Re: gps software for walking

GSV Three Minds in a Can wrote:

Yep, same here. Did you look at the screenshot of the forest track I
gave a link to a short while ago? I think that proves the point
pretty well.

Nope, care to repeat the link? (I may have lost the thread before I
got that far).

Maybe the post didn't get through. Sometimes that happens, I don't know why.
I'll repeat the whole post below.



Okay, I've taken a few screenshots to demonstrate the difference in accuracy
between the 50k and 25k maps.

Exhibit 1: Cairns

I marked a group of 4 cairns on the 50k map, then switched to the 25k map
and took a screenshot at 200% zoom. They're not only in different positions
but also in different directions. Some are even shown in different fields.
The distances between the 50k waypoints and the positions on the 25k map are
38m, 24m, 20m and 30m respectively, an average error of 28m (assuming that
the 25k map positions are accurate).
http://www.wilderness-wales.co.uk/misc/cairns.gif

Exhibit 2: Fan Fawr

I've marked the summit spot height and the trig on both map scales, the red
waypoints represent the 50k map, the orange waypoints the 25k map. It's also
at 200% zoom. The difference in the trig position is quite small, only 7.5m
(although I've seen much bigger differences in some cases). The spot height
at the summit is a different matter though, there's a whopping difference of
49.5m! Of course, you must bear in mind that I'm only comparing spot heights
on maps here, I'm not comparing the waypoints to the actual summit position.
But I'd sooner trust 25k spot height than 50k ones anyday.
http://www.wilderness-wales.co.uk/misc/fan-fawr.gif

Exhibit 3: Craig Cerrig Gleisiad

So what about the actual summit position? Well on this map (300% zoom) I've
marked three waypoints, the yellow one is a GPS waypoint I marked at the
summit cairn back in March, with excellent reception and EGNOS active,
reporting an EPE of just 1-2m. Of course, there are co-ordinate conversion
issues to consider as well, but that rarely amounts to more than 5m.
Comparing the 25k waypoint to the GPS waypoint, there's a difference of
7.8m, whereas the 50k waypoint is a massive 96.3m away! One gets the
impression that the 50k mappers just marked an arbitrary spot in the middle
of the highest contour ring. This sort of thing is quite common where the
highest point is right next to the edge of a cliff. The 25k map usually
marks the summit in the correct spot, but the 50k map usually marks it some
distance away from the edge of the cliff.
http://www.wilderness-wales.co.uk/misc/ccg.gif

Exhibit 4: A recorded track through a forest path

This is a different comparison, between a 25k map and an aerial photo. As
you can see, the GPS track doesn't follow the map track very well at all. At
first you might assume bad reception due to the trees, and that's probably a
factor, but the track still doesn't follow the map even when it exits the
forest on the west side, where the reception was probably quite good. I'd
first noticed this particular error many years ago when using a 50k map, but
although bad reception seemed the most likely cause, repeated walks produced
consistent tracks, which were consistently inconsistent with the map!

Of course, that was 50k mapping so I wasn't unduly concerned, but more
recently it bothered me that the 25k map didn't match either (although it's
actually a much better fit than the 50k map).

But when you superimpose the track onto the aerial photograph, it suddenly
becomes surprisingly accurate. Granted it's still not perfect, but it's not
far off, some bits are dead on. It's worth noting also that I wasn't walking
in the centre of the track, I was usually walking close to one side or
another, hence the "racing line" that I took around many of the corners. In
particular, notice that the track is dead accurate outside of the forest.
Clearly the mappers were very sloppy in drawing this forest path, which
proves the point that even 25k maps are not always very accurate (although
spot heights, trigs and cairns usually are).
http://www.wilderness-wales.co.uk/misc/forest-path.jpg

So, do you still want 10 figure grid refs with 50k mapping? Clearly
25k mapping is far superior, but even that isn't perfect. IMO the most
accurate waypoints are those that have been marked in the field, with good
reception conditions of course. Such waypoints also eliminate coordinate
system conversion errors. So even allowing for typical reception errors, a
field waypoint will usually be more accurate than one marked on a map.

By the way, when you do mark field waypoints, it's best to hold the GPS
above your head so that your body doesn't block the signals, and let it
settle for a short while first (to avoid "momentum" errors). Averaging the
waypoint helps too, even if only for a short time (even a 30 second average
should eliminate any spurious position blips).

Paul
--
http://www.wilderness-wales.co.uk


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