[svn r6872] Changed area definitions to something slightly more helpful

handbook/survey/gps.htm

@@ -10,6 +10,8 @@
 <h2 id="tophead">CUCC Expo Surveying Handbook</h2>
 <h1>Locating entrances by GPS</h1>
 
+<h3>Which point to fix</h3>
+
 <p>The entrance to a cave significant enough to get a number and a survey
 will eventually be marked by a numbered tag attached to a spit. This
 will then become the primary survey station - ie. the point where an
@@ -28,38 +30,53 @@ of the entrance. If you have found a group of caves close together, it
 might be better to GPS a central point rather than get quick (but less
 accurate) fixes on each entrance.</p>
 
+<h3>Taking the fix</h3>
+
 <p>Once you have chosen your point, mark it in some way (could be a spit hole
 or a cairn, for example - we aren't supposed to use paint any more) and place
 the GPS on the point. If you build a cairn, make it wide rather than high -
 tall cairns are knocked down by the depth of snow each winter. Give it a
 couple of minutes to get a fairly good fix (the first figure reported may be
-quite a way out, but after a couple of minutes things should settle). Then
-mark the point as a waypoint. If your GPS supports averaging, then choose
-"average" and leave the GPS to do its own averaging for half an hour or more.
-If it doesn't support averaging, then fix another waypoint at the same
-location just before you leave. This shows someone examining the track log
-later that all the track points relate to the same spot, and they can then do
-the averaging themselves.</p>
+quite a way out, but after a couple of minutes things should settle). Then mark
+the point as a waypoint. If you're feeling really keen, you can set it up for
+<i>averaging</i>, which gives a more accurate fix &ndash; some GPS receivers
+support this automatically, and with others you can just leave it recording a
+track log, then record another waypoint at the same place just before you leave
+so it's clear to someone examining the track log when you actually left.
+(Averaging was once crucial to getting any kind of remotely close fix, but is
+somewhat less important these days now that Selective Availability has been
+turned off.)</p>
 
 <p>While the GPS is recording your location, you can do something useful
 (like rigging the cave, doing a surface survey from the GPS point to the
-marker spit, looking for other caves, or even having lunch!) It does not
-matter (and may be slightly beneficial) if you leave the GPS longer than half
-an hour. But if you want to use it for anything else, remember to stop the
-waypoint averaging before moving the unit or changing the display page. Take
-a photo of your GPS point showing at least one of your cave entrances too.</p>
+marker spit, looking for other caves, or even having lunch!) Remember to stop
+the waypoint averaging before moving the unit or changing the display page.
+<b>Take a photo of your GPS point showing at least one of your cave entrances
+too.</b></p>
+
+<h3>Coordinate systems</h3>
 
 <p>It doesn't especially matter what display options are selected when you
-are getting the GPS fix, but it is important to use standard ones when
-writing down the reported position in the survey book. Currently we use
-"German" grid, "Austrian" datum, and "metric" units. All is not lost if you
-can't select this particular set, but it is very important that you <u>write
-down what was actually used</u>, since different grid systems can give
-results up to a couple of kilometres different!</p>
+are getting the GPS fix, but it is important to use standard ones when writing
+down the reported position in the survey book. The usual systems are either
+Lat/Long with the WGS84 datum, which all GPSses support out of the box; or
+the Austrian BMN (Bundesmeldnetz) system, which should be relative to the
+Austrian MGI datum (Hermannskogel), which is what we use for our surveys. It
+doesn't matter <i>too</i> much what combination of parameters you actually use
+as long as you record what they were, since we have conversion programs that
+can convert coordinates between the systems; but remember to record <b>which
+grid and which datum</b> your GPS was set for when you copy the fix into the
+survey file. (Besides the risk of introducing severe errors, it is an
+extraordinarily tedious task to have to repeatedly try all the plausible
+combinations of grid and datum that a given set of numbers might be in, convert
+them all to dataset coordinates and see which ones give answers in vaguely the
+right place, which is what someone will have to do if you don't write the
+details down.)</p>
 
-<p>If your GPS doesn't support Austrian coordinates out of the box, then you
-can set it up as a "User Grid" using some or all of the following cryptic
-runes:</p>
+<p>If you want to set up your GPS to use the same coordinates the survey data
+set uses, which makes life a lot easier, then these are the runes to use as a
+"User Grid" if the unit doesn't support BMN coordinates immediately (which
+Garmin ones don't, for example):</p>
 
 <div style="background-color: #BDB">
 <table>
@@ -72,6 +89,7 @@ a = 63377397.155m (&Delta;A = 739.845)<br />
 <th>Datum:</th>
 <td>Austria MGI (Hermannskogel)</td></tr>
 <tr><th>Projection:</th><td>Transverse Mercator (BMN zone M31)</td></tr>
+<tr><th>North reference:</th><td>Grid</td></tr>
 <tr><th>Grid parameters:</th><td>Central meridian 13&deg;20'E<br />
 False easting 450km<br />
 No additional scaling<br />
@@ -83,28 +101,46 @@ Scaling -2.5ppm
 </td></tr></table>
 </div>
 
+<p>(Technical note: the BMN grid is actually the same as Universal Transverse
+Mercator zone 31, but setting your GPS for UTM will give rather different
+coordinates, as the plateau is actually in zone 33. My understanding of the
+situation is that the Austrians have found it more convenient to extend one
+grid to cover the whole country, thus deviating from UTM for the areas of the
+country which are just over the grid boundary. This is an important gotcha to
+watch out for, since while all the other coordinate systems produce answers in
+recognisably different formats, UTM 33 coordinates look like dataset
+coordinates but are offset by a couple of kilometres. Having your GPS set to
+the wrong datum produces even more subtle errors - the difference between BMN
+grid + WGS84 datum and BMN grid + Austrian datum is an offset of around 500m to
+the south and 50m in altitude.)</p>
+
 <p>A good way of testing that your GPS is correctly set up is to set it WGS84
 Lat/Long and enter a waypoint for a point whose coordinates are known &ndash;
 such as the 204a tag, at 47&deg;41.456'N 013&deg;49.288' &ndash; and then
 change the settings again to use the user grid. It will now convert this point
 into the new grid; if you check its coordinates, it should come out as
-something close to 36697E, 83699N, which are the Gauss + Krugers for 204a.</p>
+something close to 486697E, 5283699N, which are the BMN coordinates for 204a.
+For use in the dataset we tend to subtract the 450km offset in the easting and
+ignore the first two digits of the northing, giving 36697E 83699N.</p>
 
-<p>Write down the figure that the GPS gives for each waypoint at the time
-(just in case some failure loses the data from the GPS memory). That's all
-you need to do at the cave. Get the GPS data downloaded to a computer next
-time you are in Base Camp (or Top Camp if someone has a laptop :-). This
-should be put in a file and a clear reference to it put in the notKH survey
-book. Don't alter the file in any way - it may be necessary to upload it to
-a GPS unit at some time to do coordinate conversion. Also copy your written
-down data to the survey book with all the other details of your cave.</p>
+<p>Write down the figure that the GPS gives for each waypoint <i>at the
+time</i> (just in case some failure loses the data from the GPS memory &ndash;
+this has happened a couple of times in recent years, much to the annoyance of
+everybody involved). That's all you need to do at the cave. Get the GPS data
+downloaded to a computer next time you are in Base Camp (or Top Camp if someone
+has a laptop :-), and as a backup (expo computers break down surprisingly often
+it seems) write it down by hand on one of the A5 cave info sheets with all the
+other details of your cave and put that in the surveys ringbinder file.  </p>
 
-<p>If you want to read about the nitty gritty of converting GPS coordinates
-to the ones used by the Kataster system, you can do no better than read
-Wookey's <a href="../../years/1996/gps.htm">Compass Points Article</a>.
-Briefly, this says "it's horribly complicated and we don't really know how to
-do it properly". However, the main point of having a GPS fix on an entrance
-is so we can find it again and be sure it is the same one!</p>
+<p>If you want to read about the nitty gritty of converting GPS coordinates to
+the ones used by the Kataster system, you can do no better than read Wookey's
+<a href="../../years/1996/gps.htm">Compass Points Article</a>.  Briefly, this
+says "it's horribly complicated and we don't really know how to do it
+properly". Things have improved a little since those days, particularly as
+without the fog of the SA variation it's now easy to find out whether your GPS
+is set up right by just GPSsing a known point and comparing the results.
+However, the main point of having a GPS fix on an entrance is so we can find it
+again and be sure it is the same one!</p>
 
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