[Tagging] Additional sub tags for survey mark

Greg Troxel gdt at lexort.com
Sun Dec 3 00:45:44 UTC 2017


Warin <61sundowner at 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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