Georeferencing an Image

Some imagery and scanned maps do not contain spatial reference information. Before these images can be used within a GIS, they need to be given a coordinate system and features on the image need to be connected to known positions on spatially referenced GIS data. This process is called georeferencing.

To georeference an image you need to set a coordinate system first. You then need to find at least four identifiable features to connect on both the non-georeferenced image and the basemap. These reference points are called ground control points.

After connecting the ground control points between the image and the basemap, the image then knows where it fits in geographical space and has the correct scale and orientation. 

The Control Point Table shows us the coordinates of the points on the image and the coordinates of the points on the basemap. The residual tells us how accurate our georeferencing was by showing the amount of error in the transformation of each ground control point. The RMSE (Root Mean Square Error) is the product of all of the residuals.