I would orthorectify and some old aerial photos with QGIS or ArcGIS; I have already read something about the use of a DEm... what's the best way to do it?

closed as too broad by Dan C, Mark Ireland, whyzar, xunilk, Kersten Feb 22 '18 at 18:23

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Orthorectification is the process of correcting aerial or orbital images for terrain geometry. The terrain geometry is given by a DEM (a DTM in the case of regular orthophotos, high-resolution DSM for true orthophotos), though you need a lot more than just that (for example, you need ground control points, camera calibration parameters, flight data, etc.).

This process, however, is very complex, and requires specialized software to do it. Softwares commonly used are PCI Geomatica, Erdas, and ENVI, and each run in the several thousends of licensing price. ArcGIS and QGIS simply don't make the cut. ArcGIS Pro actually has a standalone module for orthorectifying UAV images (or so ESRI claims, never tested it myself), but for aerial photos you're basically stuck with the ones listed above.

You can get a basic understanding of the process in this dissertation.

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