Does a gps give true distance?

walkabout · Occasional Visitor Joined: Nov 12, 2004 Posts: 31

When you get distance off a gps unit
does it allow for contours of the land?
I have been downloading data off my 60csx and and when looking
at the data I think the distances are straight line, and do not take in
to account the contours, when I look at the profiles it says I travelled
2.5 miles between tors, but when you look at the profiles
I must have travelled another mile up and down the hills.

Do I have to guess my actual miles, by adding a percentage to the recorded miles?

Thanks

Skippy · Posted: Wed Sep 27, 2006 9:04 pm Post subject:

It gives a straight line distance over flat ground.

As you observe, if you navigate to a waypoint a mile away over hilly ground then you could walk much further than that... Confused

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walkabout · Occasional Visitor Joined: Nov 12, 2004 Posts: 31

Thanks Skippy
Do you know if there is a way to work out the correct distance?
Or a rough idea?
I think the army use a table of some kind to work it out a bit more
accuratly.

Thanks

AllyCat · Posted: Thu Sep 28, 2006 1:59 am Post subject:

Hi,

You could work out the linear distance up (or down) each slope using Pythagoras' Theorem (i.e. add the squares of the horizontal and vertical distances, then take the square root). The extra distance is probably less than you think, for example 400 metres horizontally and 300 metres vertically (which is virtually a flight of steps) only becomes 500 metres, or a 25% increase on the horizontal distance. In practice, a path will normally zig-zag on such a hill, which the GPS should take into account (and the effective gradient is then much less).

However, the linear distance on a steep slope gives a poor indication of the effort expended or the time taken. For this, I suggest you Google "Naismith's Rule" (which is, or can be, included in some of the PC mapping programs). Here's a link I found:

http://www.walkingontheweb.co.uk/Walking_Distances.htm

Cheers, Alan.
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lbendlin · Posted: Thu Sep 28, 2006 3:30 am Post subject:

You also have to be careful if you want to measure distance at low speed. The inherent position error of GPS will introduce significant jitter, and can add or substract arbitrary amounts from your overall distance. You see that when you stand still - your GPS position will jump back and forth for quite a few meters/yards.
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