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":
"When a projection associated with a certain spheroid is used,
ArcView assumes that the decimal degrees data being projected was
collected in a datum based on that spheroid."
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
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
Example: I have a shapefile in AZ State Plane Feet (Central Zone,
NAD27) and want to convert it to UTM Meters (Zone 12, NAD83).
- The Datum Converter extension (located in ArcView's "samples\ext"
directory) is a port of the Molodensky 3-parameter transformation. This
extension is appropriate for conversion between most standard datums
except between NAD27 and NAD83.
- The NADCON Extension
is a port of NOAA's NADCON program for converting coordinates within the
United States between NAD27 and NAD83.
- 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.
- Use the NADCON extension to convert the resulting shapefile to
NAD83. Specify CONUS.DBF as the grid file.
- Use Projector! to convert the resulting shapefile to UTM (NAD83).
Specify "meters" as the output units and "UTM", "Zone 12" as the output
Back to AVTips