Rectify Your Scanned Map or
Digital Aerial Photo
Henry Loope - 2006
To rectify (georeference / rubbersheet) a non-georeferenced (jpg, tif) image in Global Mapper:
First list coordinates of several point locations on your image.
1) Go to File à Rectify (georeference) imagery…
2) The Batch Image Rectification box will come up and make sure you export the new file as a GeoTIFF (the other options should be checked as well) – then click OK
3) Open the file you wish to rectify (georeference) – usually a .jpg, .tif, .bmp
4) The image rectifier box will come up – you can maximize the box to make it easier to see and pick points.
Now, just pick points in the zoomed view screen where the coordinates are known (e.g. the corners of a topographic map) and click that location on the zoomed view screen.
Below that screen, the Ground Control Point (GCP) area is where you enter the coordinates for the point you have just clicked. You can enter the coordinates in DMS or in UTM.
When finished entering the correct coordinates, click on ‘Add GCP to List’ and that point will then show up below that in the table of all GCPs. Repeat the process of clicking on points on the image where the coordinates are known. The more points, the better, but you should have no less than four or five GCPs (you need at least two to rectify an image, but if you only have two, the image will likely be distorted).
You can also select the projection which the image will be exported.
When you have sufficient points, just hit OK and the image is now rectified.
To export this new file, just go to File à Export raster and elevation data à Export GeoTIFF. You now have a georeferenced image.