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updates on GPS location point - online edit of handbook/survey/gps.htm - on dev machine 'Mohawk'

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Philip Sargent
2026-02-03 12:56:44 +00:00
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handbook/survey/gps.htm
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@@ -10,6 +10,19 @@
<h3>First actions</h3> <h3>First actions</h3>
<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. <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.
<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.
<h3>Calibrate/document your phone's capabilities</h3>
<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'>&nbsp;<img width=64px src='/handbook/computing/t/F-Droid_Logo_4.svg.jpg' /></a>&nbsp;
</figure>
Install the "GPS Test" app, this one: <a href="https://play.google.com/store/apps/details?id=com.android.gpstest">GPS TEST</a>
<h4>Confirm that your phone is not going to sleep</h4>
<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>.
<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>
<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>.
<h3>Which point to fix</h3> <h3>Which point to fix</h3>
@@ -30,17 +43,22 @@ otherwise with a restricted view of the sky, then choose instead a good
landmark with a wide, clear view of the sky, and with at least two survey shots landmark with a wide, clear view of the sky, and with at least two survey shots
of the entrance. </p> of the entrance. </p>
<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. <h3>Calibrate/document your phone's capabilities</h3> <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.
<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'>&nbsp;<img width=64px src='/handbook/computing/t/F-Droid_Logo_4.svg.jpg' /></a>&nbsp; <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.
</figure>
Install the "GPS Test" app, this one: <a href="https://play.google.com/store/apps/details?id=com.android.gpstest">GPS TEST</a> and check whether your phone is using SBAS "Satellite-based Augmentation Services", which is another name for EGNOS, the differential GPS system. <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.
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). [ Philip Balister takes a screen-shot of the GPStest screen whenever he want a proper GPS location. A notebook and pencil are also valuable adjuncts when looking at lots of entrances: use the timestamp on the screenshots to correlate everything. ]
<p>Since 2018 we have differential GPS which renders historical advice obsolete (e.g. Wookey's 1996 article) but altitudes are still always inaccurate <h4>SBAS</h4>
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 Check whether your phone is using SBAS "Satellite-based Augmentation Services", which is another name for EGNOS, the differential GPS system.
<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. 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.
<p>Since 2018 we have differential GPS which renders previous advice obsolete (e.g. Wookey's 1996 article).
<h4>Altitudes</h4>
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.
<h3>Taking the fix</h3> <h3>Taking the fix</h3>
@@ -49,7 +67,7 @@ or a cairn, for example - we don't use paint any more) and place
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 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
minute to get a fairly good fix (the first figure reported may be minute to get a fairly good fix (the first figure reported may be
quite a way out is you have been moving). Then mark quite a way out is you have been moving). Then mark
the point as a waypoint (dedicated GPS devices or GPS apps only). the point as a "waypoint" in OSMand.
<h3>Camera locations</h3> <h3>Camera locations</h3>
@@ -71,7 +89,7 @@ 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. so it's clear to someone examining the track log when you actually left.
</p> </p>
<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>. <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>.
<p>While the GPS is averaging your location, you can do something useful <p>While the GPS is averaging your location, you can do something useful
(like rigging the cave, doing a surface survey from the GPS point to the (like rigging the cave, doing a surface survey from the GPS point to the
@@ -86,8 +104,8 @@ on coordinate systems</a> has been moved to a different page. If you are really
<h3 id="future">The future</h3> <h3 id="future">The future</h3>
<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. <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.
This is an excellent article and nearly all of it is still absolutely true, e.g.: This is an excellent article and nearly all of it is still absolutely true, e.g.:
<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>. 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). Unless your device can actually do 1 cm accuracy, you should be seeing those numbers change! In general, this makes most phones unusable for this purpose"</blockquote>
<p>Unfortunately RTPPP has not actually caught on [2025] 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>. <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>.
<hr /> <hr />
<a href="newcave.html">Next survey guide page</a> - 'Base Camp: getting it in to the computer'<br /> <a href="newcave.html">Next survey guide page</a> - 'Base Camp: getting it in to the computer'<br />