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In
the next step we georeference
the image using some source of location control (for example,
topographic maps,
digital roads,
or ground control points measured using GPS receivers). In this
example, location control is provided using roads depicted on
1:100,000 scale Digital Line Graph (DLG) data downloaded from
the US Geological Survey. Prominent road junctions are located
throughout the image, and their map locations measured from the
DLG file. Once a suitable set of location control points is defined
in this way, the image data are then digitally “resampled” to
fit a desired map projection.
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