Georeferencing is the process of assigning geospatial positioning information to raster data based on a defined coordinate system (essentially associating data with a specific area on Earth). Why is this necessary? Imagine you have a paper map and you use a desktop scanning device to save a digital image of the map to your computer - this map depicts a particular area on Earth but there is no way for your scanner or computer to automatically know that (1) it is a map and (2) what area on earth the map depicts. Thus, by default there is no geographic information associated with the image file and any GIS software you try to load this file into would not know where on Earth to place this map image. In order to solve this problem, it is necessary to manually generate geographic coordinate information that can be associated with scanned map image file so that GIS software can process the map image correctly and align it successfully with other georeferenced data.
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This georeferencing process is typically carried out using GIS software like QGIS, ArcMap, or ArcGIS Pro and is a powerful way to add value to scanned images of paper maps, printed aerial photos, and other geospatial raster data that is missing coordinate information by preparing the data for GIS based analysis and digitizing operations. The georeferencing process may also be combined with the process of clipping raster data to remove unwanted portions of a scanned image and with the process of image compression to reduce the raster file size.
The process of georeferencing varies slightly based on the GIS software you are using and the characteristics of the raster data you are working with. To learn how to georeference a map image file using QGIS, click on the georeferencing guide link below that matches the type of map you are working with. If the map you are hoping to georeference has geographic coordinates marked at the four corners of the map (see example A) click the link below:
Not all maps have geographic coordinates for the map corners though, and may not even supply coordinate information at all, but it is still possible to georeference these types of maps using a slightly different process. If your map does not have coordinate information for its map corners (see example B below), click on the following link to access a guide that will walk you through the process of using QGIS to georeference based on identifiable map features and an already georeferenced basemap.