Why do raster maps need to be rectified ?
You might think that the non-real
coordinates and measurement units of a non-geo-referenced map is the only problem you need to solve,
but unfortunately this is not the case.
There is another important problem: the original paper map has been subjected to a deformation from its original state.
And if the paper map has been deformed, its raster image (file) is therefore deformed too.
This means that, in order to obtain correct geometric information from a raster map, you also need to resolve this deformation, i.e. rectify the map.
But why do raster maps become deformed?
Fig. 1 - The first deformation factor depends on the accuracy applied during map creation.
Graphical errors during creation: actually this factor is not a deformation issue but it is due to human errors
that might have occurred when the map had been manually drawn on the paper. Of course these errors depend on the age of
the map: the older a map is, the bigger graphical errors it potentially contains because of the poor quality of the
instruments used. For example, Fig. 1 shows a typical error: the grid lines of that map should have been traced at
an interval of 10 cm, i.e. 200 meters for scale 1 : 2000. In this case instead, they were traced at a slightly greater distance.
The wearing effect of time: if the map is quite old, this is the main deformation reason and it’s obviously caused by
physical factors such as temperature and humidity changes, paper degradation, etc.
The scanner: raster maps are obtained by scanning original paper maps. But this process does not lack errors.
The accuracy of a scanned map obviously depends on the quality of the image scanner used, but even top quality scanners have an average
tolerance between 0.5 to 1 mm for the diagonal of a sheet sized 100 x 70 cm, and 1 mm corresponds to 2 meters for a normal map scale factor such as 2000.
In addition the scanning process can also generate a slight rotation of the map. This is due to the operation of positioning a paper map on a (plane)
scanner which is performed by aligning the border of the map with a horizontal line present on the scanner itself.
This line guarantees the orthogonality, hence the exact North direction of the resulting image. Of course, even if this operation is
performed with the maximum care, it is always subjected to a little positioning error which causes a slight rotation of the image obtained.
Not to mention when a rolling scanner is used, because this instrument significantly magnifies the positional error in the direction of rolling.
The rotation error is very subtle because it is usually very small, and therefore virtually invisible to the naked eye but, being an angle,
it causes significant linear errors for map points which are located far from the rotation pole.
Continue reading: What should I do to geo-reference and rectify a map ? ››