Mapping Hacks

Georeference an Arbitrary Tourist Map #33

Chapter 3, Mapping Your World

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HACK

You need at least three points that you have a position for and can defini-

tively identify on the map; these are ground control points, or GCPs. The

technique we use here uses the known distance between the ground control

points in the real world to derive a formula to estimate the scale of the map.

For those interested in digging into the math, the Least Squares Approxima-

tion is a common method for coming up with a solution to the linear equa-

tions used to georeference a map from GCPs. (For the truly mathematically

inclined, cartographers sometimes use quadratic and cubic approximation

methods as well.) For those just interested in getting it working, we’ve pro-

vided a JavaScript interface that will do the calculation for you.

The Web Interface

Before starting, you should have at least three waypoints that you’ve col-

lected “on the ground,” with an east and a north coordinate for each point.

The waypoints should be well separated from each other on the map you

want to georeference. This will provide a more accurate approximation of

the map’s extents, rotation, and scale.

Next, visit http://mappinghacks.com/georeference/ to load up the interactive

georeferencer. Hit the “Browse” button to view an image from your local

hard drive. Now start adding ground control points. The program doesn’t

care about the kind of values ...