![]() The GP_states_1930 layer we just added is the reference layer and it MUST be added first because it provides the spatial reference or coordinate system for your unreferenced image. When referred to as “georectification”, the process entails that a new image is created when the original is “warped”, or rectified. This process is sometimes called “rubber-sheeting” because it treats the scanned historic map as a sort of rubber sheet that can be stretched and manipulated to fit real world features. Coordinates are tied to the image by identifying common geographic features also found in the reference dataset. These coordinates are obtained from a second dataset (raster or vector) that has a properly specified “spatial reference” (sometimes also referred to as “coordinate system”). Key Concept : Georeferencing is the process of assigning coordinates to an otherwise “ unreferenced” image. On the right side of your screen appears a corresponding map of the 12 Great Plains states as they existed in 1930. On the left side of your screen in the Table Of Contents (TOC) a new layer appears represented by a coloured box below it. Navigate to ArcGIS_Lesson3\WindErosion1.gdband select the feature class called GP_states_1930 then click the Add.If you unzipped the folder in Documents\ArcGIS you should be able to find it, but if you saved it elsewhere on your hard drive, you will need to use the Connect to Folder button.) ![]() (If you have trouble locating your folder, make sure you connected the ArcGIS_Lesson3 folder to ArcMap using the Connect to Folder button.
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