I have a question I am hoping that the more experienced can help me with.
I have a local reference monument that has a published elevation. It was derived by gps observation using geiod99. I setup on it yesterday to transfer an elevation. I collected static data on it and submitted it to OPUS. The elevation came back 1.6' higher than the published elevation.
My question is what are the FEMA BFE based on and which elevation for the control monument should be used?
For what it's worth I am running additional static sessions today, one on another published local monument and one on a NGS monument to check my data.
Matt
Matt:
According to the OPUS website there appears to be some problems with Geoid12, at least in the Gulf region.
http://geodesy.noaa.gov/web/news/GEOID12_Error_Notice.shtml
The FEMA FIRM covering the area in question should state the datum. If you are doing any Elev Certs, etc, your datum must match or provide the conversion.
I have submitted numerous files to OPUS and have not been impressed with the NAVD88 results. These occupations were on known NGS control, control that was established by post processing multiple static sessions and on control that was derived from traditional leveling. In all cases the OPUS derived NAVD88 elevations did not match the published elevations by quite a bit in some cases feet. I would not reccomed using OPUS derived elevations. Many of the COR stations have elevations published to the ARP. I have had good results using one receiver and the data from several COR stations, I then post process to get NAVD88.
To the OP, if vertical is critical, then I would not forego occupying at least one, preferable more solid 1st order BM's as part of my project.
In no case, have I ever seen OPUS return results that are off in the feet range and I have submitted 1000's of data sets to OPUS as an early adapter. Many of these data submittals were often over many, many days and the results come back pretty tight consistently. My experience also covers about 1/2 of the lower 48, location doesn't seem to matter, it is a good solid system.
If you are seeing feet that is either an isolated case OR there was something else going on. Of course in areas with subsidence, it might be possible the problem is with the leveled BM's and not OPUS at all.
SHG
I revisited a project done last year it was a height mod project with 5 hour sessions. The OPUS results we not as bad as I recalled, but I would not use OPUS for orthometric heights without redundant occupations.
DATE, PID, PUB, OPUS, DELTA,
10/13/2011 DI3642 22.770 22.833 -0.063
10/26/2011 DI3642 22.770 22.775 -0.005
10/27/2011 DI3642 22.770 22.782 -0.012
10/31/2011 DI3642 22.770 22.752 0.018
10/13/2012 AL7992 27.770 27.759 0.011
10/26/2011 AL7992 27.779 27.757 0.022
10/27/2011 AL7992 27.779 27.833 -0.054
10/31/2011 AL7992 27.779 27.719 0.060
11/3/2011 AL7992 27.779 27.789 -0.010
11/7/2011 AL7992 27.779 27.682 0.097
> but I would not use OPUS for orthometric heights without redundant occupations.
BINGO, assuming those differences are feet, the mean of the differences for those 10 OPUS solutions is within .006 feet of the "truth". Can't hope for much better results than that!
SHG