I recently did some static occupations on five different bench marks that have first-order leveling on them. The occupation time was around 5 hours on each point.
The vertical differences from the OPUS elevations to published elevations were:
0.016’
0.003’
0.054’
0.107’
0.068’
I am using a Sokkia GSR2700ISX receiver on a 2.000-meter fixed height SECO tripod.
better do them again
Post-Glacial rebound???
Just for the record, were they high, low or mixed?
Howdy,
You are worried about the disagreement between first-order differentially determined NAVD 88 heights and OPUS-derived heights. Perhaps you might clarify a few issues. BTW, plotting them might help detect a tilt or bias in either the levels or geoid model.
What is the source for the heights you are using OPUS to check? When were they established? What type type monuments are they: concrete posts, 3-D rod marks, disks in bedrock? Are any located in deforming areas?
If the points are in the NGS database all these questions can be answered by examining the data sheets. You can also determine whether the points were established during the same level project. You should also insure that the status of the current NAVD 88 position is "Adjusted."
Remember that the NAVD88 compatible height from OPUS is derived "simply" by retrieving the geoid-ellipsoid separation at the site's position and applying it the computed ellipsoid height. Unlike a network solution there is no fitting involved.
Assuming that the OPUS solutions all pass the quality checking criteria and that the "published" values are shown as Adjusted, noting that you used a fixed-height pole (making a bad HI less likely), the logical answer is that the large discrepancies are due to either monument movement since its determination or a problem with the hybrid geoid model (most likely due to a paucity of data).
Did you submit the data to OPUS-DB? If so, the GPS on BM data will help in the development of new hybrid geoid models. If not, why not?
Cheers,
DMM
GPS results to actual are:
High
Low
High
Low
Low
No worried at all. In my opinion these are excellent results. Just sharing what we can now achieve compared to what GPS used to be able to provide for vertical.
:good:
This goes right along with what I said above about not posting because it just shows my ignorance... I don't find most of those discrepancies "large". My experience with OPUS has been that is about as good as you are going to get with a 5 hour session. I have run longer sessions on wide open levees on different days that give me those differences and worse. Most all of them look pretty good to me. Are you telling me that for a 5 hour OPUS session you guys get better results than those? Really? It must work better where you live than it does here.
Apologies
My error. While clearly indicated to be in foot units, I saw them as metric. Mea culpa.
Results looked good to me.
One of the few times I had to baby-sit a static setup was on top of a ridge in western NC in a snowstorm. My setup was only about 45 minutes or so but had to do it twice. My other 3 partners that day had 2 other setups to baby-sit. We were told to not use radios or cell phones so #4 (Karen) ambled around to each of us to check on us. She got to me and I had made myself a bed with pine boughs and was sound-ass asleep and getting covered in snow. She poked me with a stick and I about jumped 100' in the air. Guess she thought I was dead or something.
those results look fine to me. Are these results using the precise ephemeris? It does appear to still make a difference.
vg results..
for a 5 hr opus session being in the 3-5 cm range is a good without looking at your datasheet from OPUS
common procedure here in the land of subsidence would be 2 - 4hr+ plus sessions on consecutive days as advised by NGS for establishing heights.
