The Slovenian national orthophoto in the scale 1 : 5000 is made from the Cyclical Aerophotogrammetric Survey (CAS) images and the digital terrain model (DTM). It is the most widely used spatial source for a variety of GIS analyses and visual photointerpretation in Slovenia. It is used also as a main source for acquisition of changes in the actual use of agricultural land. Different vector or raster spatial layers can be shown on it, which may be more precise than the orthophoto. The users of the orthophoto are often not aware, that orthophoto may include positional errors. Using the geometrical connection between the original image and the orthophoto we derive the maximal radial displacement expected on orthophoto for objects of different height above or below the DTM. We show that radial displacement of higher objects, like higher rocky cliffs (height 50 m), tall buildings (like church towers) and forest edge, may exceed the permitted positional error of the orthophoto, at least in the extreme cases when these objects are located near the seam lines of the orthophoto. We also mention orthophotos made from images of unmanned aerial vehicles (drones), where the problem of its positional accuracy is even broader, as for their production the DTM with nonuniform vertical accuracy is used.
COBISS.SI-ID: 63506018