First of all, I want to get the word out as much as possible about the 2018 GPS on benchmarks campaign. A lot of you likely already know about this. For those who don't...
http://gpsworld.com/ngs-2018-gps-on-bms-program-in-support-of-napgd2022-part-5/
This is important for two main reasons...
1) The next (and final) Hybrid Geoid Model will be GEOID18, to be released hopefully in April of 2019. This will replace GEOID12B. Observing GPS on benchmarks and submitting to NGS via OPUS share will help to strengthen the new model
2) The new vertical datum NAPGD2022 to be released in 2022 as a replacement for NAVD88 will NOT be a hybrid model, it will be a strictly gravity based geopotential datum. BUT, all of the GPS on benchmarks data will help to create a more accurate transformation tool to go from NAPGD2022 to NAVD88 and vice-versa.?ÿ
I encourage any companies or government agencies to try and occupy one or more marks in your area from the lists provided as part of this effort. This must be done by August 31, 2018 to be included in the GEOID18 model. After that, it can still help with the transformation tool.?ÿ
The other day I was in the field, and I needed make a tie to NAVD88. There was a 1983 level line nearby. The three nearest marks were all rod marks, apparently undisturbed, stability B. But, each was set less than 2 feet from a utility pole. During the Basic Net A releveling that took place in the early 80's around here a lot of marks were set that way, probably thinking they would be less likely to be destroyed if they were set close to a utility pole. Of course, that was before GPS came along. So being less than 2 feet from a pole is less than ideal for a good GPS session.?ÿ
And of course there are literally thousands of good marks that are not good for GPS, or set vertical in a structure, etc.?ÿ
I inquired to NGS if it would be acceptable to set an eccentric station and do a differential leveling tie. The three points I found were all wide open, so it would be easy to set an eccentric station one turn away.?ÿ
NGS responded that yes, it is acceptable. Here is the response I received...
The short answer is yes.?ÿ NGS does have a method for accepting GPS on eccentric stations for bench marks.?ÿ In fact, I learned today that this method has been in place for some time now, and that such observations have been used to develop hybrid geoid models (like GEOID12B).?ÿ That will continue for GEOID18.
In this case, you would need to use NGS' mark reset procedure to transfer the height from the bench mark to an "eccentric mark".?ÿ If that is done, then it is possible to share or bluebook GNSS data on the mark that can be used for the future hybrid geoid model.
The reset procedures are at?ÿ https://geodesy.noaa.gov/PUBS_LIB/Benchmark_4_1_2011.pdf .
The reset location and description form is at?ÿ https://geodesy.noaa.gov/heightmod/Leveling/Manuals/Report_on_Location_and_Description_of_Reset_Bench_Mark.pdf .
The reset observation form is at?ÿ https://geodesy.noaa.gov/heightmod/Leveling/Manuals/Observations_for_Relocation_of_Bench_mark.pdf .
I would think that a station set one or two turns away from a good stable benchmark should be able to be connected to an accuracy of a mm or two using a level. Not sure exactly how the data is to be submitted so that it "connects" the existing benchmark and the eccentric stations. Maybe they will create a link on the OPUS share page or maybe a separate link on the GPS on Benchmarks page?
?ÿ
?ÿ
?ÿ
?ÿ
Glad to see you pursuing this.?ÿ I also asked during the webinar yesterday, but the answer we got then was just use a different BM from the old level line.?ÿ Of course, we know of many areas where there may not be another surving mark that is suitable for GPS within 20 km.
I guess when the Reset shows up with a PID then you can submit the share connecting it to the PID. How long does it take for a reset to appear?
Unfortunately, last time I looked at the reset procedure it required a digital level, which I think only a minority of surveyors have.
I can see that as a requirement for a real reset intended to best preserve accuracy.
But why make it so hard for a GPSonBM session that will be lucky to be within a cm anyway?
I wish there was a looser procedure for this purpose.?ÿ Maybe allowed one turn, 50 meter max balanced sights, automatic level or better, pegged same day, two level setups agree within 2 mm. Add to the share software a check box for eccentric, that suppresses the automatic transfer of horizontal coords to the data sheet.
I posted a while back about my procedure used to get myself a check, where I used the level to set both the level and the ARP to the height of the mark.?ÿ It took some fussing but I'd trust that result better than a measure-up.
https://surveyorconnect.com/community/gnss-geodesy/using-vertically-mounted-bench-marks/
?ÿ
?ÿTagging for later...
Bill
I thought your question was good and the answer was awful showing a lack of understanding for the conditions the monuments are in. Most of us that would participate if eccentric mark construction and measurement guidelines were loosened will not participate due to the process.?ÿ
I say do an OPUS share on the eccentric mark and state the loop difference to the inaccessible mark in the description. They can take it or leave it. I know they will leave it because they have their own ways but maybe more practical surveyors will find the information useful.?ÿ
Great information, thank you. The surveying club at my university has been looking at the few Priority A marks in our area (hoping to do our part and contribute) and have only found one so far that is acceptable for using GNSS...granted, one mark may well be all that we have time for. Regardless, good to know about this procedure for future reference.
I got around to re-reading the Reset procedure that John linked above.?ÿ It does include instructions for using an optical level, not only digital.?ÿ I can't readily find where I got the impression they had upped the requirement to digital.?ÿ?ÿ What is the true requirement?
