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Define Projection for a Shapefile or Geodatabase |
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Keywords: Projection, datum, coordinate system, define, spheroid, shapefile, geodatabase, ArcToolbox Category: Projections Software: ArcGIS 9.x Problem: One of the layers in my map document is not lining up with the others OR When I add a dataset to ArcMap, I get a warning message that the data does not have a coordinate system defined. Description: Features on a three-dimensional spherical surface like the Earth are depicted on two-dimensional surfaces like a paper map or computer screen using mathematical conversions known as map projections. All map projections distort real-world features in some way. A mnemonic that's often used to remember the four properties that could be distorted by a map projection is, "Projections make geographers SADD." (Shape, Area, Distance, Direction.) A valuable feature of ArcMap is something called “on-the-fly re-projection”, which means that the user can add datasets that are in different projections and ArcMap will make them line up in the same coordinate space. A critical element of this on-the-fly re-projection process is that each of the layers has its projection defined (i.e., that you, the user, tell the software how each layer is projected). Some of the datasets you come across will already have their projection defined. However, if when adding a dataset to ArcMap you receive a warning message or the layer does not line up with other layers, the dataset’s projection is either undefined or defined incorrectly. The solution is to properly define the projection using ArcToolbox (i.e., to tell the software how your data are projected). Note: The terms projection, coordinate system, and spatial reference are often used to refer to the same thing in the various ArcGIS dialog boxes. While there are slight differences between these terms, you can treat them as synonymous. Methodology: The biggest difference in defining a dataset's projection between ArcGIS 8.x and 9.x is that ArcToolbox is a stand-alone application in 8.x and is integrated into the ArcMap and ArcCatalog applications in 9.x. 1) With either ArcMap or ArcCatalog open, click on the red toolbox icon on the Standard toolbar to open ArcToolbox. Within the ArcMap application window, a pane that contains the ArcToolbox tools will be added between the Table of Contents and map display area as shown below. The ArcToolbox window will contain different toolboxes depending on which ArcGIS extensions are loaded on the computer.
Three of the more commonly used toolboxes are: Analysis Tools, Conversion Tools, and Data Management Tools. 2) Navigate to the Define Projection tool under Data Management Tools > Projections and Transformations and double-click on it.
3) In this example, we will use a shapefile of Pennsylvania census tracts downloaded from the Census web site. Click on the Browse button to specify the Input Dataset or Feature Class.
ArcMap will first check to see if the dataset already has its projection defined. If it doesn't, the program will then scan the dataset's coordinates to see if they all fall in the range of -180 to +180. If the coordinates are in that range, ArcMap will assume that the data are in latitude/longitude (also referred to as Geographic) coordinates. That is the case in this example. While the assumed projection may be correct, it is good practice to define the projection explicitly. 4) Click the Properties button to define the dataset's projection.
The Spatial Reference Properties dialog box opens showing the projection properties of the dataset as it's currently defined. The dialog box provides three ways to define a dataset's projection: 1. Select: Select from a list of commonly used coordinate systems. 2. Import: Import the coordinate system definition from another dataset. Use when you know that your dataset is in the same projection as another dataset. 3. New: Create a new coordinate system. On rare occasions, your data may have been projected in a proprietary or uncommon coordinate system. This option allows you complete reign over the components of the coordinate system definition. In this example, the Select option will be used. The coordinate system will be chosen to match the metadata information provided on the Census web site. 5) Click Select, then browse to the coordinate system through either the Geographic Coordinate Systems or Projected Coordinate Systems folder. In this example, the desired coordinate system is reached by navigating to Geographic Coordinate Systems > North America > North American Datum 1983.prj.
Many of the commonly used coordinate systems such as the State Plane and UTM systems can be accessed through the Projected Coordinate Systems folder. Upon selecting the desired coordinate system, its detailed properties should appear in the Spatial Reference Properties dialog. 6) Click OK to dismiss the Spatial Reference Properties dialog. The name of the selected coordinate system should now appear in the Define Projection dialog.
7) Click OK again to accept the selected coordinate system. A dialog box will report whether the projection definition was assigned to the dataset properly. Click Close to dismiss this dialog. Note: Remember that this recipe is used simply to tell the software how a dataset is projected in cases when that metadata is missing or incorrect. It is critical that ArcMap have correct projection information for each data layer in order to perform on-the-fly re-projection. In some situations it is necessary to permanently re-project a dataset from one coordinate system to another. See the Reprojecting data recipe for details on this process. Links: |
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| Authored by: Jim Detwiler Modified: 1/14/05 |
