GEOG 486
Cartography and Visualization

Defining a Projection

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Defining a projection refers to the process of using projection metadata to describe what spatial reference system was used to georeference a particular data set. Most GIS software packages will allow you to import data that does not have projection information already attached. However, without this information, the software may not properly register or display the data (see Figure 7.cg.7, below). If your dataset’s projection information is not defined, the software will not be able to change either the coordinate system that the data are stored in or reproject the data into another map projection. Defining the projection does not actually transform the data, but identifies its spatial reference. Many data misalignment problems are caused when a dataset’s projection or coordinate system is either not defined (and the computer cannot correctly guess what projection and reference system it uses), or is defined incorrectly (i.e., using incorrect parameters).

misaligned projections
Figure 7.cg.7 In this example, the road network displayed in blue was defined as using the MN State Plane Coordinates-North Zone projection. The road network displayed in green did not have a projection defined, but was in fact projected using the MN State Plane Coordinates-South Zone. Because the green layer did not have a defined projection, the GIS was unable to use the project-on-the-fly feature to register the datasets (see the Coordinate Transformations vs. Projecting-on-the-fly vs. Reprojecting Concept Gallery item for more information on these different coordinate system manipulations).