2.6 More about georeferencesA geore

2.6 More about georeferences
A georeference is a service object, which stores the relation between the rows and
columns in raster map(s) and the ground-coordinates (X, Y). A georeference is
needed for raster maps and uses a coordinate system. If you work with several raster
maps of the same area, and you want to overlay them (e.g. with the Cross operation),
it is important that all maps have the same number of rows and columns, and the
same pixel size. The pixels in both maps thus refer to the same position on the
ground. Therefore all raster maps of the same area should have the same
georeference. Let us look at a georeference.
Georeference Cochabamba is a georeference corners. By defining the X and Y
values of the corner of the raster map, ILWIS is able to find the correct coordinates
for each pixel. A georeference corners assumes that the raster map is oriented to the
north. The X-coordinate increases from the left column to the right column, and the
Y-coordinate increases from the lowest line to the upper line. The X and Y values
relate to the coordinate system that we have seen in the previous exercise.
A georeference corners has information on the number of rows and columns in the
map, the coordinate system, the minimum and maximum X- and Y-coordinates of the
corners of the map, and the pixel size.
A georeference of type corners is used for instance for maps which are converted
from vector to raster (i.e. rasterized vector maps).
For such calculations you can also use the Command line of the Main window.
For satellite images, scanned aerial photographs and other imported images, which
may not be north-oriented, we have another type of georeference: a georeference
tiepoints.
ILWIS