All geographic data is created in some coordinate system, and in the context of some datum. A datum includes ellipsoid information and a datum definition. For example, the Gauss-Kruger Conformal projection system used in Germany uses the Bessel ellipsoid and Potsdam datum definition.
You can create a completely new global coordinate system or use a predefined coordinate system as the basis for creating your own. You can also modify or delete any coordinate systems that you define. You cannot delete any of the predefined coordinate systems supplied with AutoCAD Map 3D toolset. If you modify a predefined coordinate system, you save it as a new coordinate system and the original one remains.
Some coordinate systems only specify the ellipsoid; these are non-geodetic. Coordinate systems that specify the complete datum are geodetic. When you create a coordinate system, it can be geodetic or non-geodetic.
Determine whether a coordinate system is geodetic or non-geodetic before you assign it to a drawing. See About Geodetic Transformations. If you have data in a drawing that uses a non-geodetic coordinate system, but do not know what datum the data came from, you might introduce errors if you convert the drawing to a geodetic coordinate system.
There are two possible sets of coordinate systems you can use with regards to HPGN systems.
There is the set of systems that use the HPGN datum used in the geodetic transformation NAD83_to_HPGN. This datum covers all the HPGN grid files for the United States. An example of a coordinate system using such datum is HPGN (HARN) Virginia State Planes, South Zone, US Foot called “VAHP-SF”.
There is also a set of local systems that use individual HPGN datums for each of the states of the USA. For instance, the datum HARN/VA used for the system “HARN/VA.VA-SF” which is the equivalent of the VAHP-SF system.
This transformation uses all the USA HARN las/los grid files typically found in the folder c:\ProgramData\Autodesk\Geospatial Coordinate Systems\Usa\Harn. These grid files overlap in areas bordering the states where two grid files intersect to cover the same bordering regions. For source points located away from the region of overlap, there is only one grid file covering that region and the coordinate system engine successfully finds it in the list of the geodetic transformation NAD83_to_HPGN.
For source points on the border of the state where there is an overlap between two grid files, the engine uses the grid file that is recorded first in the list. You can use the MAPCSLIBRARY command to edit the transformation NAD83_to_HPGN and arrange the grid files so the ones you want to display are moved toward the top of the list.
For example, say there is a point (-82.500000000000, 36.875000000000) located in Virginia, but it's very close to the Kentucky border. The coordinate system for that point is VAHP-SF. However, if the grid file for that region of Kentucky is higher in the transformation list than the grid file for Virginia, the Kentucky grid file gets displayed. You'd have to edit the NAD83_to_HPGN transformation list if you wanted to switch the grid file that gets displayed.
The Individual State HPGN datum was created to reduce the time you'd spend editing the NAD83_to_HPGN transformation list to re-order grid files. This datum and its related transformation is split into approximately fifty datums and transformations to cover every state in the United States. In the process, new coordinate systems were created, like HARN/VA.VA-SF, which is equivalent to VAHP-SF, for the state of Virginia. Its datum is HARN/VA and the transformation specific to the area of Virginia is “NAD83_to_HARN/VA” which has a single grid file, vahpgn.l?s.
Therefore, when using the datum HARN/VA.VA-SF, the point near the Kentucky border, (-82.500000000000, 36.875000000000), will always display the correct, Virginia grid file.