All you ever wanted to know about geodesy

If you've ever wondered what the difference between a coordinate system and a datum is, or what the difference between OSGB36 and WGS84 is, or what is the relationship between the ellipsoid and the geoid, or even what is the difference between a TRS and a TRF, then I recommend the excellent Ordnance Survey paper A guide to coordinate systems in Great Britain. It's an extremely well written explanation from first principles of the underpinnings of GPS - the coordinate and reference systems which are used to locate a point on the planet. Although the title suggests it is UK-centric, much of it is in fact globally applicable.

Categories : Peak District

Getting accurate height datums

My Garmin eTrex Summit has a built-in altimeter, and although it can auto-calibrate from the GPS fix, it isn't that accurate and it takes a while to do. The Ordnance Survey have been gradually removing the spot height and benchmark information from the 1:25,000 maps as mostly they aren't used for surveying any more. Both trig points and benchmarks need 'maintaining' by being resurveyed at intervals as they are subject to ground movement, so it is understandable that the OS isn't bothering if they don't use them any more. However they are still good enough for calibrating an altimeter which isn't accurate to less than a metre anyway, the problem is actually finding the information! I was browsing around the web looking for something else entirely and I found that The Planning Inspectorate provides a map facility that allows you to drill down on th the OS MasterMap maps which still have the height information on them. Go here and click on the "Select a Point" or "Select an Area" icons and keep drilling down to you see the individual property boundaries on the map. From that point on you will be able to see all the spot height and benchmark data, to the nearest centimetre, and there are a lot of them, so you should be able to find a convenient one to use for calibrating.

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Categories : Tech