GIS Cookbook: Recipe - Define Projection for a Shapefile or Geodatabase
|Keywords: Projection, datum, coordinate, system, define, spheroid, shapefile, geodatabase, transform, ArcToolbox, wizard, unknown, |
Software: ArcGIS 8.x
Problem: When I open my geographic data file, I get a warning message that the file 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.
1) Start ArcToolbox from its location in your Start Menu hierarchy.
|The ArcToolbox window|
2) There are four primary options in the toolbox: Analysis Tools, Conversion Tools, Data Management Tools, and My Tools. Navigate to the Projections option underneath the Data Management Tools.
|The various Data Management Tools, including projection tools|
3) There are two Define Projection Wizards, one for coverages/grids/TINs and one for shapefiles/geodatabases.
|The Projection tools|
See Pitfall 1
Double-click the Define Projection Wizard (shapefiles, geodatabases) option.
In this example, we will use a traffic analysis zones file for the Sacramento Council of Governments planning area.
|The Define Projection Window|
4) You are asked to select the data set for which you want to perform this operation. Using the Browse button (or type the path and file) to the right of the select window, navigate to your data set. In this case, the traffic analysis zones were selected.
|The Define Projection Window with the selcted file displayed|
Once selected, your data set should appear in the bottom half of the pop-up window. The right-hand side column displays the Coordinate System that has been defined for your data. If there is a coordinate system already defined, you're done. However, if you have not used this data set yet, it is likely that it says "Unknown". It is also possible that the GIS has assumed a coordinate system. If either you're coordinate system is unknown or assumed, you should proceed through the process of explicitly defining your coordinate system.
5) Select your data set in the window and click Next->.
|There is no coordinate system assigned yet|
6) The next window shows the details of the coordinate system for your data. If no coordinate system has been defined, it will be gray and blank. Click the button Select Coordinate System.
|Select the method of coordinate system assignment|
You are presented with several options on the Spatial Reference Properties window.
1. Select Select a pre-defined coordinate system. This is the most likely course of action. Your data is probably in a common coordinate system that ESRI has included for you as pre-defined.
2. Import Import a coordinate system and X/Y/Z and M domains from an existing geodataset (e.g., feature dataset, feature class, raster). If you have another data set that you know has the same projection, you can simply import that coordinate system here.
3. New Create a new coordinate system. In 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.
See Pitfall 2
In our example, a .txt file was included in the publicly available download from www.sacog.org that described the projection of the data (State Plane CA Zone II units feet Datum NAD83).
7) So, we click Select
|Starting the navigation to the proper coordinate system|
This takes us to a browse screen with two folders to browse: Projected Coordinate Systems and Geographic Coordinate Systems. Our coordinate system for the traffic zone data is a projected coordinate system.
8) Double-click the Projected Coordinate Systems or Geographic Coordinate Systems folder.
Several options are available. Navigate through the folders until the desired coordinate system is located. For the example file, the State Plane folder is opened and then the NAD 1983 (Feet) folder, until a list of state plane projection options appears. For this example, we choose NAD 1983 StatePlane California II FIPS 0402 (Feet).prj, since that is the projection identified in the .txt file that accompanied the data download.
|The proper coordinate system for our traffic zones data|
9) Single-click the proper coordinate system and then click Add. You are returned to the previous screen showing the chosen coordinate system and its details. Click OK.
|The Spatial Reference Properties window has been populated|
10) Now the coordinate system details are waiting to be attached to your data set. Click Next.
|The Coordinate System has been attached|
11) You are shown a final summary. Click Finish if you're satisfied.
|The final summary window|
Sacramento Council of Governments data download page - The WWW location of the data used in the example
|Authored by: Rob Farrell Modified: 2/26/05|
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Cookbook: Ben Sprague, Ethan Sundilson, Carlin Wong, Sam Ying