NADs Revisited

PLP Staff



In V6N3 it was shown that the default spheroid of PC (as well as Host) ARC/INFO's PROJECT command is CLARKE1866. This can be problematic when exchanging data with other applications (such as ArcView and Idrisi) which do not default to that spheroid. For example, the following parameters would typically used to project lat-long data to UTM Zone 12:

INPUT
PROJECTION GEOGRAPHIC
UNITS DD
PARAMETERS
OUTPUT
PROJECTION UTM
UNITS METERS
ZONE 12
PARAMETERS
END
But the resulting projection is not the same as that used by ArcView. It is actually equivalent to NAD27, whereas ArcView's UTM settings use NAD83. Not even the assumptions regarding lat/long are the same. The following parameters are the equivalent of ArcView's projection from DD to UTM Zone 12:

INPUT
PROJECTION GEOGRAPHIC
UNITS DD
DATUM WGS84 THREE
PARAMETERS
OUTPUT
PROJECTION UTM
DATUM NAR_C THREE
UNITS METERS
ZONE 12
PARAMETERS
END
Because the GRS80 spheroid is practically identical to WGS84 in the continental U.S., the following parameters will lead to the same values:

INPUT
PROJECTION GEOGRAPHIC
UNITS DD
DATUM NAD83 NADCON
PARAMETERS
OUTPUT
PROJECTION UTM
DATUM NAD83 NADCON
UNITS METERS
ZONE 12
PARAMETERS
END
Thus, should you wish to convert NAD27 UTM Zone 12 data to DD for use in ArcView, you may use the following parameters:

INPUT
PROJECTION UTM
DATUM NAD27 NADCON
UNITS METERS
ZONE 12
PARAMETERS
OUTPUT
PROJECTION GEOGRAPHIC
UNITS DD
DATUM NAD83 NADCON
PARAMETERS
END
Note also that ArcView cannot "unproject" according to Clarke1866; therefore, you will need to use the above parameters (in reverse) to bring DD data created for ArcView into your PC A/I project.

A good rule of thumb regarding the appropriate parameters for abstract data (e.g. lat/long lines) is to ask yourself how you want the features to "appear" relative to a particular projection. For example, USGS 7.5' quads (except the newest ones) use NAD27 lat/long frames; if you were to use the above parameters to project a lat/long frame, it would match the NAD83 ticks.

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