On 21-Nov-17 08:32 PM, Mark Wagner wrote:
On Tue, 21 Nov 2017 10:47:30 +1100
Warin <61sundow...@gmail.com> wrote:
Hi,
There have been attempts in the past to add sub tags to
man_made=survey_point
https://wiki.openstreetmap.org/wiki/Tag:man_made%3Dsurvey_point
To me there are 2 'types'. they are quite different;
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.
If consideration is given to
https://wiki.openstreetmap.org/wiki/Seamarks/Categories_of_Objects#Control_Points_.28CATCTR.29
then expanding survey_point in a similar manner could be
survey_point:configuration=triangulation/benchmark
I use 'configuration' rather then 'type' or 'category' as it is more
specific as to what is meant.
Any thoughts?
Are there any other configurations?
Survey marks in the US tend to come in three types:
1) Benchmarks, which are typically a bronze disk solidly attached to
the top of something. Both the horizontal and vertical positions are
usually well-surveyed.
2) Vertical control points, which are typically a bronze disk affixed
to the side of something. The vertical position is well-surveyed, but
the horizontal position is only recorded well enough to let a surveyor
find it.
I think of these as both kinds of benchmarks.
The accuracy of the horizontal and vertical location are different and could be
tagged (somehow - I've not investigated this ... )
The orientation could also be tagged .. I think these are already tags for
orientation within OSM for other features.
3) Reference marks. Typically, a bronze disk attached to something
that gives directions to a nearby survey mark. The position isn't
recorded beyond what's needed to find it, if that.
Humm .. If the survey mark itself is mapped there should be no reason to map
these 'reference marks'?
It would be like mapping the sign carrying the street name.
(There are also horizontal control points, but these tend to be things
like flagpoles and smokestacks which have been surveyed and documented
for use as survey marks.)
Those would form the same function as a trig point.
And could be tagged the same way?
Should have some indication of accuracy.
Any thing that is mapped in OSM could be used the same way as a true trig point,
but the accuracy would be the key thing between a hill top and a trig point as
the trig point would be determined with much higher precision.
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