Feature Coordinated Connections One of the greatest challenges in the THO effort was georeferencing all maps to a common coordinate system. Unfortunately, most georeferencing software is designed to transform coordinates for U.S. maps that are created with the North American Datum of 1927 (NAD27) or more recent horizontal datums. Many maps selected for use in THO use a datum predating NAD27. Of the 3,318 source maps, 745 used an antiquated datum such as the North American Datum (1913), the U.S. Standard Datum (1901), or other datums available prior to development of regional geodetic datums. Most antiquated datums used by the historic maps selected for the project are actually a datums that use the Clarke 1866 ellipsoid. The exceptions to this generalization are the Walbeck 1820 ellipsoid and the Bessel The Texas Historic Overlay is integrated into the existing statewide GIS of the Texas Department of Transportation (TxDOT) for ready access by staff and approved consultants. 1841 ellipsoid. In 1870, the U.S. Coast Survey adopted the Clarke 1866 ellipsoid for use for geodetic purposes including mapping. This ellipsoid was used as a local datum until the adoption of the first geodetic datum of the United States, the U.S. Standard Datum of 1901. In 1913, the U.S. Standard Datum was renamed the North American Datum (NAD) with its adoption by Canada and Mexico. To transform a coordinate system to the selected common coordinate system, mapping experts developed three-parameter Molodensky transformations for each historic datum encountered. The process works by calculating the shift along the x-, y-, and z-axes from one system to another. To calculate this shift, the team undertook a substantial research effort to find coordinates for features represented in historic and modern form. Resource Ready Completed in early 2007, the THO is integrated into TxDOT's existing statewide GIS, which is built on ArcGIS. All selected historic maps were georeferenced using vector registration overlays representing historic coordinate systems or geographic features with resulting images in GeoTIFF and MrSID formats. Once the maps were registered and resampled, PBS&J used the Positional Accuracy Assessment Generator (PAAG), an ArcObjects application created by PBS&J, to calculate the accuracy of the spatially referenced historic maps. Source map extents were digitized as polygons in geodatabase Continued on page 22