Reference your raster!

At this point you should have:

1. a mashup of the area you want to focus on, and it should be marked with 1-pixel marks at four or more visual landmarks, widely scattered across the image. Note: For work in GRASS, I recommend that you save that image as an indexed-color .TIF file.

2. a spreadsheet file in which you have recorded the exact coordinate of each landmark, and which calculates the decimal version of the degree-minute-second (D-M-S) coordinates.

Now, using Qgis, you will georeference the mashup by generating a “world” file and a “points” file of the mashup image. The original and new files will be named something like:
mashup.tif
mashup.tfw
mashup.tif.points

Start Qgis. NOTE: my screenshots are from my Mac, so it will look a little different, but QGIS seems to run the same way on Mac, Linux, and Windows.

Accessing the Georeferencer tool in QGIS

Call up the georeferencer from the Plugins menu.

Georeferncer

Browse through your files to find your mashup, wherever you saved it.

Raster to-be-referenced

Once you select your mashup, the georeferencer tool will open a window onto that file so you can place georeferencing points.

Zooming in on the file

So zoom in to where you remember the landmarks to be;

unreferenced image: zooming in

And zoom in further;

zoomed in close enough to drop a GCP

and once you are close enough in, pick the “add point” tool from the buttons at the top of the main georeferencer window. Once you have picked this tool, move your cursor to the middle of the marked pixel (in my case yellow, to contrast with the background). Click on the pixel, and a coordinate-input dialog box opens up. This is where you need to enter the decimal coordinates that you calculated from the Google Earth D-M-S coordinates. So you will need to start up your spreadsheet software and open up the spreadsheet you had created back on the “make-a-mashup” page.

entering the coordinates

NOTE: when you input data, notice that the georeferencer simpy asks for X and Y coordinates. Under most circumstances, X = Longitude east or west; Y = Latitude north or south. But the convention which Google Earth follows is to give the latitude (say, 34 N) first, then the longitude (for example 69 E). So make sure to put the latitude number into the X field and the longitude number into the Y field.

create the World File

When you have put in your data for all your points, finish by clicking either “create world file” or “generate world file and load layer”. This step will generate mashup.tfw and mashup.tif.points. Your raster is referenced! Now you can load it in QGIS or any other GIS software you might use!