Projection of the earth’s surface on the film or image surface is distorted both through relief and distance from the centre of the image. The production of true to scale aerial photomaps is carried out by ortho-rectifying the geo-referenced aerial photos (thus producing digital orthophotos - DOP), mosaicked to one seamless image and then cut in predefined ortho-tiles.

Orthophoto and DTM

Single image rectification for relatively flat terrain with few height differences is an inexpensive but less accurate option. The production of digital orthophotos, with its application of differential rectification, on the other hand satisfy high accuracy requirements. A digital terrain model is used to rectify the position of each image pixel in order to determine the exact position of heights and depressions, ie peaks and troughs. Thus orthophotos and client GIS-data are referenced directly to one another.

True Orthophotos

For high resolution orthophotos (GSD=10cm and smaller) the topographic details (such as civil engineering structures eg bridges, dykes etc) have to be taken into account during rectification. Thus breaklines will be added to the DTM which is transformed to a digital surface model (DSM) in order to achieve the quality required.

In city areas it is essential that blind spots (occluded or obscured areas) be minimised. When the DTM is complemented with building outlines the building edges are rectified accurately. The blind spots can be determined and rectified using the content of overlapping aerial images. This is known as true-orthophoto.

Mosaicking and Radiometric Balancing

Orthophoto projects often cover an extensive area and therefore are tiled at regular intervals. These are stitched together like a mosaic from many digital orthphotos. At the seamlines individual photos have colour variations which can be eliminated through shadow edge correction along the seam lines. Radiometric colour balancing ensures homogenous representation over the entire image.

The program OrthoMaster (INPHO GmbH) is used for ortho-rectification and mosaicking.