Introduction

The task of understanding map projections, datums, and coordinate systems can be daunting. There are several sources of information available, some of them being:

• Cartographical Map Projections
• ESRI's Virtual Campus (there is one free section)
• The Geographer's Craft: Coordinate Systems Overview
• The Geographer's Craft: Geodetic Datum Overview
• The Geographer's Craft: Map Projections
• GIS Lounge: Projections and Coordinate Systems
• Reprojecting geographic features
• USGS: Map Projections

There are also several software packages that attempt to make the reprojection process as simple as possible if you know the start and end projections. ESRI's software packages offer a variety of help topics for performing the task in ArcInfo, ArcView, or ArcGIS. This is a list of software (mostly public domain) that will reproject digital geospatial data.

The following are some basic definitions:

Projection:

A map projection is the method for converting the three-dimensional surface of the Earth to a two-dimensional representation. There are several projections available, and the one you choose depends on the extent and location of your data. All projections somewhat distort distance, direction, scale, area, shape, or a combination of these.

Datum:

Datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. There have been hundreds of datums developed since the time of Aristotle, and an incorrect choice can result in positional error of up to several hundred meters.

Coordinate System:

The coordinate system is based on the projection, datum, and units that you choose. It is used as a reference for navigation and for the location of points in space.

Using ArcInfo

There are several different ways to project, reproject, or define the projections of data in ArcInfo. Some of the commands are:

• PROJECT (Grid, Arc)
• PROJECTCOPY (Arc)
• PROJECTDEFINE (Arc)
• PROJECTGRID (Grid)
• MAPPROJECTION (ArcPlot)

You should use the HELP command to read about the different uses of these commands as each one has several subcommands. Projection information about a dataset is stored in the PRJ file (prj.adf).

The help documents also provide information about the supported projections, datums, and coordinate systems (search the index for "projection").

Some of the above commands are described in more detail in the Procedures section.

Using ArcView

ArcView 3.x offers the Projection Utility extension. This utility will reproject shape files, but it will not alter the original shape file - it creates a new shapefile. The projection utility will create a projection file (.prj) or read the information from a projection file you have created. For the correct format of the .prj file, see the procedures section or the FAQ on "Projection metadata" in the ArcView help documents (Fig. 1). You can also search the help documents for "Projection Utility" (Fig. 2).

Fig. 1. The Projection metadata FAQs.

Fig. 2. The help documents for the Projection Utility.

Using ArcGIS

ArcGIS includes the GeoProcessing Wizard, which will reproject data on the fly during analyses between two layers with different projections or coordinate systems (Fig. 3). ArcMap also allows you to change the projection and coordinate system of the data frame through the data frame properties (Fig. 4).

Fig. 3. The help screen for the GeoProcessing Wizard.

Fig. 4. The "Data Properties" screen, "Coordinate System" tab.

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