N.B. per utenti Italiani: questo sito è dedicato alla versione internazionale di CorrMap in Inglese. la versione Italiana è disponibile dal sito dedicato

# What should I do to geo-reference and rectify my map?

From the questions "Why do raster maps need to be geo-referenced?" and "Why do raster maps need to be rectified?" we’ve seen why a raster map needs to be geo-referenced and rectified. Now the question is: What does it mean, exactly, to geo-reference and rectify a raster map in order to get real and correct geometrical information from it? Well, it means performing the following four operations:

1. Assign to the raster map its real map (or surveyed) coordinates (Fig. 1).
2. Correct the orientation error (rotation, Fig. 2).
3. Transform the measurement units from pixels into the real units, e.g. meters, yards, etc. (Fig. 3).
4. Rectify the deformation of the map (Fig. 4).

Fig. 1 - We need to move the raster origin to its real coordinates.
Fig. 2 - We need to rotate the raster so that it matches the real North direction.
Fig. 3 - We need to scale the raster for transforming its units from pixels to real units (meters, yards, etc.).
Fig. 4 - We need to rectify the map deformation.

It depends on the purpose of your task whether to apply all four of these operations or just some of them. For example, if you need to simply obtain your raster coordinates converted into the real map reference system, you just have to perform the 3 first actions. Whereas, if you also want to get correct geometrical information such as coordinates and distances, you need to perform action 4 as well.

Also, you might want to geo-reference your map into its original reference system (national grid or whatever), or you might simply want to calibrate your map adapting it to your land (topographical) survey. You also need to take into account the origin of your map, i.e. how it has been created. This means that, before deciding how to proceed, you should answer questions such as the following:

• Has the map being obtained by a land survey, or by an aerial photographic survey?
• Is there a grid of control points available for the map to be geo-referenced and rectified?
• Does the map provide grid lines with their reference system coordinates?
• Do I need to retrieve very precise geometrical information in order to solve a boundary dispute?
• Can I recognize some map points in the field so that I can survey them?

Confused? Don’t worry, CorrMap has the answers to all these situations. In fact it provides 6 "transformations" suitable for each of them. For example:

• If you only need to transform your raster into the real map reference system ...
Does your map provide grid lines with reference system coordinates?

Yes ... then apply the Affine transformation.

No ... then apply the Barycentric transformation.

• If your map comes from a land survey and you also want to rectify it ...
Does your map provide grid lines with reference system coordinates?

Yes ... then apply the Grid transformation.

No ... then apply the Trilateral transformation.

• If your map comes from an aerial photographic survey and you want to rectify it ...
Is a grid of control points available for the map?

Yes ... then apply the Rubber-Sheeting transformation.

No but you can find and survey some map points in the field … then apply the Homography transformation.

There are many other possible combinations of "map-type / task-purpose", CorrMap can help you to solve all of them.