Starting with the 2009 orthophotography acquisition the Virginia Base Map Program (VBMP) is delivering data in the North American 1983 HARN Datum. Previously, the North American 1983 Datum (NAD83) was used without the HARN upgrade. A High Accuracy Reference Network (HARN) is a statewide upgrade in accuracy of NAD83 coordinates using Global Positioning System (GPS) observations. Generally the HARN upgrade differs from the NAD 1983 Datum by approximately one foot but varies across the state between zero and 3.2 feet.
The mix of datums you are most likely to encounter are NAD83 and NAD83 HARN. The proper geographic transformation between these two datums is the NADCON “VAHPGN” grid transformation method developed by the National Geodetic Survey. In ArcGIS this transformation is referred to as NAD_1983_To_HARN_Virginia.
The datum shift can be corrected in ArcGIS by using the proper geographic transformation in a map document's data frame properties dialog box. The transformation may also be required when using some geoprocessing tools. See this document for more details.