Good thing I’m not a surveyor
Just messing around looking at city coordinate systems, I found this from Aurora, CO: Microsoft Word – City of Aurora Requirements for Survey Control Drawings March 2019.docx (auroragov.org).
The thing that jumped out at me was this:
7. If State Plane Coordinates NAD 83/92 are used and converted to Project Coordinates (Ground)
include a note stating the Grid Factor (sea level factor combined with scale factor). Show the Grid
Factor to nine decimal places. If Project Coordinates (Ground) were truncated, note what number
was subtracted from the north and east coordinates. Project coordinates are to be shown to four
decimal places. (Emphasis added)Now the sea level factor was used with NAD27 because the geoid and ellipsoid were considered coincident at Meades Ranch. That is not true for NAD83.
In essence, sea level factor uses orthometric height and elevation factor uses ellipsoid height. The difference between the two under NAD83 is usually around 30 meters, but it can be more than 50 meters. Taking the statement at face value, a surveyor would use the NAVD 88 height to approximate height above sea level instead of the ellipsoid height.
The term “combined with” is also vague as there are many ways to “combine” two numbers. Is is really too hard to say “multiplied by”? It’s almost as bad as saying that the published coordinates are based on the Colorado State Plane Coordinate System with nothing further.
For me, as I struggle to learn the mathematics that supported surveying, the inconsistency in terminology from publication to publication made the job harder. Perhaps that’s not true for surveyors learning on the job.
This piece makes it clearer than I ever could: Elevation Factor | GEOG 862: GPS and GNSS for Geospatial Professionals (psu.edu)
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