[Talk-us-massachusetts] Fwd: GPS on Benchmarks Campaign from the National Geodetic Survey

[Bill Ricker]

> The implication is that they don’t have good information about the
> > location of most of these.
>

Ahh.  The not-lately-recovered and the SCALED VERTCON that may be a trifle
tricky with only old pre-paving descriptions ...

> I was skeptical of this suggestion (that navigation-solution positions
> owuld be helpful), but their page indeed says they want non-survey-grade
> measurements of marks that are marked "SCALED" meaning they guess the
> location from a topo map.  I don't think there are that many of those.
>



> They also want recovery notes for marks that have not been recovered in
> 30 years.
>

That makes sense. (Although quite a few of those may be destroyed.)

They do want survey-grade ellipsoidal height measurements of benchmarks.
>

And contrary to popular informal usage, "benchmark" is a technical term for
a specific subset of survey control points, specifically *vertical* control
points. Which formerly were set in benches, to have a level surface, go
figure.
Most of the disks we see are triangulation points - horizontal control
points only, and thus not BM.

(The MassDOT survey-disk/map viewer will let you select VERTCON or HORZCON
or BOTH only. Most are Horizontal control only. )

marked "scaled".  An example of scaled is
> https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=MY2109
> There are fairly few scaled ones, and from a very quick look they seem
> to lean to true benchmarks (vertical control) along roads and railroads.
>

Since VERTCON only had only rough horizontal survey to begin with, those
being strong in the SCALED category makes sense.
Roads and Railroads VERTCON markers are for civil-engineering  when they're
being laid so not surprising that they've not been carefully preserved or
recovered.

I'll send a screenshot offlist to Andy and Peter and happy to send it to
> anyone else who wants it; it's too big for the list  (not a complaint!).
>
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