Map Technical Notes

These notes provide a brief description of the data and tools used to produce the walks/travel map shown in Walks Map.

Measurement of Distance Travelled

The calculation of diatance travlled from a GPX file is computed is many different ways by different tools. The measurements made from one file (20150728.gpx harcamlow) are (November 2016, km):

Garmin Basecamp: 41.4
Google earth, length: 46.6
Google earth, ‘range total’: 39
Leaflet-GPX default (3d): 46.55
Leaflet-GPX 2d: 41.31
uTrack flat distance: 41.3
uTrack real distance: 46.5

So main difference is from ‘flat’ or ‘sloping’ distances. Since elevation is very noisy in most gpx measurements, the difference is large. uTrack reports total climb as 8,126m, which is almost Everest scale – so clearly very wrong.

For this site the leaflet-gpx has been tweaked to use the flat (2d) distance rather the sloping (3d) distance.

It is noted that the computation of the flat earth distance in leaflet-gpx uses a fixed earth radius (6371km). While the earth radius varies from equator to pole the difference is small for this application and dwarfed by GPS measurement errors.