How do I convert a Datum that has been defined as "Modified (ground) coordinate system based on..."?

Question

I am working in Autocad. I have some 100 Lat/Long Coordinates. The destination drawing defines it's datum as such:

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BASIS OF SURVEY:

HORIZONTAL DATUM: MODIFIED (GROUND) COORDINATE SYSTEM BASED ON THE NEW MEXICO COORDINATE SYSTEM OF 1983, CENTRAL ZONE, NAD 1983(92) US SURVEY FEET AND WAS ESTABLISHED BY 0.P.U.S. GPS OBSERVATIONS, IN ACCORDANCE WITH THE NATIONAL GEODETIC SURVEY'S SPECIFICATIONS FOR O.P.U.S. OBSERVATIONS.

VERTICAL DATUM: NAVD 1988

COMBINED FACTOR = 0.999611828

COMBINED RECIPROCAL FACTOR = 1.00038832274

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It contains a comparison table. A comparison taken at random is such:

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Northing - 747878.008

Easting - 1719902.164

Elevation - 4556.84

LaLatitude 33°03'17.51520"

Longitude - 105°59'33.89390"

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I am currently using Corpscon6 to do conversions between Latitude/Longitude and New Mexico Central State Plane Coordinate system, which is what this appears to be based off of. The problem is, a raw conversion between the two creates an error of about 500 feet, which is too much of a margin of error for the use this is being put to.

My question is, does anyone know any mamathematicalormulas or anything I can do that would convert what Corpscon6 puts out to something that would be usable in the autocad datum?

Answer 1

The problem you are dealing with is ground v. grid coordinates.

The UTM or State Plane "grid" is based at the ellipsoid and not at the ground surface. At the ellipsoid, 1 m on the ground equals 1 m at the grid. The ellipsoid is usually below the geoid or ground surface, therefore, 1 m at the ellipsoid is slightly more than 1 m at the geoid.

You'll have to research these principles to understand exactly the mathematical principles.

To address your immediate issue, you are dealing with ground coordinates, but you want grid values. The ground coordinates equal the grid coordinates times a constant (ground adjustment factor).

To go from your modified state plane coordinates (ground) to grid coordinates, multiply the ground values by the combined scale factor (.9996...). To go from grid coordinates to ground values, multiply the grid coordinates times the reciprocal (1/.9996) of the combined scale factor. This is the GAF (ground adjustment factor).

If your data is Lat/Lon., use corpson to convert to SP. Then multiply coordinates by the combined scale factor. You'll then be using State Plane grid values, which will work for you in CAD.

Good Luck.

Mike

Mapping Director

Desert Archaeology, Inc.

Answer 2

Hey!  Good luck with that one.