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Guidelines for using projection files
Before setting or converting coordinate systems YOU MUST KNOW THE CURRENT COORDINATE SYSTEM. Not setting the correct initial coordinate system is probably the most common mistake that leads to the terrible illness known as overlay gap disorder. Don't let this happen to you! Always set the appropriate (geographic or projected) coordinate system (using instructions below) for your source file before using ArcMap to project on-the-fly or to reproject shapefiles.
Arc Catalog
Prj Files
Import Method
Select Method
Copy and Paste Method
Teale Albers (CA)
Reproject Shapefiles
ArcCatalog
ArcCatalog provides a few simple tools for associating a given geographic or projected coordinate system with any spatial data file (shapefile, coverage, geodatabase layer). Every spatial data file that we work with should be associated with a projection file in this way, unless there is some specific reason not to provide this association. This will allow us to more efficiently conduct transformations and do projections on the fly.
Prj Files
Projection (prj) files work 'behind the scenes' to register a shapefile or coverage to a coordinate system. These are text files. They appear under the "prj" file name in a coverage folder or as a ".prj" extension file with the same name as a shapefile.
Import method
Importing is the easiest way to associate a coordinate system with a spatial file in ArcGIS. To use this tool open ArcCatalog. Click on the file you want to register. Choose File..Properties from the File menu (or right-click on file and select properties from shortcut menu). Make sure the 'Fields' tab is selected. Click on the 'Shape' field. Below in 'Field Properties', click on the button next to 'Spatial Reference'. The Spatial Reference Dialog Box should appear. Now click on the button labeled 'Import'. Navigate to the spatial data file that you know shares the same coordinate system with the current file you are registering and click 'Add'. Click 'OK' twice. ArcCatalog just created a prj file for your spatial data. You're done.
Select method
Follow the above instructions until you get to the 'Import' button. At this point click on the 'Select' button instead. You will be given the option to choose between geographic and projected coordinate systems. Select the appropriate coordinate system and click 'Add'. Now click 'Ok' twice. You're done. Read below for more information on coordinate systems:
Geographic coordinate systems are based on the angles created by connecting lines from the center of the earth to the prime meridian and the location of interest. An example would be 37.5678 degrees north (from the equator) by (-)119.4678 west (from the prime meridian). Projected coordinate systems are are used to calculate the location on a 2-dimensional map surface. They are given in units of linear distance (feet, meters, etc.) from an origin point (which may be the intersection of the prime meridian and equator, or another intersection with false northing and easting). For most local government data sets we will be dealing with projected coordinate systems.
Copy and Paste method
You can bypass ArcCatalog by copying a prj file associated with a spatial data layer that shares coordinate systems with the one you need to register. Then paste it into the same folder with the file you want to register. In the case of a coverage call the file "prj" and put it into the same info coverage folder. In the case of a shapefile give the filename the same name as your shapefile, but with a .prj extension.
Teale Albers
To set this projection either import the coordinate system from a file that is already registered (has a prj file) to the correct coordinate system or select by choosing Projected Coordinate Systems..Local..Teale Albers. Click 'Add'. Then click 'Ok' twice. You're done.
Reproject Shapefiles Diagram
ArcMap can be used to reproject data from one coordinate system to another. For example an unprojected shapefile (geographic coordinates) can be converted into a projected shapefile. Also, a shapefile can be changed from one projection to another (State Plane to Albers). Keep in mind that this is not always necessary. With
ArcGIS's projection on-the-fly it is possible to keep all datasets in their original coordinate systems and allow the projection engine to overlay them for you. However there are a few reasons for reprojecting: 1.) processes (including screen drawing) work faster, 2.) specific projections may be helpful for calculating areas or distances.
To reproject shapefiles use the instructions below:
The first step is to identify the current coordinate system of your source shapefile. Is it in a geographic coordinate system (Latitude & Longitude in degrees) or a projected coordinate system (in feet, meters, etc..)? Which coordinate system is it in? Read the metadata. Once you have identified the initial coordinate system, set it in the file properties in ArcCatalog using instructions above for Selecting, Importing or Copying projection files. For example, if you are working with ESRI CD data (L:\Other\ESRI\) Select Predefined..Geographic Coordinate Systems..
Okay, you have set the initial coordinate system. Step 1 is over. Next you will work in ArcMap to set the final coordinate system in the Data Frame Properties. There are couple of ways to do this. One way is to set the coordinate system in the properties manually (right-click 'Layers'..properties..Predefined..etc..). The other way is to load a dataset that is already associated with the final coordinate system as your first data layer in the document. The data frame properties always default to the first layer that is added. For example, let's say you want to convert your ESRI CD unprojected shapefile to match the County (CA State Plane Zone IV NAD
1983) projected coordinate system. Open ArcMap or create a new map document and add data from a County data set (such as the county boundary) that is properly registered (has a .prj file) in ArcCatalog. The dataframe will now be set to CA Zone IV. Next add your unprojected shapefile that you want to project. Right-click the unprojected shapefile layer and choose Data..Export Data. Click the radio box titled, "Use the same coordinate system as the data frame". That part is critical.
Your new shapefile has been projected to the final projected coordinate system!
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