ArcView and Datums: The Real Story

It is extremely important for ArcView users to understand that ArcView's projection engine does not inherently support datum transformation. The following is from the online help under "datums","working with datums":

Thus, for example, if your data in Decimal Degrees (DD) were collected in NAD27 and you're attempting to project your view to a projection using NAD83, you will not get correct coordinates. Similarly, if you were to attempt to use ArcView's Projector! extension to convert between two projections based on different datums, you will not get correct results.

ArcView 3.2 comes with a standalone projection utility (Windows only) that supports most common datum conversions, including NADCON (but not Canada's CNT). It has a wizard-based interface but can also be invoked from the command prompt. The following example converts a shapefile from State Plane NAD27 feet to UTM NAD83 meters:

projutil -Q -ID . -OD . -IF az_sp.shp -OF az_utm.shp -IC NAD_1927_Arizona_Central -OC NAD_1983_UTM_Zone_12N -IG 108001
Other ESRI software currently capable of projection and datum transformation include Arc/Info, PC Arc/Info, Data Automation Kit (DAK), ArcCAD 14, and Atlas GIS. For users lacking access to these software packages, two ArcView extensions exist for converting DD data between datums:

Example: I have a shapefile in AZ State Plane Feet (Central Zone, NAD27) and want to convert it to UTM Meters (Zone 12, NAD83).

  1. Use Projector! to convert the shapefile to DD (NAD27). Specify "State Plane - 1927", "Arizona, Central" as the input projection, and "decimal degrees" as the output units.
  2. Use the NADCON extension to convert the resulting shapefile to NAD83. Specify CONUS.DBF as the grid file.
  3. Use Projector! to convert the resulting shapefile to UTM (NAD83). Specify "meters" as the output units and "UTM", "Zone 12" as the output projection.

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