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number of satellites - online edit of handbook/survey/gps.htm

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Philip Sargent
2026-02-24 18:29:19 +00:00
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handbook/survey/gps.htm
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@@ -68,7 +68,7 @@ Check whether your phone is using SBAS "Satellite-based Augmentation Services",
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).
<p>Differential GPS (SBAS) has existed since 2018, but it is often not actually available on a phone. The number of satellites now 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.