Home | Index | Getting software | Mashup HowTo | Georeferencing | Reprojecting rasters
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. For work in GRASS, I recommend that you save that image as an indexed-color .TIF file (I will explain the reason for this format later).
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 coordinates.
Now, using Qgis, you will georeference the mashup by
generating a "world" file and a "points" file of the mashup image. With
these two new files, GRASS can import the mashup image into a GIS
"Location" and recognize the geographic coordinates of the mashup. The
world-file will have the same filename as your mashup, except that .tif
will be replaced with .tfw; the points-file will have the same name as
the original with .points added to the end. So the original and new
files will be named something like:
mashup.tif
mashup.tfw
mashup.tif.points
Now 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 both Windows and other operating systems.
Call up the georeferencer from the Plugins menu.
Browse through your files to find your mashup, wherever you saved it.
Once you select your mashup, the georeferencer tool will open a window onto that file so you can place georeferencing points.
So zoom in to where you remember the landmarks to be;
And zoom in further;
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 yellw, 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 dms coordinates. So you will need to start up your spreadsheet software and open up the spreadsheed you had created back on the "get data" page.
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.
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. You can now import this file into GRASS.