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<br />The consequences of using different projections are apparent if you compare two maps that show the same <br />region but use different projections and coordinate systems. <br /> <br /> <br />The map first map uses geographic coordinates. The second map is projected to the North Carolina State <br />Plane system and uses a planar coordinate system, If you look carefully, you can see a difference in the <br />shapes of the counties meso You will also notice that the X and Y coordinates of a point are different in the <br />two maps even though they represent the same point (Durham), Some software packages require manual <br />conversion of data layers into the same projection and coordinate system before they can be overlayed, other <br />software can do this automatically. <br /> <br />Digital data are stored in real world coordinates, e,g, one foot on the map equals one actual foot on earth, <br />Scale only becomes important when the data are plotted or displayed. Plotted or displayed digital data treats <br />scale the same way as on paper maps, A certain number of paper or screen units correspond to a consistent <br />number of real world units, When scale is used to describe digital data, it usually refers to the scale of the <br />source data, In most cases, this is the scale of the paper map or aerial photography from which the data were <br />generated. The consequences of scale in relation to digital data are easily seen. <br /> <br /> <br />For example, the map here shows two hydrology layers, both registered to <br />. the same coordinate system but of different scale. The dark lines layer was <br />derived from a 1:24,000 scale map and the dotted lines one from a 1:100,000 <br />scale map, The differences between the two coverages are barely noticeable, <br /> <br />60f9 <br /> <br />3/17/98 10:05 AM <br />