[Tagging] Topographic Prominence for Peaks

Colin Smale colin.smale at xs4all.nl
Thu Sep 27 07:19:40 UTC 2018

On 2018-09-27 07:17, Graeme Fitzpatrick wrote:

> & when you say survey with GPS, is that accurate enough for an altitude reading? With my Garmin GPS (which admittedly is 10 - 15 years old, but _wasn't_ a cheap one!), I can calibrate it in the back yard at 6m ASL, go for a day trip & when I get home, it displays the exact same spot as anything between -5 & +30m ASL :-( When out driving, I've also seen the altitude display change by 100s of m's instantly, when the road is virtually flat.

...bearing in mind that ele=* is supposed to be expressed in WGS84 /
EGN96 not relative to a local sea level datum, so if your GPS already
applies a "correction" to show you heights relative to sea level, you
have to back those corrections out, or put the datum in the OSM

GPS is not very accurate in the vertical direction, due to the altitude
of the satellites and the geometry involved. In the horizontal
directions, you are surrounded on all sides by satellites which can give
a fairly accurate fix. In the vertical plane the angles to the
satellites are limited to the half sphere you can "see" without going
through the earth.
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