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Horizontal accuracy for GPS

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Philip Sargent
2026-02-03 16:49:38 +00:00
parent 3de8f06b90
commit bba32fdcc5
9 changed files with 75 additions and 1 deletions
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handbook/survey/gps.htm
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@@ -27,6 +27,16 @@ Install the "GPS Test" app, this one: <a href="https://play.google.com/store/app
<h3>Which point to fix</h3>
<div class="onright">
<figure><a href='/1623/196/l/Screenshot_20250723-144450.html'><img src='/1623/196/t/Screenshot_20250723-144450.jpg' /></a>&nbsp;<br />
<caption>at 1623/196</caption>
</figure></div>
<div class="onright">
<figure><a href='/1623/196/l/IMG_20250723_144434.html'><img src='/1623/196/t/IMG_20250723_144434.jpg' /></a>&nbsp;
<a href='/1623/196/l/IMG_20250723_144406.html'><img src='/1623/196/t/IMG_20250723_144406.jpg' /></a>&nbsp;
<br />
<caption>1623/196 tag and entrance photo</caption></figure></div>
<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
@@ -84,7 +94,13 @@ the point as a "waypoint" in OSMand.
<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.
<h4>Accuracy</h4> <div class="onright"> <figure> <a href='/handbook/survey/l/insert-ringed.html'><img src='/handbook/survey/t/insert-ringed.jpg' /></a>&nbsp;<br /><caption>Click for explanation</caption></figure></div>
<p>Google maps or whatever will report your location accuracy using the "PDOP" number, but this is not what you think it is. What you need is the probability that your location reading is accurate: the estimated horizontal accuracy "EH Acc". In this image (at 1623/196) this is <b>11.2m</b> i.e. there is only a 68% chance that this lat./long. is within <em>11.2m</em> of the "real" location ! Even though the HDOP in this case is an apparently very good 0.8m. This is the instantaneous accuracy - this can be substantially improved by averaging. Click on the image to the right to get a fuller explanation.
<h4>Averaging</h4>
<p> If you're feeling really keen and have a dedicated GPS device or sophisticated GPS app,
you can set it up for <i>averaging</i>, which used to give a more accurate fix &ndash; some GPS receivers
support this automatically, and with others you can just leave it recording a