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120 lines
9.8 KiB
HTML
120 lines
9.8 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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<p>Your current phone may <em>just not be able to do what we need</em>: some phones are just brain-dead for this purpose. Buy another phone.
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<h3>Calibrate/document your phone's capabilities</h3>
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<figure class=onright width=20><a href='https://github.com/barbeau/gpstest#readme'><img width=64px src='/handbook/computing/t/gpstest.jpg' /></a><a href='https://github.com/barbeau/gpstest#readme'> <img width=64px src='/handbook/computing/t/F-Droid_Logo_4.svg.jpg' /></a>
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</figure>
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Install the "GPS Test" app, this one: <a href="https://play.google.com/store/apps/details?id=com.android.gpstest">GPS TEST</a>
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<h4>Confirm that your phone is not going to sleep</h4>
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<blockquote>Keep the device very still. If the position shown by GPSTest slowly refines then stops updating within a minute or two, or shows only the last digit changing slowly, <em>then your device cannot be used for averaging</em>.
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<p>A device with 1 metre accuracy still shows the 6th and 7th decimal place digits of the longitude and latitude updating every second or so (that is a roughly 10 cm wobble every second). You should be seeing those numbers change! In general, this makes most phones unusable for averaging because the chipset just won't do it.</blockquote>
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<p>If you record a track using OSMand, and keep your phone still while you do it, some phone chipsets just decide that you are not moving and just turn off. So you do need to check whether your phone does this <em>before taking your phone to expo</em>.
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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>If you are doing surface prospecting and do not have daylight-capable survey kit with you (actual physical tape measure, compass/clino) then <em>estimate</em> the distance and direction from your GPS recorded location and the cave entrance, e.g. "10m towards entrance in direction 025 degrees". You do have a notebook with you of course. This will be transcribed onto your "New Cave" form. Your phone does have a compass capability: download the app and use it. Take a photo of the GPS point with the entrance in view behind. If you have set up your phone correctly (ahem), your phone camera will record the compass direction of the photo.
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<p> A notebook and pencil are essential when looking at lots of entrances: no, your memory is not good enough. Takes a screen-shot of the GPStest screen whenever you want a proper GPS location and upload the screenshot to the digital wallet for a new cave. Use the timestamp on the screenshots to correlate everything. A screenshot is proper evidence, your memory is not.
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<h4>SBAS</h4>
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Check whether your phone is using SBAS "Satellite-based Augmentation Services", which is another name for EGNOS, the differential GPS system.
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You need to record if SBAS is in operation whenever you record the position of a cave entrance (and ideally whenever you take a photo too, but that would be unrealistic). A screenshot of the GPStest screen does this for you.
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<p>Since 2018 we have differential GPS which renders previous advice obsolete (e.g. Wookey's 1996 article).
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<h4>Altitudes</h4>
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Altitudes are still (2026) always very inaccurate (~20m variation sometimes) and GPS devices don't generally tell you how inaccurate they are. (But the GPStest screen does tell you.) We do not bother to record GPS altitudes as they are spurious: instead we use the latitude and longitude and get the altitude later - from a 1m accurate laser scan of the plateau.
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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" in OSMand.
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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 (2026) 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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<h3 id="future">The future</h3>
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<p> <a href="https://www.cavinguk.co.uk/info/locatingsurveys.html">Location fixing: How to obtain a fixed point for a cave survey</a> (2020) .Galileo High Accuracy Service (HAS) was due to become available in 2024, which would offer RTPPP for free around the world - but it's still not really working for most people.
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This is an excellent article and nearly all of it is still absolutely true, e.g.:
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<p>Unfortunately RTPPP has not actually caught on [2026] and it is still only available for experts and phones supporting L2 and L5 frequencies have been slower to appear than we hoped. Read Wikipedia on <a href="https://en.wikipedia.org/wiki/Real-time_kinematic_positioning">Real-time kinematic positioning (RTK)</a>.
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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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<br />
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<div class="text-box">
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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 />
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<a href="/handbook/survey/gps.htm_edit">Locating Entrances</a> - expo recommended methods (this page)<br />
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</div></body>
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</html>
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