Trouble calculating distances using GPS (1 Viewer)

I am having trouble calculating the distance between two GPS points using online calculators, as they appear to give different readings. I have been doing some research on weaver nests in Sudan and I need to know the length of an irrigation channel to work out the nesting density. I assumed this would be easy, as I have the GPS position of each end of the channel. However, the following distances were given by these websites.
1.78km (ig.utexas.edu)
1.75km (moveable-type.co.uk)
1.32 km (boulter.com)
1.35km (gpsvisualizer.com)
1.35km (Jan.ucc.nau.edu)
1.35km (rechneronline.de)
I recognised that there would be some level of inaccuracy, but there are clearly two different measurements here, and it is not clear which to use.
One end of the channel is: N 13deg 38.775min, E033deg 35.158min (or 13.64625, 33.585967). The other end is: N 13deg 38.214min, E033deg 34.677min (or 13.6369, 33.57795).
I originally assumed that the problem was to do with the format used, such as whether it was in degrees, minutes, and second, or degrees with decimalized minutes (as mine are above). However, I also came across ‘Jeon’s laboratory’ (jeonlab.wordpress.com) that discusses how errors can occur in calculations of short distances due to the level of precision used. Apparently, “Floats have only 6-7 decimal digits of precision.” However, it was a bit hard for me to follow what it all meant, and I realized I was a bit out of my depth.
I was hoping that someone here would know something about this problem and be able to suggest which of the above measurements is correct. Any help would be appreciated.
Thanks

Tom

Have you tried the measuring tool on google earth?

1,353 metres between those two points according to GE. But the second point (13.6369, 33.57795) isn't even on the channel, let alone at the end of it. The ditch is 1,622 metres on GE from your first point (northeast end) to 13.635934, 33.575326 (southwest end).

Thanks Nutcracker, I will have a look at Google Earth. Three of the online tools I tried had options to show the positions on a satellite map, but all of them showed them differently, and all incorrectly. Only one had the correct endpoint of the channel, with the other in the wrong place. It made me lose confidence in the accuracy of them all.
Edit: I have tried measuring directly from a satellite map using several different websites, and each time I get the same result (1.63km, as you found above). This looks a lot more reliable. Thanks for you help Nutcracker.

Tom

Not sure if this will work, don't know how detailed the Global Map will be.

Download Garmin BaseCamp.

BaseCamp loads to a Global Map

After you extract the file and the program is open go to Find > Location Coordinate > input the data > Recenter > right click New Waypoint.......do the same for the other coordinate; creating a Waypoint......then choose one of the Waypoints > right click Begin Measurement and drag it to the other Waypoint or where you want to go.

The distance is shown as you drag the mouse.

Good luck my friend.

From ebird, they mention this in the help pages: "or online tools like Runningmap.com or the Gmap-pedometer," -- a google search for those two names likely will give something useful.

Niels

Doing it the old manual way!

Difference in latitude is 0.561' which equals 1.38 km

Difference in longitude is 0.481' which at 13N is 0.924 km

Pythagoras gives us 1.389 km

Mono said:

Doing it the old manual way!

Difference in latitude is 0.561' which equals 1.38 km

Difference in longitude is 0.481' which at 13N is 0.924 km

Pythagoras gives us 1.389 km

Click to expand...

Something wrong with this. The Pythagoras calculation for these distances is:

((1.38*1.38)+(0.924*0.924))**0.5 (square root of the sum of the squares)

which is 1.66km. This assumes that the Earth is flat (which might be a near enough approximation for distances as short as this), and that the degree->distance calculation is right.

Another way of getting the answer is with Wolfram Alpha, which is rather rigorous on most numerate matters.

This direct link for the search:

http://www.wolframalpha.com/input/?i=distance+13.64625N,+33.585967E+and+13.6369N,+33.57795E

gives the answer 1.35km.

Keith

I find Google Earth very accurate for measuring fenceline lengths and paddock areas. The points have to be correct though and your second point is not at the end of the channel.