Import a Projected ESRI .shp File

This topic comes from the case files of Manifold Technical Support. A customer is using Manifold System Release 4.50.

The customer has an ESRI "shapefile" in .shp format. Even though .shp format is highly unsuited for interchange of projected information, this file contains data that has been projected. Lucky for our customer, he has a detailed specification of the projection parameters used with this file. Our customer writes:

Our customer has three files, the usual set used in "shapefile" format. These consist of a .dbf, a .shp and a .shx file as seen in Windows Explorer:

The following sequence illustrates the procedure used. Click on the thumbnail at left to see the full screen shot.

	We begin by launching Manifold and choosing File - New. We choose Import ESRI Shapefile and provide a convenient name. In this example we also use "regdist", like the originating shapefiles.
	In the import wizard we browse over to the shapefile using the Browse button and click Refresh to see the fields in the shapefile. We check Use abstract coordinates to import a projected shapefile. Click Finish to import the file.
	The result is an import of an abstract file. Note the status bar readout shows X and Y in meters.
	Choose File - Properties. In the Map Properties tab, double-click into the cells for Geographic and Geographic Coordinates and change the values to yes. Press OK.
	Manifold has now been told the map is geographic, but it still does not have the projection right. Note the status bar now reads out in degrees.
	Now, a trick: Choose Tools - Projections to launch the projections dialog. Convert from Albers Equal-Area Conic to Albers Equal-Area Conic using the parameters given by our customer in both sides of the dialog. Yes, the "central meridian" is the same as the longitude origin. Press Convert. This is something of a trick because we are using this dialog to avoid entering the projection properties manually. It's easier just to use the projection dialog in this way.
	The result is a Manifold map in Albers Equal-Area Conic projection. Save the workspace to update the .mfd/.mdb Manifold files correctly.

Once we have a valid Manifold map in the correct projection, we can do with it what we desire.

ESRI .shp format is a very old format that does not normally store information on projection and projection parameters that are used. If you save projected files into .shp format you must make note of what projection was used so that the files can later be used. If you publish such files, you must provide this information to users via some other means. When importing a .shp file, if you don't have a "readme.txt" file or some other documentation that provides the required projection information, projected .shp files cannot be used. That's why shapefiles are unsuited for interchange of projected data sets... it's too easy to lose the auxiliary information that must be kept track of manually. We strongly suggest that shapefiles be used only for interchange of unprojected data in ordinary latitude and longitude degrees.

What if I don't know the projection parameters used in this .shp file? You're out of luck. Since .shp files don't save the projection used, if the creator of the file did not tell you this information the file is useless. Next time, get the projection info, get the data as an unprojected shapefile, or use a more modern format that keeps track of projection information so you don't have to chase around doing so manually.

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