Hello, I recently ran into this issue. We are using CHCNAV i73 receiver with a local CORS. The MSL heights using EGM96 are about 30cm higher than Trimble using the same corrections. I wanted to check if anyone else faces such issues and how can we correct it if at all. In Trimble there is a parameter called "Project Height". What is the significance of it?
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Regards
Manybe a difference in antenna types and offsets?
30cm? That's a big error, do you have a way to check which one is correct? A local bench mark?
It sounds like a measure up/measure point error.?ÿ
The project height is something that needs to be entered to make the DC files work. Frankly, I don't pay much mind to it, it's always been my thought it is more for setting up a scale factor; I decide on a scale factor before I leave the shop.?ÿ
My guess is it's a field software job settings issue; a 30cm blunder in measure-up is pretty obvious. How's the horizontal check between the two systems?
Modifying the Project Height in Trimble Access won't affect field-observed heights/elevations - it is used to compute grid/ellipsoid distances for 2D COGO routines or 2D points. The job datum + geoid (if applicable) controls observed elevations.
Don't know about India, but in Puyallup, we don't use MSL; our datum is NAVD88
Maybe you're seeing 2 different datums?
Like John said, can you check a local bench mark?
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FWIW,
Using coordinates for an IGS site in Bengaluru, India I ran the UNAVCO GEOID HEIGHT tool https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html
I specified the EGM2008 and the EGM96 models. The resulting geoid heights were -85.59 and -85.24 meters respectively.?ÿ
@loyal?ÿ
Good article. I also recommend a 2018 FIG paper ??Towards ?ÿThe Modernization of Indian Vertical Datum? by Dr S K Singh.
The 1909 IVD was based on nine tidal gauge stations assuming they represented the same water level.?ÿ
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The 1909 IVD was based on nine tidal gauge stations assuming they represented the same water level.?ÿ
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Seems the same assumption was made about 26 tidal gauge stations a hundred plus years ago.
Does one controller have a geoid file and the other doesn??t?
Other possibility is one has correct measure up point (center of bumper, ARP, etc.) and the other doesn??t.
Well, that is suspiciously close to 30 centimeters. I would give a 85 percent probability that it's the answer to the problem.?ÿ
Yes they could be using different Geoid files
The 1909 IVD was based on nine tidal gauge stations assuming they represented the same water level.?ÿ
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Seems the same assumption was made about 26 tidal gauge stations a hundred plus years ago.
Of course, they knew better. Interesting decision.
As pointed out by others above, there are a number of possibilities, all of which need to be addressed and verified.
Vertical Datum(s)
GEOID Model
Antenna Calibration data
Measurement from GRP to ARP
Validity of CORS position
Baseline solution statistics
All of this is pretty easy for those of us in the US, thanks to the NGS.
Loyal
Mean-Sea-Level means different things to different people and has a different value for your particular place on earth.?ÿ
EGM96? That??s been superseded. Include observations on orthometric BM and solve for rotations in least squares.
IMO. That??s how I did it back the day with geolab.?ÿ
@loyal?ÿ
I agree. It is challenging to address the OPs question w/o more info.
While the possibility of a difference in EGMs is indicated by the UNAVCO tool, the article you linked highlighted the fact that these global models are not sufficiently accurate to address local/regional gravity issues.?ÿ
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The link to this article is:
https://fig.net/resources/proceedings/fig_proceedings/fig2018/papers/ts06e/TS06E_singh_9497.pdf
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@dougie NAVD is based on MSL.
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NAVD88 is based on a single reference monument with an assumed value that makes the elevations some approximation to height above MSL, but there is no direct connection to mean sea level at any location.
NGVD29 was originally called height above MSL, and was based on a long-term average at several locations, which actually don't have the same sea level when measured carefully.