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PCCC GIS I Exercise 2 (AV9.1) - Locate your car with GPS, map it with GIS v2, Apr 2007 - page 15 / 31





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Step 8 - Change projection of county shapefiles to NJ State Plane 1983 NAD Discussion - As mentioned earlier, there are 2 primary ways to view data in GIS: 1) unprojected and 2) projected data. Your ArcMap map view is nothing more than a set of X,Y,Z coordinate axes, and therein lies a potential problem: Arcview does not always keep track of the units of measurement along the coordinate axes. Since different projection systems use different units of measurement as well as different datums (the “origin”), the same data sets (e.g., a point representing your house) could appear in two different locations on the same map. Below is an example of what used to happen in AV3 when your house, expressed in both lat-long and NJ State Plane coordinates, was added to your map view: The point, even though it represented the SAME HOUSE, appeared in TWO DIFFERENT PLACES:

600,000 feet East , 600,000 feet North

NJ State Plane coordinates (units in feet) very large numerically


  • -

    74 deg West, +40 deg North

NJ Lat-lon coordinates (units in degrees) very small numerically



Despite the fact that AV8/9 has “on the fly” projection-matching capability, certain GIS functions that you may frequently use, including Geocoding and the creation of 3D models, require that shapefiles be defined in the same coordinate system as that of the view. Therefore, you will define and change the projection of your county shapefiles from lat-long to NJ State Plane, and save the new ones with slightly different names.

Step 8a. Define the CURRENT coordinate system of a layer Procedure:

  • Create a county_SP folder inside your Exr02 folder

  • In the ToC of your map view, rt-click on county_roads > Properties > Source

Note that the current coordinate system is GCS_Assumed_Geographic_1, North American Datum (NAD) 1927. Use your internet browser to visit http://www.census.gov/geo/www/tlmetadata/metadata.html. In the first paragraph, note that Census 2000 TIGER/Line files have a 1983 NAD datum, not 1927. We want to define the projection correctly before we re-project it to the NJ State Plane coordinate system.


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