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.
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.
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 8.1'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 1927) 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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