In most cases, geographic reprojection affects only the latitude and longitude of the referenced data, not the elevation. This is because there is no algorithm available to convert the elevation for most combinations of source and destination datum. An exception is when one of the datums is NAD27 and the other is NAD83. In that case, Promis.e assumes that the vertical datum that corresponds to data in the NAD27 datum is the National Geodetic Vertical Datum of 1929 (NGVD29), and the vertical datum that corresponds to data in the NAD83 datum is the North American Vertical Datum of 1988 (NAVD88).
However, you may have data that is in the NAD83 datum but it uses the NGVD29 vertical datum, or you may have data that is in the NAD27 datum but it uses the NAVD88 vertical datum. Therefore, you can choose a different vertical datum when a geographic coordinate system (GCS) that uses a NAD27 or NAD83 datum (or HPGN or their equivalent) is selected from the GCS library. When this type of GCS is selected, the Geographic Coordinate System Properties dialog displays Coordinate System Modifiers section with the Vertical Datum property as illustrated below:
If you have set Reproject Elevation to "Yes" in the Reprojection Settings dialog, the product uses the VERTCON algorithm and the data identified by the vertcon.gdc geodetic data catalog to convert the elevation data.
For example, a GCS using the NAD83 datum is selected but the vertical datum is set to NGVD29. If the model that uses this GCS is referenced to a model using the NAVD88 vertical datum, and Reproject Elevation is set to Yes, the product will adjust the elevation of every point in the design file by the VERTCON-calculated elevation difference between NGVD29 and NAVD88 at that point's latitude and longitude.
Elevation adjustments can be performed only when the data is 3D. However, the product allows you to select the vertical datum in 2D files (as long as the datum is NAD27, NAD83, HPGN, or their equivalent) for documentation purposes.