Re: gps software for walking

On Fri, 18 May 2007 07:47:53 +0000 (UTC), Chris Malcolm
<cam@xxxxxxxxxxxxxxxxx> wrote:

|!Dave Fawthrop <invalid@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
|!> On Tue, 15 May 2007 16:22:25 +0100, "Paul Saunders"
|!> <pvs1@xxxxxxxxxxxxxxxxxxxxx> wrote:
|!
|!> |!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.
|!
|!> Ok I'll believe you.
|!
|!> So my requirement, to find my rocks in a featureless moor, should now read
|!> "able to enter a 10 figure (1 m) OSGB grid reference on the fly and use the
|!> correct algorithm to transform it into something else.
|!
|!I assume the primary requirement is to be able to visit a rock, take
|!its co-ords, and that it be as easy as poss for others to find it
|!using possibly other GPS units?

Actually the rocks/cup and ring stones have been surveyed using OSGB to
within 1 m by English Heritage. If I find a new one people will need OSGB
grid references, if only because the will me using OS Explorer maps.

|!Since there's no nearby cliffs, tree
|!cover etc. to give reception problems, my guess is that pretty much
|!any good reputable walking/hiking GPS will do. In conditions of good
|!sat view there's not much to choose between them. And you don't even
|!need mapping. I say walking/hiking because some of the dumbed down car
|!drivers' units lack map grid referencing.

Been there done that, looked at the GPS which says within 10 m accuracy
then and tramped round a 10 m square/circle of bracken and not found it.

|!And you don't want really sophisticated transforms from WGS84 Lat Long
|!to OSGB grid because that only applies when making paper map
|!transitions and will throw off any unit not using the standard simple
|!algorithms. What's more, the current published standard walking paper
|!maps don't even support the accuracy of the simple algorithms in
|!use. You need special surveyor's high res maps for that kind of
|!translation accuracy to matter. Those exist for urban areas, but
|!probably don't for featureless moors.

Any surveyor with better equipment than I can afford can do much better
than 1 m. Time Team Ch 4 have a GPS man who surveys to some fantastic
accuracy, looks to me like 1 cm to me with a no doubt very expensive thing
on a pole. He found that a ?5m? circular earthwork was not in fact
circular.

|!I'd say if sometimes the rocks are close enough for the usual 3-5m
|!accuracy to be ambiguous, then you need a note of local info such as
|!the layout of the rocks, rather than straining to increase the mapping
|!accuracy. Because on a day when by bad luck you have a bad overhead
|!sat configuration you'll lose that anyway.
|!
|!If pencil sketches in notebooks or notes like "the westernmost rock of
|!the cluster" aren't enough then a photo should do the job. If a hand
|!held photo would have too much perspective distortion then a quick
|!holding the camera aloft and looking down on a long monopod should
|!do. That's easily extended in scope by the use of a wide lens and a
|!really long monopod if necessary.
|!
|!But I'm just guessing because I don't really know why you're asking
|!for this rather awkwardly high grid ref accuracy from the GPS :-)

Because as I said above English Heritage have surveyed them to 10 figure
OSGB/1 m and my trusty *non* WAAS Yellow Etrex is not good enough.

I really need WAAS/EGNOS and the right algorithm for OSGB
--
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