How to convert (reproject) data from UTM NAD27 to UTM NAD83 ArcGIS 9 FAQ series August 2004 Project data from one DATUM to another. • DATUM is the equation used for the earth’s shape and may be used with geographic or projected coordinate systems. • Change in projection often requires change in DATUM. Project data into other coordinate systems (and datums) • Use the “project” or “project raster” tools under “Data Management Tools” - Look in the “Projection and Transformation” group. • Use ArcToolbox in either ArcMap or ArcCatalog. Use the “project” or “project rastor” tools under “Data Management Tools” Look in the “Projection and Transformation” group. The feature project tool allows you to specify a geographic transformation, which you want to do if changing datums as well as projections. Use drop-down menu to select from choices. Process may take a while if the feature data set has many polygons or nodes. Project Raster Data “Project raster” tool does not support geographic transformation option. (Because the process for rasters is more like a rubber- sheet process.) It has an additional parameter to describe the type of sampling to use. Basically category data should be “nearest neighbor” and continuous or floating point data should be “bilinear.” See the “help” to learn more. Raster data Projecting Rasters may take CONSIDERABLE time (hours). If you see in Windows Task Manager that ArcGIS is “Not Responding” – that means it is calculating in the CPU; Don’t Kill it - give it more time.