Why do raster maps need to be geo-referenced ?
Before answering this question, it is worth clarifying the meaning of the term "geo-referencing".
As the two parts of this compound word tell us, geo-referencing means to give a geographic (geo) reference system (referencing) to a map.
But why do we need to give a reference system to a raster file map?
Fig. 1 - It does not help if the raster map contains reference points or grid lines showing their coordinates, they’re simply numbers drawn on the map.
Simply because a non-geo-referenced raster map does not have any reference system.
This is also true when a raster map contains some reference points or grid lines provided with coordinates directly
written in the map, as shown in Fig. 1. This is because these coordinates are simply numbers drawn on the map, not
the coordinate values of their corresponding points.
The easiest way to understand this problem is by importing a raster map into a CAD software which supports raster image files,
such as AutoCAD(*). As soon as you activate the import menu command, the CAD system asks for the following parameters
in order to insert the image into your drawing:
Insertion point: you need to specify X-Y-Z coordinates at which to insert the lower-left vertex of the image.
Scale: the scale factor to assign to the image.
Rotation: the rotation angle to apply to the insertion point in case you want to rotate the map
instead of inserting it perfectly orthogonal to the drawing axis.
What values do you provide for these 3 parameters?
Fig. 2 - The map just inserted simply has coordinates 0,0,0 in its lower-left vertex.
Normally you don’t know. So, you end up by simply accepting the default values suggested by the CAD system
itself, i.e.: 0,0,0 for the insertion point coordinates, 1 for the scale factor and 0 for the rotation.
As a result, you will soon find out that your raster map is completely unusable for getting real geometric data because:
Fig. 3 - The coordinates of any point in the map are simply the number of pixels in horizontal and vertical direction from the lower-left vertex.
The coordinates of the map lower-left vertex (Fig. 2) are simply: X = 0, Y = 0, Z = 0.
Therefore, if you ask for the coordinates of a reference point in the map, as shown in Fig. 3,
you don’t get the real coordinates written on the map, but you simply obtain the number of pixels in
horizontal/vertical direction between the lower-left vertex of the map and that point. Quite unusable, isn’t it?
The unit of measurement of the raster map is pixels, not the real units (meters, yards, etc.). This implies that
you cannot obtain real distances from the map either. For example, the map shown in Fig. 4 contains grid lines at
an interval of 200 meters, but inquiring about the distance between two consecutive grid lines, we do not obtain
200 but a totally different value, it is simply the number of pixels between the two grid lines.
Fig. 4 - Also we cannot obtain real distances from a non-geo-referenced map.
Well, all this is the reason why raster maps need to be geo-referenced.
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