This tutorial will teach the user how to define the projection for their GIS data. You will then reproject your data to a more suitable format. The sample data is a few city blocks in Burnaby, BC and is using the NAD83 Geographic Coordinate System with the units in longitude /latitude degrees. Geographic coordinate units are not constant (1 degree of longitude varies in size from 0 at the north and south poles to 1/360th of the circumference of the Earth at the equator). This is because it is actually a 3 dimensional model of the Earth. Due to this, these coordinates cannot be used to measure area or distance. You should not use data in geographical coordinate to do an analysis. We will first define the projection of our data, and then reproject the data to a Projected Coordinate System. Projected Coordinate Systems are a representation of the 3 dimensional Earth on a 2 Dimensional surface. This means there will be distortion in area, shape, distance and/or direction. The coordinate units however are constant and are defined through a Cartesian or planar x,y surface. The first thing you should notice is the coordinates of the data. Look in the bottom right hand corner, and as you hover your mouse over the geographic data the coordinates will change. You should see that the coordinates are in longitude /latitude decimal degrees. Under the “XY Coordinate System” tab, press the “Select…” button to choose a predefined coordinate system. In your Coordinate Systems folder go to Geographic Coordinate Systems >, North America >, North American Datum 1983.prj and Click OK. This creates a projection file (.prj) stored with the shapefile. None of the data is altered, it is simply defining the projection. Now that ESRI ArcGIS knows the projection of the data, we can reproject it to a Projected Coordinate System for analysis purposes. Go to “Data Management Tools >, Projection and Transformations >, Feature >, Project” and open this tool. The next option “Output Coordinate System” is where we define the reprojection information. Press the “Select…” button to choose a predefined coordinate system. In your Coordinate Systems folder go to Projected Coordinate Systems >, Utm >, Nad 1983 >, NAD 1983 UTM Zone 10N.prj and Click OK. Once this has completed, view the output file in ArcCatalogs the preview tab. You will see the coordinates have changed this time to UTM meters which are suitable for an analysis in Burnaby, B.C. The user should know how to define and reproject coordinates systems in ArcCatalog and understand its importance. Source.