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How to take any unreferenced map and georeference it for use in
QGIS.<br>
<br>
<br>
Needed:<br>
GIMP<br>
Google Earth<br>
QGIS<br>
An unreferenced raster.<br>
<br>
1. GIMP<br>
<br>
Load the image into GIMP. <br>
<br>
Size the image canvas such that the total width is no larger
than 2048 pixels. Once in QGIS larger sized raster images can cause
performance problems. 2048 pixels is a compromise between file size
and image readability.<br>
<br>
Enable the grid and grid snap. Using a uniform distance from the
edge of the raster make 5 small registration marks (a simple 3 or 4
pixel line width cross will do). Simply use the grid lines to
measure the distance from the edge. To be even more precise, just
think in multiples of 2048. ie 512, 1024, 1536 pixels. Three marks
go evenly across the top, from top left, middle, to top right. Two
marks go along the bottom at the 1/4 and 3/4 positions respectively.<br>
<br>
Use GIMP's export feature to save to a .png file. <br>
<br>
2. Google Earth<br>
<br>
Import the image. Move the image to the desired location. Google
Earth has the ability to make a raster layer transparent. Use this
feature to help locate and scale the image precisely.<br>
<br>
Once satisfied with the placement, draw a path between the 5
registration marks so that they form a large "W". Zoom in closely
to help with precise positioning of the W's vertexes.<br>
<br>
Export the "W" to a .kml file.<br>
<br>
3. QGIS<br>
<br>
Import the "W" .kml file as a vector layer.<br>
<br>
Open the Georeferencing tool and load the raster.<br>
<br>
Starting at the top left, zoom in on the registration mark and
click in its center. Then, select the "From Map Canvas" option. Zoom
in on the top left vector of the "W" and carefully select the
endpoint. Repeat for the other four registration marks. Start from
the top left, tracing the "W" as you would print it, ending at the
top right. <br>
<br>
Once all five marks are registered with coordinates (they appear
on the bottom of the screen), click the "Transformation Settings"
tool (a wrench icon on the menu). Click on the folder icon next to
the "Output Raster:" text input box and just accept the
"filname-modified" for the output filename. I recommend using this
convention to avoid confusion later on. Your CRS should be the same
as your project. The other default settings should be good to use. <br>
<br>
To complete the transfer to the QGIS canvas press the green
"play" button on the menu.<br>
The image should automatically appear in the correct location.<br>
<br>
This procedure can be used for any unreferenced raster map,
sketch or aerial image. Many images can be stacked on top of one
another using the transparency feature of QGIS and Google Earth. The
performance of QGIS does slow down when multiple images are active
at the same time.<br>
<br>
<br>
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