Defining Projections and Reprojecting Data

Defining Projections and Reprojecting Data

(performing data transformations)

Use ArcCatalog to determine if all your data files share the same coordinate system, projection, and datum.

If they don’t you may have a problem. ArcMap is a fairly “smart” program in that it will “reproject on the fly.” What this means is — if the data files have been assigned a coordinate system, projection and datum — ArcMap will reproject them in the Map Display so the layers will align correctly. However, this does not mean that your data files have undergone a transformation to a new projection. You must change the projection in ArcTool box if you want to do meaningful analysis of the data (if you are creating a looky-lu map you can leave the layers in different projections.)

Sometimes the data is in a certain projection but ArcMap does not know what that projection is. (For example, a .shp file will have a .prj file associated with it that tells it what the projection is, if this file is missing, ArcMap will not know the projection.) If this is the case, you will have to use ArcToolbox to define a projection for that data file.

Defining a Projection

Defining a Projection

1. Double click on “Define Projection” - the following dialog box will open:

Browse to find the Ca_faults file.

Add the Ca_faults.shp in the next dialog box.

The next window will define the projection (you should be working off of the D drive not the U drive).

In the next window, press Spatial Reference Properties button, then in the next window press “Select”

Then click “Geographic Coordinate Systems”, then “Add”, double click on “North America”

Then select

then “Add.”

In the next window read the details dialog window to make sure it has the correct project then click “OK.” You have just Defined a Projection for the ca_faults.shp.

Changing Projections

Now let’s reproject all the Earthquake Data to a projection that is more appropriate for your project.

Select “Project” from the Toolbox and double click to open it.

In the following window you will: 1) hit the browse button and navigate to your cities_dtl.shp file and 2) select it and hit “Add”.

Now select the browse button for the Output Dataset or Feature Class dialog box. Navigate to your projected folder and rename your file “cities_dtl_p”

Hit Save and OK.

In the next window you will select your new projection.

When the Spatial Reference Properties box opens hit select. This time select Projected Coordinate Systems and hit “Add” , Continental and then hit “Add” North American and hit “Add”. Now you must decide. Do you want a conformal, equal area, or equidistant map? Or do you want a State specific projection?

Select the one most appropriate for your map and hit OK. Verify in the next window that the projection is correct and hit OK. You now should be back to this window.

Hit OK. A progress box will open and tell you when it is completed.

Reprojecting more than one file at a time.

Select Batch Project

Browse and add all the files you need.

Select the correct “Output Workspace” and the appropriate Coordinate System and hit OK.

Depending on the number and size of the files this can take a long time so be patient. As long as the status window is running it is working.

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