International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 7-4-3 W6, Valladolid, Spain, 3-4 June, 1999 
GCPs and the method of measuring their pixel coordinates. The 
localisation accuracy usually lies in the 0.5-1 pixel range. 
3.2.2. Geocoding 
Through geocoding, image data are transformed to a common 
reference map projection, e.g. defined through topographic 
maps being used. This is the standard procedure for remote 
sensing images. Displacement errors caused by topographic 
relief are removed through the integration of a DEM (in such a 
case, also the terms orthorectification and differential 
orthorectification are used). 
2. As the above transformation yields an irregular raster of 
points, a proper grey value interpolation/resampling has to 
be applied in order to get a regular raster. 
Grey value 
Figure 5 shows a general scheme of the image geocoding 
procedure. Based on a DEM and an optimised imaging model 
(see above), geocoding can be performed in two basic 
processing steps: 
1. First, for each output pixel which defines a co-ordinate 
triple (East, North, Height) in the output map projection, the 
corresponding location in the input image is determined 
through a so-called map-to-image transformation. 
2. Subsequently, a grey value is interpolated/resampled from 
neighbouring input image pixels and assigned to the related 
output pixel. 
3.2.3. Parametric Image-to-lmage Registration 
Alternatively to geocoding, image data can be registered in 
image geometry. Based on parametric methods this can be done 
through a so-called map-to-image transformation of geocoded 
image data into the geometry of a reference image. In the 
approach implemented at the Institute of Digital Image 
Processing, the images to be registered first have to be 
geocoded using the standard geocoding method. The 
transformation of the geocoded images to the geometry of 
another selected reference image is then done in a direct 
resampling approach in two steps (see Figure 6): 
1. For each geocoded input pixel the corresponding output 
pixel location, i.e. column and line values related to the 
reference image, is calculated using the transformation from 
map coordinates to the reference image. 
Fig. 5. Digital image geocoding workflow. 
Fig. 6. Workflow of transformation from map to image 
geometry.