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63 lines
3.3 KiB
HTML
63 lines
3.3 KiB
HTML
<html>
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<title>CUCC Expo Surveying Handbook: GPS</title>
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<link rel="stylesheet" type="text/css" href="../../css/main2.css" />
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</head>
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<body>
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<h2 id="tophead">CUCC Expo Surveying Handbook</h2>
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<h1>Locating entrances by GPS</h1>
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<h3>Which point to fix</h3>
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<p>The entrance to a cave significant enough to get a number and a survey
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will eventually be marked by a numbered tag attached to a spit. This
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will then become the primary survey station - ie. the point where an
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underground survey will start, and the point to which a surface survey
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should go. It's worth thinking about where you would put such a tag right
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from the start. Unlike the first rigging bolt (often used as the first
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point of a survey in the past) it should be sited with a particular
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view to its visibility and accessibility without having to put on SRT
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kit. If such a point has a clear view of the majority of the sky, then
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this is the point to use for a GPS fix too.</p>
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<p>If however, you are dealing with a cave at the foot of a cliff, or
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otherwise with a restricted view of the sky, then choose instead a good
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landmark with a wide, clear view of the sky, and within one (or maybe two) survey shots
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of the entrance. If you have found a group of caves close together, it
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might be better to GPS a central point rather than get quick (but less
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accurate) fixes on each entrance.</p>
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<p>Yes, you will have to manually do a surface survey leg using your survey instruments between the GPS point and the cave entrance tag station, and record that in your cave survex file.
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<p>We now (2018) have differential GPS which is much more accurate than
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in the past (e.g. Wookey's 1996 article) but altitudes are often very inaccurate
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and GPS devices don't generally tell you how inaccurate the altitude is.
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<h3>Taking the fix</h3>
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<p>Once you have chosen your point, mark it in some way (could be a spit hole
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or a cairn, for example - we aren't supposed to use paint any more) and place
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the GPS on the point. Don't build a cairn, they don't last for decades and we have survey stations that old. Give the GPS device a
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couple of minutes to get a fairly good fix (the first figure reported may be
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quite a way out, but after a couple of minutes things should settle). Then mark
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the point as a waypoint. If you're feeling really keen, you can set it up for
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<i>averaging</i>, which gives a more accurate fix – some GPS receivers
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support this automatically, and with others you can just leave it recording a
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track log, then record another waypoint at the same place just before you leave
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so it's clear to someone examining the track log when you actually left.
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Averaging the readings like this is vital if you want an even vaguely accurate altitude reading.</p>
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<p>While the GPS is recording your location, you can do something useful
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(like rigging the cave, doing a surface survey from the GPS point to the
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marker spit, looking for other caves, or even having lunch!) Remember to stop
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the waypoint averaging before moving the unit or changing the display page.
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<b>Take a photo of your GPS point showing at least one of your cave entrances
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too.</b></p>
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<p>We all use the same coordinate system WGS84 these days, so <a href="coord2.html">the extensive discussion
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on coordinate systems</a> has been moved to a different page. If you are really interested you can read
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<a href="coord.htm">Olaf's article</a>too.
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<hr /></body>
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</html>
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