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

[Greg Troxel]

Andy Anderson <aanderson at amherst.edu> writes:

> Here’s a direct link to their map of “found” markers, not a whole lot in Massachusetts in comparison to, say, Connecticut, Rhode Island, and Vermont:
>
> 	https://noaa.maps.arcgis.com/apps/opsdashboard/index.html#/a61976366d1f4a19b3bc8a2d5a0d5498
> <https://noaa.maps.arcgis.com/apps/opsdashboard/index.html#/a61976366d1f4a19b3bc8a2d5a0d5498>
>
> I know where a few are off the top of my head, so I’ll add them when I get a chance to revisit those sites.

Interesting that it is so sparse.  I have submitted a few recoveries but
they were long ago.

There are two separate things going on.  One is that NGS would like to
know that a mark still exists and can be found using the description in
the datasheet.   That doesn't require any particular equipment, and that
sounds like what you are intending to do.

The other is the point of "GPS on Benchmarks".  Benchmarks by definition
are vertical controls, and many have published elevations in NAVD88.
These heights are rdferenced to gravity in that they are determined by
leveling loops, moving horizontally along equipotentials and measuring
the distance along the plumb line.

The NGS is moving to a height reference that is based on ellipsoidal
heights as determined by GNSS -- which therefore are not related to
gravity -- and a gravity model that relates ellipsoidal heights to
orthometric heights, more or less meaning distance along the plumb line
to the standard equipotential (the modern equivalent of "mean sea
level").  But, there are many benchmarks that have NAVD88 heights but
not ellipsoidal heights.

So what they really want is a survey-grade setup for a long time (4
hours? -- maybe longer -- I'm not really clear on details) using a
dual-frequency carrier phase receiver, a calibrated antenna, and then a
solution processed by their online tool OPUS that aims for
millimeter-level ellipsoidal heights.  I have borrowed mid-90s vintage
equipment (GPS only, but L1/L2) like this and done a 36-hour
observation.  I think my solution quality would be right on the edge of
what's good enoguh for GPSonBM, had the mark I measured been a BM.
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