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walkabout Occasional Visitor
Joined: Nov 12, 2004 Posts: 31
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Posted: Wed Sep 27, 2006 8:15 pm Post subject: Does a gps give true distance? |
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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?
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Wed Sep 27, 2006 9:04 pm Post subject: |
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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... _________________ Gone fishing! |
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walkabout Occasional Visitor
Joined: Nov 12, 2004 Posts: 31
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Posted: Wed Sep 27, 2006 9:15 pm Post subject: |
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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.
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AllyCat Frequent Visitor
Joined: Feb 23, 2005 Posts: 376 Location: Catford, London, UK
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Posted: Thu Sep 28, 2006 1:59 am Post subject: |
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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. _________________ Garmin GPS72H/76/60/45, Etrex H, Mapsource v6.5.
Acer N50,HP114,Loox N560,Dell x50,CF/SD cards to 4/32GB.
RoyalTek,Holux236,Navman B10 & Copilot(Globalsat) BT GPS,TomTom5/6.
Memory Map (v5.4.2 & v5.1.3 OS & Euro), GPS gate,OSGPSconverter. |
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lbendlin Pocket GPS Staff
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
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Posted: Thu Sep 28, 2006 3:30 am Post subject: |
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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. _________________ Lutz
Report Map Errors here:
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philpugh Lifetime Member
Joined: Dec 28, 2005 Posts: 2003 Location: Antrobus, Cheshire
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Posted: Thu Oct 26, 2006 12:50 pm Post subject: |
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Skippy wrote: | 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... |
Isn't it (to be a tad pedantic) distance over the surface of the ellipsoid used to estimate the surface of the earth? This changes very slightly between different co-ordinate datum.
There is also some variability introduced by the positional 'wandering' caused by atmospherics, local geography, satellite geometry etc... All these effects may be small but do mount up when walking. But hilly ground will certainly give rise to distance errors (always an under estimate) |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Thu Oct 26, 2006 1:41 pm Post subject: |
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philpugh wrote: | There is also some variability introduced by the positional 'wandering' caused by atmospherics, local geography, satellite geometry etc... All these effects may be small but do mount up when walking. |
The wandering goes one way then the other so surely the net result is that when measuring distance the position error would cancel itself out over time rather than being cumulative? _________________ Gone fishing! |
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lbendlin Pocket GPS Staff
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
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Posted: Thu Oct 26, 2006 7:47 pm Post subject: |
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yes, over a period of 48 hours. But who is walking THAT slow? _________________ Lutz
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Thu Oct 26, 2006 8:23 pm Post subject: |
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There is a recorded instance of "slow" walking, OK no GPS involved as far as I know, have a look HERE Mike |
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Border_Collie Pocket GPS Verifier
Joined: Feb 01, 2006 Posts: 2543 Location: Rainham, Kent. England.
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Posted: Thu Oct 26, 2006 8:33 pm Post subject: |
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Quote: | Lloyd Scott is known across Britain and around the world as the walking Deep Sea Diver, |
He'd be alright with my device then. When setting up a Route from Home (Kent) to POI Basildon Hospital, it shows a distance of 10 miles. The fact the road distance is around 38 miles, I guess it routes under the Medway and Thames. :P _________________ Formerly known as Lost_Property
And NO that's NOT me in the Avatar. |
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philpugh Lifetime Member
Joined: Dec 28, 2005 Posts: 2003 Location: Antrobus, Cheshire
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Posted: Mon Oct 30, 2006 5:21 pm Post subject: |
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Skippy wrote: | philpugh wrote: | There is also some variability introduced by the positional 'wandering' caused by atmospherics, local geography, satellite geometry etc... All these effects may be small but do mount up when walking. |
The wandering goes one way then the other so surely the net result is that when measuring distance the position error would cancel itself out over time rather than being cumulative? |
No - your position could be averaged if you stood still for 12/24/48 hrs (take your pick) but your distance will be cumulative - i.e. each second your GPS would calculate your position and work out how far you have travelled by comparing this to the last position, calculating the distance and adding it to the last cumulative trip figure. It doesn't have any way of knowing if it is real movement or just random variations. Perversley the slower you are travelling the worse this error will be. So a walker on the mythical ellipsoid surface could find he had "travelled" further from A-B than a cyclist / motorist / aviator (in that order) doing the same journey as the additional distance will depend upon the total time taken for the journey. |
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