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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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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf8" />
<title>GPS location</title>
<link rel="stylesheet" type="text/css" href="/css/main2.css" />
</head>
<body>
<H1>GPS location</H1>
<div class="centre"><img alt="" src="/1623/196/i/Screenshot_20250723-144450.jpg" />
</div>
<p>Screenshot<br /><a href='?to_be_rotated'><b>&olarr;</b>&nbsp;</a>rotate this image<br /><br /><br /></br /><br /></p>
23 July 2025 14:44
<p class="caption">Photo &copy; Philip Sargent, 2025</p>
<hr /></body>
</html>

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cave_data/1623-196.html
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@@ -17,6 +17,7 @@ the form documented at
<cave>
<non_public>False</non_public>
<unexplored>False</unexplored>
<official_name>Schwa H&ouml;hle 196</official_name>
<area>1623</area>

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entrance_data/1623-196.html
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@@ -28,7 +28,7 @@ though, you do not need to do a data import as it happens automatically -->
<approach>Path from Top Camp to Steinbruckenh&ouml;hle (204) goes past this cave. Close to <a href="/1623/194/194.htm">194</a>, <a href="195/195.html">195</a>, <a href="/1623/165/165.htm">165</a>.</approach>
<underground_description></underground_description>
<photo><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;</photo>
<a href='/1623/196/l/IMG_20250723_144406.html'><img src='/1623/196/t/IMG_20250723_144406.jpg' /></a>&nbsp;<a href='/1623/196/l/Screenshot_20250723-144450.html'><img src='/1623/196/t/Screenshot_20250723-144450.jpg' /></a>&nbsp;</photo>
<!-- marking options: P = Paint,
P? = Paint (?),
T = Tag,

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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

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf8" />
<title>Estimated horizontal accuracy</title>
<link rel="stylesheet" type="text/css" href="/css/main2.css" />
</head>
<body>
<H1>Estimated horizontal accuracy</H1>
<div class="centre"><img alt="" src="/handbook/survey/i/insert-ringed.jpg" />
</div>
<p>
In this image the Estimated horizontal accuracy is 11.2m i.e. there is only a 68% chance that this lat./long. is within 11.2m of the "real" location ! Even though the HDOP in this case is an apparently very good 0.8m and 20 satellites have been used to calculate this position. This is the instantaneous accuracy - this can be substantially improved by averaging.
<p>So for this particular measurement, there was a 1/3 chance that the tag is more than 11m away from this GPS data shown by the phone. This is not good enough for finding cave entrances on the plateau (there are often half a dozen holes within 10m) and certainly no good for surveying.
<p>The Help text in the app explains this in more detail. On the web read:
<a href="https://barbeau.medium.com/measuring-gnss-accuracy-on-android-devices-6824492a1389">Measuring GNSS accuracy on Android devices</a>
<blockquote><em>
Get the estimated horizontal accuracy of this location, radial, in meters.
<p>
We define horizontal accuracy as the radius of 68% confidence. In other words, if you draw a circle centered at this location’s latitude and longitude, and with a radius equal to the accuracy, then there is a 68% probability that the true location is inside the circle.</em></blockquote>
<p>TTFF is "time to first fix". This means I would have had to wait <em>at least</em> 2 seconds after stopping moving before I would have any hope that a photo taken with the camera on this phone would get an accurate fix embedded in it.
<p>
GPStest online <a href="https://github.com/barbeau/gpstest/blob/master/FAQ.md">documentation</a> explaining all the information on the screen.
<p class="caption">Image: Philip Sargent, 2026</p>
<hr /></body>
</html>

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