument as to why another algorithm would be better.

Call for voluntary help. I could use help from someone with statistical ability and some appreciation of what can and cannot be done using numerical methods to analyse sample tracks and devise a proposed, better method of calculating height calculation.

In the meanwhile, here are some known things you can do to improve accuracy.
a. turn on gps in advance of walking, for example in your house but in view of the sky, to allow the unit to figure out its position ("finding the ephemeris"). Turn it off after not more than 12 minutes. When you then turn it on at or immediately before the walk starts let it have a good view of the sky (reduces high initial errors in readings)
b. when carrying the gps position it so it always has a good view of the sky. This reduces trackpoint height measurement errors, which can accumulate and give spuriously high figures.
c. in general leave the gps on and in good view of the sky for the whole walk. (reduces start errors)

Remember also that no matter what solution is arrived at, there is no perfect answer as the problem is similar to that notorious one of measuring the length of a coastline, which depends on size of the ruler. However that said, there could be a better algorithm with an explicable rationale.
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