GPS Explained

Good page

One small thing WAAS is not enabled in Australia and is primarily an aircraft aid anyway.
I realise you have probably cut and pasted some stuff but I constantly see units advertised locally as WAAS enbled and it will give accuracy of ***meters with it.
Not here it wont and as far as I can tell never will.


Still a very worth whle tutorial and pity its not a sticky to help stop all the multiple similar posts

Hello Doug - Good info from various Garmin websites. There are some shortcomings though - I looked in the Glossary of terms and couldn’t find the meaning of "#**#+!s", its doesn’t appear in any dictionary I have either, any clues?:)

Also I am not sure which "#**#+!s" your were referring to but the comment "I have had some #**#+!s try and tell me
a GPS unit does not recognise or take into account corners and hills" is partly wrong and partly right.

Corners are of course recognise/recorded by GPS units otherwise we would all have a collection of straight lines. Corners are also taken into account by a GPS for measuring distance, though settings made on GPS may effect the number of points recorded on a corner and therefore exact distance may not be recorded (i.e. lines drawn between points may "cut" the corner). I am not sure about Garmin’s but Magellan’s have an Auto detailed setting which whacks in lots of points at any significant changes of direction (eg a corner) and less points on straighter sections to improve accuracy of distance measurements and the actual track plot if required.

Altitude is also, as we all know, recorded by GPS units and can be used to show vertical profiles (this was not disputed in previous forum thread) but as has been discussed before slope is not used by your average GPS to measure distance traveled along the ground - this is illustrated by your vertical profile images - the scale along the bottom of your graphs does not change even when steep slopes are encountered. Excluding other variables the distance traveled will therefore be greater than your Graph/GPS indicates whether you like it or not. For example if you were to plot the vertical profile of someone climbing a 1km vertical cliff face the distance travelled would be recorded as 0km on your graphs.

The distance along the bottom of your graphs is correct for the track plot on a flat surface (and takes into account corners) but does not take into account the additional distance traveled when going up and down hills - which may or may not be bugger all in any case.

Cheers
Greg

Hello Doug, In another possibly vain attempt to convince you that your average GPS (or average GPS mapping program such as Mapsource/OziExplorer) doesn’t take into account the altitude of recorded track points to calculate distance, I have conducted a little experiment which I encourage you to replicate so as to advance your knowledge on a subject you obviously have a great interest.

I opened the original track file you have made available on you “GPS Explained” webpage. Each track point has an altitude and the total distance of the track (calculated by Mapsource) was 253km. I then changed all the altitude readings for each of the 1742 track points to 0 and saved the file under a new name. I then opened this file in Mapsource. Guess what distance the track file then showed in Mapsource? If you guessed 253km you are right.

Can you explain this? My conclusion is that altitude is not used in distance calculations by Mapsource. I did same test in OziExplorer, same result (though OziExplorer shows a distance of 252.09km for both original and edited track files). Initial discussion on this subject was actually about whether GPS units (not gps mapping software as demonstrated here) used altitude when calculating distance but it is apparent that they don’t either.

May pay to change certain comments on your “GPS Explained” page to reflect the facts (both in respect to main subject and the level of experience and knowledge of your fellow GPS users), but that of course is your choice.

Cheers
Greg