Past construction and survey practices have resulted in the use of multiple local coordinate systems for measuring and reporting the horizontal position of wells and other facilities and loc' '_ns on the Hanford Site. Regulatory reporting requirements mandated by the Tri-Party Agreement a_Ad the increasing use of spatial data analysis in support of environmental studies have significantly , increased the need to provide horizontal location data in a single coordinate system that is applicable across the entire Site. This report describes the development of a coordinate transformation process and algorithm and its application to the conversion of the horizontal coordinates of Hanford Site wells from the various local coordinate systems and datums to a single standard coordinate system, the Washington Coordinate System of 1983, South Zone 1991 (WCS83S). This work was undertaken as an interim measure to supplement ongoing resurveying efforts so that the best currently available well location information can be reported and made accessible in WCS83S coordinates as required by Washington State law. The coordinate transformation algorithm, implemented as a computer program called CTRANS, uses standard two-dimensional translation, rotation, and scaling transformation equations and can be applied _o any set of horizontal point locations. For each point to be transformed, the coefficients of the transformation equations are calculated locally, using the coordinates of the three nearest registration points (points with known locations in both coordinate systems). The report contains a discussion of efforts to verify and validate both the software and the well location data, a description of the methods used to estimate transformation and registration point accuracy, instructions for using the computer program, and a summary of the Hanford well conversion results for each local coordinate system and datum. Also included are the results of using recent U.S. Army Corps of Engineers survey data to obtain estimated measures of location errors in wells for which the local coordinate data source is undocumented, unverified, and therefore of unknown accuracy. The estimated average error in the local coordinate data of these wells was about 14 meters for wells in the 100-F and 600 areas, about 150 meters in the 100-KE/KW area, and 2 meters or less in all other Hanford areas. Because of the large errors in the local coordinates of the 100-KE/KW wells, a complete resurvey of these wells is recommended. An important side benefit of this well conversion effort has been the development of the CTRANS coordinate conversion program and the associated registration point files for the different local coordinate systems on the Hanford Site. CTRANS runs on UNIX w,grkstations and on personal computers with a Windows interface. CTRANS has been and continues to be used extensively to convert local coordinates of waste sites and other points of special interest.