[Qgis-user] How do I register a Google Map?

[G. Garibaldi]

How to take any unreferenced map and georeference it for use in QGIS.


Needed:
GIMP
Google Earth
QGIS
An unreferenced raster.

1. GIMP

     Load the image into GIMP.

     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.

     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.

   Use GIMP's export feature to save to a .png file.

2. Google Earth

     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.

     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.

     Export the "W" to a .kml file.

3. QGIS

     Import the "W" .kml file as a vector layer.

     Open the Georeferencing tool and load the raster.

     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.

     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.

     To complete the transfer to the QGIS canvas press the green "play" 
button on the menu.
The image should automatically appear in the correct location.

     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.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20140206/a4b53d44/attachment.html>