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geographical information system image. In general, the
term " image " means a picture taken at a
given moment but for the map maker the image must be
adapted to meet his requirements : knowledge of existing
and updated information will allow him to do just that.
The image must therefore be superimposable on other
data in the information system. To do this, it is necessary
that the image can be held in the same system of co-ordinates
that the plans already use and that it does not present
any distortions due to relief, as if the shot had already
been taken in minute detail and constituted a single
large photo of the area concerned. The photos are therefore
georeferences, " corrected " and " pieced
" to be superimposed on constantly evolving maps.
This is an orthophotoplan. When it is a matter of a
unique shot, it is orthophotography.
Uses. Orthophotography complements perfectly more
traditional information such as land registers, town
maps and topographical surveys. It can be used with
the tools of CAO-DAO and of map-making. Holding these
images in systems of co-ordinates, such as Lambert
II, allows their superimposition on other geographic
databases and the creation of composite drawings (images
+ sections). The visual quality allows for a wide
band of use, at present being able to go from 1/250e
to 1/10 000e. The orthophotoplan allows the user to
locate everything which is visible to an observer
placed vertically at every point in the territory.
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