Re: 1m Accuracy - Do you believe it?

In message <ebihip$j1s$1@xxxxxxxxxxxxxxxxxxxxx>, Paul Saunders <pvs1@xxxxxxxxxxxxxxxxxxxxx> writes
Chris Hill wrote:

I've got a Garmin Forerunner which I take running, and I can overlay
its GPS data onto Google Earth via SportTracks. The degree of
accuracy is sometimes quite amazing - I obviously know exactly which
paths or roads I've run down and the Garmin data can sometimes be
accurate to within, say, two or three metres, putting me exactly
where I know I've been. On the other hand, when going through trees,
gullies, etc., it can be out by ten metres or more.

Is it ten metres out erratically or consistently?

There are certain areas that are consistently accurate, but they tend to be high-up and exposed. So it could be down to the accuracy of the maps or alternatively because GPS reception is particularly good and free from reflections, etc. Hard to say.

You should be aware that
aerial photographs are not always perfectly aligned, especially in hilly
areas where distortions can occur. There are several instances I've found
in Anquet where paths traced on the maps don't match the aerial shots, Fan y
Big is one for example, Ysgyryd Fawr another.

The same problem would probably occur in Google Earth, although they use
different aerial photos. Take a look at Big Ben for example, on one side of
the road the buildings tilt north, on the other side they tilt south (and
the shadows fall in different directions too) showing that aerial photos
don't always point straight down.

Yes - and sometimes the colour balance changes abruptly, too, indicating that the image has been stitched together (very well, in most cases) from two or more sources.

On the subject of comparing GPS data, after this year's Grindleford
Gallop, Adrian and myself were able to compare data from my Garmin and
his Magellan for the same route, which we walked/ran in a group. The
similarity between the two routes was extremely good on certain
stretches. Unfortunately, I've since lost his .gpx file, but if he's
reading and wants to send it to me again then I'll do a comparison and
put a picture up somewhere.

I've done the same walks multiple times and had remarkably similar tracks.
If those tracks don't match the paths then it's quite possible the paths are
wrong, not the GPS, although it could be in certain areas if there's a
consistent source of GPS error (like a cliff causing signal reflections).

I finally got round to posting some pictures at;

http://gpsoverlaidongoogleearth.fotopic.net

(checked the T+C at Google Earth and I can't see that I'm doing anything wrong there..,)

Some explanations;

In all cases, the blue line(s) is the GPS data.

Buxton01.jpg is the end of the Goyt Valley Challenge route (the second track on the left is where I turned the GPS on at the start). It's certainly got me on the correct side of the road, and even has sufficient detail to show where I've skirted round T-junctions.

Buxton02.jpg is coming down the Long Hill Rd into Buxton. Still pretty good, and shows us crossing the road, but seems to lose accuracy at the bottom-right where we move into tree cover.

GG01.jpg compares myself and Adrian Godwin's GPS data for a bit of the Grindleford Gallop. I think mine is the one which cuts the corner at the top-left. And again one could possibly observe some loss of accuracy near trees, or maybe that's reading too much into it...

GG02.jpg another bit of the Grindleford Gallop. A whole new swathe of the Peak District has just appeared in detail on Google Earth, though sadly not enough to cover the whole of this route.

Run01.jpg bit of one of my regular running routes. This was one of the narrow lanes that I referred to earlier, and considering the width it's got me pretty much spot-on. Helps to look at this one full size as the line is quite indistinct.

Run02.jpg another bit of the same route, same lane.
--
Chris Hill

.

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