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<html>
<head>
<title>CUCC Expo Surveying Handbook: GPS coordinate systems</title>
<link rel="stylesheet" type="text/css" href="../../css/main2.css" />
</head>
<body>
<h2 id="tophead">CUCC Expo Surveying Handbook</h2>
<h1>GPS and coordinate systems</h1>
<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 (&Delta;A = 739.845)<br />
1/f = 299.1528128 (&Delta;f 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&deg;20'E<br />
False easting 450km<br />
No additional scaling<br />
Grid boundaries at 11&deg;50' and 14&deg;50'</td></tr>
<tr><th>EFEC coordinate conversion<br />equation with respect to WGS84:</th>
<td>Offsets &Delta;x = -575m, &Delta;y = -93m, &Delta;z = -466m<br />
Rotations &omega;<sub>x</sub> = 5.1"; &omega;<sub>y</sub> = 1.6", &omega;<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 &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 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 &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 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>

137
handbook/survey/gps.htm
View File

@ -1,7 +1,5 @@
<!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>
@ -25,11 +23,15 @@ 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
landmark with a wide, clear view of the sky, 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>
<p>We now (2018) have differential GPS which is much more accurate than
in the past (e.g. Wookey's 1996 article) but altitudes are often very inaccurate
and GPOS devices don't tell you how inaccurate the altitude is.
<h3>Taking the fix</h3>
<p>Once you have chosen your point, mark it in some way (could be a spit hole
@ -54,134 +56,13 @@ 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>We all use the same coordinate system WGS84 these days, so <a href="coord2.html">the extensive discussion
on coordinate systems</a> has been moved to a different page. If you are really interested you can read
<a href="coord.htm">Olaf's article</a>too.
<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 (&Delta;A = 739.845)<br />
1/f = 299.1528128 (&Delta;f 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&deg;20'E<br />
False easting 450km<br />
No additional scaling<br />
Grid boundaries at 11&deg;50' and 14&deg;50'</td></tr>
<tr><th>EFEC coordinate conversion<br />equation with respect to WGS84:</th>
<td>Offsets &Delta;x = -575m, &Delta;y = -93m, &Delta;z = -466m<br />
Rotations &omega;<sub>x</sub> = 5.1"; &omega;<sub>y</sub> = 1.6", &omega;<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 &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 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 &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 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>

2
handbook/survey/hints.htm
View File

@ -59,7 +59,7 @@ might get rubbed off.</li>
<li>Take at least one spare pencil!</li>
<li>Return instuments to the carbide dessicator before you fall asleep
<li>Return instuments to the dessicator before you fall asleep
otherwise they are likely to fog up the next time. Don't leave them in the
cave, both for the above reason and because the next survey may need them
somewhere else entirely.</li>

34
handbook/survey/how.htm
View File

@ -1,7 +1,5 @@
<!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: Underground</title>
<link rel="stylesheet" type="text/css" href="../../css/main2.css" />
</head>
@ -147,38 +145,6 @@ before and after the trip).</li>
do the above, written from experience in Austria.</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: underground</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>

3
years/1996/gps.htm
View File

@ -11,6 +11,7 @@
15, March 1997, pp 11-15</font>
<center><H2><A NAME="art4">GPS For Expedition cave location</A></H2>
<br><br>
<I>Wookey</I></center>
<P><I><b>GPS units are now cheap enough that cavers can afford to consider
@ -28,7 +29,7 @@ accuracy in the field could be formed. Here I describe what was done, the
methods used, and the results obtained.</b></I>
<P>I scrounged a Garmin GPS45 from my mate Ian Harvey (see his review in
<A HREF="http://www.chaos.org.uk/survex/cp/cp11/cpoint11.htm#art4">Compass Points
<A HREF="http://www.chaos.org.uk/survex/cp/CP11/CPoint11.htm#Art_4">Compass Points
11</A>) to take on Cambridge UCC's annual jaunt to Austria. The expedition
area is high up in the mountains in very unhelpful karst terrain where
getting about is greatly impeded by dwarf pine and small cliffs, as well as