GPS accuracy in forests

[dilligaf]

Been taking a little GPS with me to calibrate the speedo. (the speedo is about 10% high at normalish speeds). But after runs in the hills it tells me I had maximum speeds 20 or 30k greater than I thought I was doing. I suspect this is a problem with the GPS and not my speedo being out the other way at speed. (the gps did once have the bar of Meredith pub doing 430k). Anyone else experience this?

[seiko1]

I run GPS in country Vic and it is shocking with streets.
Always miles out in Bannockburn and Mortlake and Terang but speed hasn't been an issue cause the
Truck is a wee bit slower than your bike

[hoffy]

dilligaf !

[seiko1]

dilligaf !

What about him?

[dilligaf]

dilligafayngaf

[craig]

.. 'ayngaf............. .........lost me.....

[dilligaf]

does it look like i give a fuck about you not giving a fuck

[Disco]

Most GPS's have a satellite screen (my two have anyway) maybe do a run with it on this page and make sure you have at least 3 satellite's to triangulate your position.....Unless there is heavy tree overgroth I think it should work fine....

[dilligaf]

Thanks. I'll try that. The tree growth is fairly heavy.

[6maniac]

So what's the problem, mate ? Did you forget to draw a map of where the treasure was buried ?

[esie]

When we are out geocaching we find that under heavy tree cover the GPS signal moves around, as it bounces of braches, leaves and hills, therefore not giving an accurate position. If you want to test your speedo using GPS, you would be best to do it on a straight flat road with no tree cover whatsoever.

[mike-s]

Another thing that affects it is hills & buildings. that is why sometimes you also get weird shit happening with gps's when you go through a segment of a city where there are a mass of tall buildings next to each other (prime example = castlereigh street in sydney cbd)

[aardvark]

Every time I've buried a body in the forest I've marked it on the GPS. I've always been able to find my way back to them.

[mike-s]

No worries Ivan, you keep on keeping on!

[drunkmeerkat]

With all GPS receivers, the position accuracy is related to the strength of the signal from the satellites, and the number of satellites in view. Position degradation can be caused by signals being bounced off trees and buildings as mentioned above (called multi-path effects), but also due to the receiver noise.

You can't do too much about the multi-path stuff, but you can improve the signal quality with a good quality active antenna. Does the unit you are using have a connector for an external antenna ? An active (powered antenna) will be better than a passive one.. you can pick them up for around $30..

This will help you pick up more satellites, and improve the quality of the signal coming in...

Another issue which may be affecting your speed reading, is the accuracy of the timing information. The receiver calculates speed = change in position / change in time. If the GPS is only logging position once per second (1 Hz), any problems with the accuracy of the timing will result in speed error (you only need 50 milliseconds of timing error for 20% speed error).. Thats why GPS receivers which log at 5 or 10 times per second, can average the 5 or 10 speed calculations over 1 second, and provide a seemingly smooth speed calculation...

If you need any more info re: receivers or antennas, let me know..

cheers.