Mapping Hacks

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Chapter 3, Mapping Your World

#33 Georeference an Arbitrary Tourist Map

HACK

The World File

The simplest way to make use of this transformation is to put it in a sepa-

rate file that accompanies the image. The format is just the list of six num-

bers, separated with line breaks. This is called a world file, often seen in the

company of TIFFs as a TIFF world file (.tfw). A world file can accompany

any image format with GIS raster data in it. A common convention is to take

the first and last letters of the file extension, followed by a “w,” for the

world file extension—so a JPEG would travel with a .jgw, or an ESRI BIL

raster with a .blw.

Once you have a world file describing an image, you can load it into your

favorite GIS program. QGIS is a good choice for quick-and-easy viewing of

GIS data. Also, the GDAL toolkit understands world files and can use them

to convert raster data between different formats and different coordinate

systems, as described in “Convert Geospatial Data Between Different For-

mats”

[Hack #68].

The Matrix Transformation

The six numbers (coefficients) in the world file are values from a 3x3 affine

transformation matrix. The matrix describes a linear transformation between

one 2-D coordinate space and another—say, from pixels to latitude and

Figure 3-39. Georeferencing an image with the JavaScript client