Warin <61sundow...@gmail.com> writes: > Triangulation (or 'trig point') that are visible over quite some > distance (say over 2 km), used to triangulate a position without > having to go to the mark. Usually a pole standing on top of a > rise/hill. > > Benchmarks that are visible on the surface but cannot be sighted at > any distance. They can be small brass plaques fastened to the ground > or engraved into stone. These are used by surveyors by placing a > tripod over the mark, thus have to be locally approached.
This may be the UK usage, but does not match US practice. I have seen UK trig points (e.g. Arthur's Seat, Edinburgh), so I do understand what you mean by that. Certainly the notion that the mark is visible from a great distance is an important property, and Andrew Harvey's comment about looking at nautical aids to navigation makes lots of sense. In the US, historically there were two kinds of controls. The monuments are very similar, both brass disks. One is a "bench mark", which is a vertical control (and the horizontal coordinates are not necessary known precisely). The other is usually just called "horizontal control" or perhaps long ago "triangulation station", and these are for the horizontal network. Some points are both. Essentially all of them have a Permanent Identifier (e.g., "MY6064") that one can use to look the control up in a database, and except for a few ancient ones, the PID is inscribed on the mark. Probably because we have more trees, the UK-style trigpoint did not become popular and there were sometimes 15m towers erected over marks to see from and be seen. Now of course people set up geodetic GPS antennas over marks, with an optical plummet and measuring rod. Overall, I lean to minimal description of physical appearance in the major key until we have a clear reason to get into describing purpose. So for the usual US disc: manmade=survey_point survey_point=brass_disc ref=us:ngs:MY6064 with ref being country:authority:identifier (or country:state:authority:identifer, like us:ma:massdot:123 if it turns out to be needed). I can certainly see survey_point=trigpoint to refer to the UK-style concrete pillars. I am omitting information about horizontal vs vertical control, order, etc., but that can be found using the ref as a foreign key in the right database. Please don't conflate trigpoint = pillar and flush brass disc = benchmark. That may be what the words mean in the UK, but that will be confusing to professional surveyors and geodesists in the US. Greg (osm user gdt)
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