| 7.2.6 Map
database management
A great number of different
functions and operations fall under such a heading. Some of them are common
to many other graphic tools (i.e. CAD systems), others are specific of
GIS environments. To the fist class belong the operations for adding, deleting,
moving and rotating features. To the second appertain functions such as
projection change, conflation and georeferencing. Source maps may come
in very different projections; so they need to be converted to a common
coordinate system by means of projection changes. To rectify and georeference
a satellite image, the input raster must be resampled to a new grid, where
the coordinates are spaced according to the new common coordinate system.
In general, GIS incorporate a subsystem to accomplish this operation through
a more or less friendly user interface. In spite of the functionality of
the GIS UI, all the above operations are time consuming and frequently
frustrating even for users familiar with the matter.
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Click on the preview
of each image in order
to see a larger version
of the photo 
Click on the preview
of each image in order
to see a larger version
of the photo
|
In these two pictures we
can examine a method for raster data rectification. In the lower left window
of each one, we have the red vector "coverage" (island of Salamina,
near Piraeus), that is established in a certain projection system &
its elements have "real" coordinates. The black & white (upper
of it) picture is a raster (scanned) image of the same area & we want
to transform it from its ¡paperâ coordinates to the real ones. Adjusting
frames in these two small windows, we define the areas we want to overlay
& then we put the first 4 links (green arrows in the right window of
the 1st picture). After the first registration (which brings us to the
2nd picture), we put more links to achieve better transformation, trying
also to have low RMS values, by deleting and keeping the most proper ones
(see the two upper windows). |
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