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85 lines
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HTML
85 lines
6.0 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>First actions</h3>
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<p>First of all, you need to <a href="/handbook/computing/myphone.html">set up your phone</a> in "expo mode" for recording locations on the plateau reliably. This is not just some technical settings, it also means a particular style of useage whch you need to learn.
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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 with at least two survey shots
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of the 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>Since 2018 we have differential GPS which renders historical advice obsolete (e.g. Wookey's 1996 article) but altitudes are still always inaccurate
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and GPS devices don't generally tell you how inaccurate they are. Also phones are now much more complicated than the dedicated GPS devices used in the past. It is now
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<a href="/handbook/computing/myphone.html">the phone itself</a> we have to worry about, not just GPS. Phones try to be "helpful" these days and do not tell you what they are doing.
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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 don't 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 provide permanent locations. Give the GPS device a
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minute to get a fairly good fix (the first figure reported may be
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quite a way out is you have been moving). Then mark
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the point as a waypoint (dedicated GPS devices or GPS apps only).
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<h3>Camera locations</h3>
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<p>Even if you have no intention of using your location or recording a track, the camera in your phone will record locations of your photos which are extremely useful to future expeditions - for reasons which only become apparent when you yourself try to work out what someione did 10 years previously.
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<figure class=onright><a href='/handbook/computing/l/camera-ne-track.html'><img src='/handbook/computing/t/camera-ne-track.jpg' /></a><figcaption>Camera photo locations are not<br> on the track!</figcaption>
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</figure>
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<p>Your <em>camera</em> will use the same location settings as the rest of your phone, but sometimes with a bit of a delay. We have lots of examples of geo-located photos where the recorded location is alctually the location of the <em>previous</em> photo because someone has taken a quick photo but the phone hasn't had time after waking up to get a location, so it uses the previous one! And doesn't tell you!!
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<p>So when taking a photo of an entrance, always take one photo; delete it, and take another. This will give your phone a chance to get synchronised properly.
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<p>ALSO: always take 3 photos of any entrance, the obvious one about 10m away, a scene-setting one from 20 or 30m away, but also a really close one of 3 to 5m away, so that we can see if rocks have moved around the entrance and also for a much better identification in future. If there is a tag, <em>always</em> take a close-up photograph of it so that the letters are readable.
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<h4>Averaging</h4>
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<p> If you're feeling really keen and have a dedicated GPS device or sophisticated GPS app,
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you can set it up for <i>averaging</i>, which used to give 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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</p>
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<p>Averaging used to be very important, but today (2025) we have so many GNSS satellites in the sky that ionospheric effects are the significant error. These change slowly over 5 hours or so, so avergaing for a minute or and hour does nothing useful. You would need to average over several days. The solution for a fast fix is <a href="/handbook/computing/myphone.html#future">to use RTK</a>.
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<p>While the GPS is averaging 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 />
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<a href="newcave.html">Next survey guide page</a> - 'Base Camp: getting it in to the computer'<br />
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<a href="/handbook/computing/myphone.html">Your phone on expo</a> - Do not select the "high accuracy" location setting on your phone<br />
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<a href="/handbook/essentials.html">GPS essentials on the plateau</a> - Safety information<br />
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<a href="/logbookentry/2024-07-21/2024-07-21c">Photo GPS</a> - Is unreliable unless you follow procedure<br /></body>
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</html>
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