mirror of
https://expo.survex.com/repositories/expoweb/.git/
synced 2024-12-11 19:12:21 +00:00
188 lines
9.6 KiB
HTML
Executable File
188 lines
9.6 KiB
HTML
Executable File
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
|
|
<title>CUCC Expo Surveying Handbook: GPS</title>
|
|
<link rel="stylesheet" type="text/css" href="../../css/main2.css" />
|
|
</head>
|
|
<body>
|
|
|
|
<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
|
|
underground survey will start, and the point to which a surface survey
|
|
should go. It's worth thinking about where you would put such a tag right
|
|
from the start. Unlike the first rigging bolt (often used as the first
|
|
point of a survey in the past) it should be sited with a particular
|
|
view to its visibility and accessibility without having to put on SRT
|
|
kit. If such a point has a clear view of the majority of the sky, then
|
|
this is the point to use for a GPS fix too.</p>
|
|
|
|
<p>If however, you are dealing with a cave at the foot of a cliff, or
|
|
otherwise with a restricted view of the sky, then choose instead a good
|
|
landmark with a clear view, and within one (or maybe two) survey shots
|
|
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 you're feeling really keen, you can set it up for
|
|
<i>averaging</i>, which gives a more accurate fix – 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!) 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. 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 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>
|
|
<tr>
|
|
<th>Ellipsoid:</th><td>Austrian (Bessel 1841)<br />
|
|
a = 63377397.155m (ΔA = 739.845)<br />
|
|
1/f = 299.1528128 (Δf x 10,000 = 0.10037428)
|
|
</td></tr>
|
|
<tr>
|
|
<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°20'E<br />
|
|
False easting 450km<br />
|
|
No additional scaling<br />
|
|
Grid boundaries at 11°50' and 14°50'</td></tr>
|
|
<tr><th>EFEC coordinate conversion<br />equation with respect to WGS84:</th>
|
|
<td>Offsets Δx = -575m, Δy = -93m, Δz = -466m<br />
|
|
Rotations ω<sub>x</sub> = 5.1"; ω<sub>y</sub> = 1.6", ω<sub>z</sub> = 5.2"<br />
|
|
Scaling -2.5ppm
|
|
</td></tr></table>
|
|
</div>
|
|
|
|
<p>Note: The table above used to incorrectly give the y rotation as 5.1".
|
|
But there's a slightly more accurate version in the
|
|
<a href="coord.htm#ftnBMN">Coordinate Systems section</a>.</p>
|
|
|
|
<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 –
|
|
such as the 204a tag, at 47°41.456'N 013°49.288' – 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 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 <i>at the
|
|
time</i> (just in case some failure loses the data from the GPS memory –
|
|
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 the
|
|
short introduction to <a href="coord.htm">coordinate systems</a>, which briefly
|
|
says "it's horribly complicated and we use computer programs to do it properly".
|
|
(A rather outdated first attempt at this can also be found in Wookey's
|
|
<a href="../../years/1996/gps.htm">Compass Points Article</a> from 1996, which
|
|
briefly says "it's horribly complicated and we don't really know how to do it
|
|
properly".) Overall things have significantly improved since the early 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>
|
|
|
|
<hr />
|
|
|
|
<ul id="links">
|
|
<li><b>Expedition Handbook:</b>
|
|
<ul>
|
|
<li>Expo Handbook <a href="../index.htm">Introduction</a></li>
|
|
<li><b>Surveying:</b>
|
|
<ul>
|
|
<li>Surveying <a href="index.htm">Overview</a> and index of
|
|
topics</li>
|
|
<li><a href="what.htm">What is a cave survey?</a></li>
|
|
<li><a href="why.htm">Why am I doing this?</a></li>
|
|
<li>Methods: <a href="how.htm">underground</a></li>
|
|
<li>Pitfalls to avoid, <a href="hints.htm">hints'n'tips</a> to
|
|
make life easier</li>
|
|
<li>Methods: <a href="ontop.htm">surface</a></li>
|
|
<li>Base Camp: <a href="getin.htm">getting it in</a> to the
|
|
computer</li>
|
|
<li>Base Camp: <a href="drawup.htm">drawing it up</a>, writing
|
|
the description</li>
|
|
</ul></li>
|
|
<li><a href="../look4.htm">Prospecting guide</a></li>
|
|
<li><a href="../rescue.htm">Rescue guide</a></li>
|
|
<li><a href="../rigit.htm">Rigging guide</a></li>
|
|
<li><a href="../photo.htm">Photography guide</a></li>
|
|
</ul></li>
|
|
<li><a href="../../infodx.htm">Index to info/topics pages</a></li>
|
|
<li><a href="../../indxal.htm">Full Index to area 1623</a>
|
|
<ul>
|
|
<li><a href="../../areas.htm">Area/subarea descriptions</a></li>
|
|
</ul></li>
|
|
<li><a href="../../index.htm">Back to Expedition Intro page</a></li>
|
|
<li><a href="../../../index.htm">Back to CUCC Home page</a></li>
|
|
</ul>
|
|
</body>
|
|
</html>
|