Because ESRI .shp format files ("shapefiles") do not store projection information they are most often used to save information in unprojected form where the contents are coded in coordinates using values that represent degrees latitude and longitude. On occasion we may encounter shapefiles that encode projected information. In such cases the contents of the shapefiles will be coded in coordinates using values that represent meters or feet in some projected coordinate system.
To use such files we must know the projection and projection parameters used to encode the shapefile. Because the shapefiles themselves do not contain this information we must receive it through other means such as a "readme.txt" file that accompanies the shapefiles or other documentation.
This example shows how to import a projected shapefile where we know what projection was used to encode the shapefiles. This example assumes a text file accompanies the shapefile to tell us what projection it is in.
Step 1: Determine correct projection
Our first task is to find out what projection the shapefile is in.
In Windows Explorer we can see that the "shapefile" is actually three files: a mexico.shp file, a mexico.dbf file and a mexico.shx file. In addition, we see that there is a readme.txt file that accompanies the shapefiles. If we are lucky, this text file contains information about the shapefiles such as the projection this shapefile is to use.
We can either pop open the text file in Notepad, or, using File - Import - Comments import it into Manifold as a comments component as seen above.
If we open the comments component we see the readme.txt file tells us that the projected shapefile was projected using Orthographic projection centered at 24 degrees latitude and -102 degrees longitude, using the WGS84 datum.
These are "magic numbers" in that if the author of the drawing did not provide them to us we would have no way of knowing from the shapefile itself which projection and parameters to use.
Experienced user may object that this example begins right away with a page out of fantasy-land, in that it is rare that someone who is so thoughtless as to publish projected information in shapefiles will have the courtesy and foresight to provide a "readme" file that tells us what projection to use. But that happens surprisingly often, or, if the information is not in an accompanying text file it will be available in some "metadata" file or in some web page associated with the site from which we downloaded the shapefile.
People who use legacy formats normally get street-smart very quickly about the need to do a little detective work to find out what projections are intended if projected data is in play. For the sake of this topic we assume that such detective work has found the necessary information in the form of an accompanying text file.
Step 2: Import shapefile using defaults
We will import a projected shapefile that shows Mexico. This example is not accompanied by a sample file on the Manifold CD.
In Manifold, choose File - Import - Drawing. In the Import dialog browse over to the directory in which the shapefile is located. Choose SHP Files (*.shp) in the Files of type box and open the mexico.shp file.
Manifold will automatically open the .shp file as well as the other two files normally used in SHP format. In the Import SHP File dialog we see this drawing has but one field, a column called Place_name. We press OK to import the drawing.
The result is a new drawing in the project pane. We can open the drawing by double-clicking on it in the project pane.
When opened the drawing appears OK. If one does not have a practiced cartographic eye one might not notice that the shape of Mexico is characteristic of projected maps as opposed to the "wide" appearance of countries in these latitudes as seen in unprojected Latitude / Longitude views. If we move the cursor over the drawing the coordinates seen in the status bar will show there is something not yet right about the coordinates in this drawing.
We can see that even though the drawing has been imported using Latitude / Longitude degree coordinates the coordinates shown in the status bar as the mouse moves over the drawing are values like 99517 and 56020, which are impossible values for degrees. Since shapefiles do not provide projection information but are mostly used for unprojected drawings, Manifold uses a default of Latitude / Longitude to import shapefiles. For correct use within Manifold, the correct projection must be specified for this drawing.
Step 3: Specify correct projection
To specify the correct projection we launch the Edit - Assign Projection dialog and change parameters as necessary.
This is a familiar dialog used within Manifold for several similar purposes. In this case the purpose is to specify what projection parameters should be used to interpret the coordinate values in the drawing. The dialog shows the current interpretation applied, that the drawing is in Latitude / Longitude projection using degrees as the unit of measure for coordinates.
We expand the Azimuthal heading and choose the Orthographic projection.
We specify 24 as the Center Latitude and -102 as the Center Longitude. The datum is already WGS84 so there is no need to change the datum. Press OK.
There is no change in the appearance of the drawing. However, if we move the mouse over the drawing we can see there is a change in the status bar readouts.
The status bar now reports the drawing is in Orthographic projection. The position of the cursor is reported in values that make sense for longitudes and latitudes in Mexico.
Regional Settings
When importing data from text file formats like .csv or from .dbf or shapefiles the Regional Options in the system have to match settings within the file that is being imported. This is because such simple formats are too stupid to understand that in different countries people use different symbols to denote decimal points (that is, either a dot character or a comma). To change Regional Options, do the following:
§ Go to the Control Panel and open the Regional Options applet.
§ Go to the Numbers page,
§ Ensure that the Decimal Symbol is set to '.' (dot),
§ Ensure that the List Separator Symbol is set to ',' (comma),
§ Press Apply to apply changes if there were any,
§ Import the .csv file or shapefile desired.
§ After import, restore the original settings of Decimal Symbol and List Separator Symbol if you like.
Another option is to invoke the Data Sources applet (hidden within Administrative Tools folder of the Control Panel if you're running Win2K) and create a file DSN for the .csv file you want to import. After creating the DSN configure it to use the comma character as a data separator.
Notes
A recent addition to shapefiles is the use of a .prj file that saves projection information. However, this is a recent addition that is not found with most projected shapefiles that are available. A further problem is that .prj files do not provide unambiguous projection information and so place users at risk of importing shapefiles that use .prj and still getting inaccurate projection information. If we are using projected shapefiles that are accompanied with .prj files, we should still make every effort to find out what projections are intended and then confirm the result of the import as in the example above.
A sample file for this example is not provided on the Manifold CD because of the risk of confusing users with a projected shapefile should someone try to open the file without reading this example first.