Geographic information system (GIS) databases are often distributed in different map projections using a variety of geodetic datums. Map projections are attempts to portray the surface of the earth or a portion of the earth on a flat surface. A geodetic datum is a geometric model describing the size and shape of the earth and the origin of the coordinate system used to map the earth. There are a number of map projections and datums, each of which is useful when dealing with particular scales of data or different regions of the world. In South Carolina, the most common map projections you will find are Universal Transverse Mercator (UTM), State Plane, and Geographic. The data provided on this CD-ROM are in State Plane coordinates, with the North American Datum of 1983(NAD83). This projection was chosen because it is the preferred projection of South Carolina's coastal management program. Other South Carolina data sets that can be downloaded from the web, such as those at the South Carolina Department of Natural Resources, are distributed in UTM.
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With this wide variety of data sources, it is often necessary to project data sets into different coordinate systems so that all data in the GIS will overlay. To do this, there is a projector extension already created and available within every ArcView package. To use it, make the projector extension available in the Project View. Go to the file manager and navigate to the ext folder by using the path shown to the left. Copy the prjctr.avx file to the ext32 directory. |
From the Project window, add the Projector! extension from the File/Extensions menu. Go back to your View window and choose Properties from the View menu. Set map units to the current units the data in your view is stored in (for example, State Plane would be in feet and UTM would be in meters). The Projector! extension needs this information in order to run. All of the data contained on this CD-ROM is in the State Plane projection using feet as the units of measurement. Note in your View window, there has been an additional button, the Projector! 
button, added to your button bar. This button allows the user to create a new shapefile from the active theme with different projection parameters.
For this example, let's convert the Charleston County boundary to geographic coordinates. To do this, add the shapefile to the view, make it the active theme, and click on the Projector! button . This will give you a box to choose the input projection parameters. As previously mentioned, the data on the CD-ROM is in State Plane projection for South Carolina, with units in feet and a datum of NAD83.
After inputting these parameters you are asked to choose output units (for geographic projection, we will choose decimal degrees).
ArcView recognizes that there is only one projection that accepts decimal degrees as units; therefore, it defaults to Geographic. You are next prompted as to whether or not you would like to recalculate area, perimeter, and length. Answer yes to this question. Finally, you are prompted for an output file name and are asked whether or not you would like to add this data set into your current view or another view. You now have a new data set in a different projection system.