Datums and inaccuracy?

I’ve been testing out datums and accuracy, what I’ve found is Transdat is off by a few meters in height - at least it seems that way ?

I’ve been using a local reference station coordinates location for the tests.

Stn ID, ETRS89_X, ETRS89_Y, ETRS89_Z, ETRS89_Lat (N DD MM SS.SSSSSS), ETRS89_Long (E/W DDD MM SS.SSSSSS), ETRS89_H, OSGB36_E, OSGB36_N, Ortho_h, Ortho_Datum, TranModel, Km square, Hi, Current antenna/radome type code, Current receiver type code

LIAR ,4093715.6080,-371674.7420,4860662.2963,N 49 58 09.077804,W 005 11 15.895822,100.4240,171510.908,12540.043,47.426,Ordnance Datum Newlyn,15,SW7112,0.0000,LEIAR20 LEIM,TRIMBLE ALLOY

It seems odd the easting and northing are more or less spot on but the height is way off ?

Looking again, I think it’s down to Ortho_h vs Ellips Height even then I’m not sure it’s correct ?

Hi David,

Thanks for your patience.

Yes, by default Reach receivers utilize the ellipsoidal heights only, that is why there is such a difference in the Z coordinates.

Thanks @artem.fomenko But this is regarding pure coordinate conversion and no Emlid devices used.

I still believe Transdat maybe ‘incorrect’ but I not sure why. I would expect to get 47.426 or maybe 100.424 for height I can’t explain where 50.3733 comes from ?

Anyone ?

Although I’m not familiar with your software, Cartesian Coordinates (XYZ) give true Geodetic coordinates on the Earth’s ellipsoid. For the USA and using the NGS NSRS, the ellipsoid would be the GRS80 Ellipsoid. Your software is computing the ellipsoid height, which one is unknown. This is not orthometric height (approximate ground height). To determine orthometric heights, you would need a geoid model for your area. The software doesn’t specify which ellipsoid per the pic. The National Geodetic Survey here in the USA has many conversion tools freely available.


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