[Tagging] Feature Proposal - RFC - survey_point:benchmark

[Kevin Kenny]

On Sun, Mar 8, 2020 at 8:42 AM Anne-Karoline Distel <annekadistel at web.de> wrote:
> I've been surveying benchmarks for the past four months and I would like
> to propose an alternative to benchmark=yes for survey points:
> https://wiki.openstreetmap.org/wiki/Proposed_features/survey_point:benchmark
> The reason being that I would like to also propose
> survey_point:hexagonal_bolt and survey_point:ground_bolt with it.
>
> Definition: Ordnance survey point usually chiselled in stone with its
> typical horizontal bar and arrow below on vertical surfaces, dot with
> arrow below on horizontal surfaces. Now often replaced by hexagonal
> bolts in walls or bolts in the ground.
>
> Thank you for your time,

A 'control station' is any fixed mark used by surveyors to establish a
frame of reference. They're often placed specifically for the purpose
(often called 'monuments' in that case), but visible and stable marks
such as church steeples and fire towers have been used as stations.

In common speech, it's typical to call all the monuments 'benchmarks',
but to be technically correct, a 'benchmark' is limited to a 'vertical
control', a fixed point at a known elevation. The horizontal position
of a benchmark may or may not be established to survey accuracy.

'Ordnance survey' is UK-specific, as is marking benchmarks with the
Broad Arrow (a heraldic pheon, historically used to label Crown
property). The US, for instance, has no Ordnance Survey. We have the
US Coast and Geodetic Survey, the US Geologic Survey, many state
surveys (New York's is under the Department of Environmental
Conservation in the Adirondacks, the Department of Transportation
elsewhere), and many oddball ones
(https://forums.geocaching.com/GC/index.php?/topic/73161-1934-us-supreme-court-border-survey-mark/
- even the Supreme Court has ordered surveys). In the US, many
benchmarks have been placed by private surveys as well, because large
private operators also need coordinate frames. The best known are
perhaps http://www.wintertime.com/OH/GC/Disney/disneymarks.html

There's a fairly comprehensive discussion about the types of markers
used by the US government at
https://www.ngs.noaa.gov/web/about_ngs/history/Survey_Mark_Art.pdf

Right now the `survey_point=*` key is a mess. In fact, I'd write it
off as a total loss. It conflates several ideas:

Purpose:  Vertical control, horizontal control (or both). There are
also 'reference marks' - additional monuments placed to establish the
location of a primary mark lest the primary be lost or destroyed;
'azimuth marks' - used to establish a sighting direction from the
primary mark immune to geomagnetic variation, and stations that were
used in surveys to map the geomagnetic field, the gravitational
potential, the tidal variation, and so on. In addition, some control
stations had 'witness marks' that were placed only to alert people to
the presence of the station.
https://flickr.com/photos/ke9tv/29681317420 is one form these could
take. The trig point that it warns of is
https://flickr.com/photos/ke9tv/29348215163.

Form: I understand that the commonest form of a UK survey mark is a
stone tablet (or lettering and symbols of the same form chiseled in
native stone. While the US has used stone tablets, disc-shaped bronze
tablets are the commonest form here. Other forms frequently observed
are rods, pipes, bolts, drill holes, and clay cones.  In diggable
soil, a second mark was often installed underground, and was actually
the primary reference. The surface mark would be plumbed above it and
offset by a known vertical distance. Some of these
formerly-underground markers have since been exposed by damage. In
addition to form, material might be interesting: is a tablet of stone,
bronze, concrete, fired clay, ...?  Many tablets and rods were affixed
to drill holes by pouring hot lead, Babbitt metal, sulphur or bitumen
in the hole, and sometimes the hole and fill material are all that
remain (and the centre of the hole is still a usable horizontal
control).  You also seem to consider shape important - disc-shaped,
rectangular, or hexagonal tablets may have particular meaning to you?

Accuracy: Many surveys placed markers to different orders of accuracy.
Standards varied by agency and time.

Operator: Who placed the mark? Who now controls it?

(There are no doubt other attributes.)

Since all four of these can vary independently, it seems unwise to
group them all under 'type'

A more-complete description might be: 'BLACK (OD1740) horizontal
trigonometric control station, marked by a copper nail and washer
stamped "N.Y. / V.C.", position established to first-order accuracy
and thought to be of better than normal stability, placed by the
Adirondack Survey (Verplanck Colvin, state surveyor), now controlled
jointly by the National Oceanographic and Atmospheric Administration
and the New York State Department of Environmental Conservation.' or
'ALANDER (MZ2083) geomagnetic station, marked by a disc-shaped bronze
tablet, placed by US Coast and Geodetic Survey and now controlled by
NOAA, originally not usable for positional control but now located to
second-order horizontal accuracy'

My guess is that the Wikidata gnomes already have a schema for this
that considers attributes I've missed. I'd surely not mind if OSM were
to have a better set of keys to describe these, but 'type' is a poor
choice.

-- 
73 de ke9tv/2, Kevin