CE 397 GIS in Water Resources


Projection Information

Datum Conversion

The difference between NAD27 and NAD83 is that NAD 27 does not include Alaskan islands and Hawaii.

To convert from NAD27 to NAD 83

WGS84 and NAD83 are essentially the same so it is not necessary to convert the datum.

Units

There are a few possible units for geographic projection:

Other projections may also have these units:

Parameters

It is optional to state the datum to be used for all projections except those which support data on a sphere only.

The term name is simply means name of the datum. The term method is the datum conversion method used to convert between the specified datum (e.g. NADCON).

The classification none is used when projecting data on a spheroid or sphere to a projection which supports data on a sphere only

You can also specify the spheroid to be used. However, do not use the SPHEROID subcommand if DATUM is stated in the parameters.

The default is Clarke 1866. AN other spheroid which can be used is GRS80. Look in Help "Map Projections " for a complete list of spheroids.

NOTE:ARC/INFO does not convert between spheroids unless transforming between datums.

Projections:

AZIMUTHAL

Here is a list of parameters for azimuthal projection.

The latitude of true scale is usually the Equator, but it doesn't have to be. The radius of the sphere of reference must be a real number. You should note that Azimuthal is supported only on a sphere.

CONICAL

Here is a list of parameters for a conical projection.

The longitude west of the Prime Meridian and the latitudes South of the Equator are negative. The false easting and northing are real numbers and can be zero. The latitude of origin defines the origin of the y coordinate for the projection.

CYLINDRICAL

Here is a list of parameters for a cylindrical projection

The longitude west of the Prime Meridian and the latitudes South of the Equator are negative. The azimuth must be entered as an angle in decimal degrees. The scale factor of the central line is a real number, usually one.

ZONE FOR UTM

Each UTM zone has its own central meridian and spans 3 degrees west and 3 degrees east. There are 60 zones. You can look in the HELP in ARC under the Map Projection for a table with all the zones and their spans.

Examples

The following projection file, called alb_lamb.prj, converts from Albers projection to a Lambert Projection

Another example is the following file, called geostate.prj, which converts from geographic coordinates to stateplane.

These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 1997 Center for Research in Water Resources.