mirror of
https://expo.survex.com/repositories/expoweb/.git/
synced 2024-12-04 23:52:23 +00:00
69 lines
3.2 KiB
HTML
69 lines
3.2 KiB
HTML
<html>
|
|
<head>
|
|
<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 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
|
|
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>
|
|
|
|
<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.
|
|
|
|
|
|
<hr />
|
|
|
|
|
|
</body>
|
|
</html>
|