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Z15

Mark is Suitable for GPS

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I have noticed some recovery reports by Geocaching enthusiasts that are improperly reporting marks set vertically in buildings as "SUITABLE FOR GPS".   Suitable for GPS means that a surveyor can occupy the point with a clear vision of the sky from 20° (for 360°) above the horizon so as to be able to collect data with a tripod or bipod setup.  A mark set vertically in the wall of a building or other structure IS NOT suitable for GPS.   

Edited by Z15
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I agree, but ...

You aren't always concerned with lat-lon.  I've been doing OPUS Share submissions that I hope will be accepted for the GPS on Benchmarks program, and wondered if there would be some way to use those vertically mounted disks, for height only and not lat-lon.  If you set up the GPS antenna say 50 ft away in the clear, and use an automatic level set at the height of the mark to set the ARP at the height of the mark, anyone who knows how to use the equipment should be able to match within a millimeter or so, well within the expected 2-4 cm accuracy of a 4-hour GPS session.  I don't see any way to tie elevation only for the share submission to the PID in the NGS system.

A procedure like this would improve the availability of stable points for GPS vs Leveling comparisons (the point of GPS on Benchmarks), because the other stable old marks are too often not suitable for use.  They tend to be on railroad bridges too close to an active track (unless you have the budget for a RR flagman), or on culverts and bridges near a stream with a lot of trees, with more trees when the RR line is abandoned and thus easier to access.

I looked at the data for 25 marks used to create Geoid12B in my area, and was shocked at how many concrete posts they used as opposed to more stable mountings.  Only 3 were Stability B.  The above paragraph may explain why-posts are more often out in the open. 

A side issue is that poured-in-place concrete posts are called C stability, and precast ones are D. I'd have to see evidence before I believed that was warranted.  From what I've seen, the precast ones (if they don't get broken) at least guarantee a depth, whereas poured ones that I've seen out of the ground aren't necessarily as long, leaving me wondering about frost heave.

Edited by Bill93
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That way leaves open the chance of blunders.   If we could not find a suitable BM to set up on and had to use the one in a bldg. for vertical control, we would just set a new control point in the open and carry the elevation (double run) to the new point and use that new elevation to constrain our network.  Done that many times but we were only using the data for our projects.  GPS was not good enough for elevations on our projects back then so we ran digital levels.  Many projects we only provided horizontal (project datum) control but required the consultant to provide the elevation control for mapping and plans. 

Since OPUS was just coming of age back circa 2002  my boss (old school PS) was skeptical and would process the survey with our Leica software.  After a year of him spending days doing what OPUS did in 2 min he finally conceded OPUS was better and saved time.  But as I said, at the time we were only charged with providing data for our project control.  My boss and I took early retirement from the DOT in 2002.

Edited by Z15
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