@MASTERSTHESIS\{IMM2004-03257,
    author       = "M. {\O}. Nielsen",
    title        = "True orthophoto generation",
    year         = "2004",
    keywords     = "Orthophoto, Digital Surface Models, Aerial photography, Photogrammetry, Color matching",
    school       = "Informatics and Mathematical Modelling, Technical University of Denmark, {DTU}",
    address      = "Richard Petersens Plads, Building 321, {DK-}2800 Kgs. Lyngby",
    type         = "",
    note         = "Supervised by Assoc. Prof. Keld Dueholm, {DTU}.  During the project, imagery and surface models are used for testing and illustration. These data have kindly been provided by BlomInfo A/S.",
    url          = "http://www2.compute.dtu.dk/pubdb/pubs/3257-full.html",
    abstract     = "This Master Thesis investigates methods for generating true orthophoto imagery from aerial photographs and digital city models.

The thesis starts by introducing the theory for generating orthophotos, followed by a comparison of orthophotos with true orthophotos. Methods and problems that arise when extending the theory to true orthophotos are treated. On the basis of the investigation, an overall method for creating true orthophotos is devised. The remaining chapters treat the steps of the method in details, and evaluate the results.

The true orthophoto rectification is divided into four general steps: Rectification, color matching, mosaicking and feathering. Creating the image mosaic is found to be the most crucial part of the process.

Three methods for mosaicking source images are tested. They all rely on simple pixel score techniques used for assigning pixels from the source images. The best method found uses a method where the score is calculated as a combination of the distance to the source images  nadir points and the distance to obscured areas. A histogram matching algorithm is used for giving the source images the same radiometric properties, and feathering is applied along the seamlines to hide remaining differences.

The method is tested on a range of areas, and the overall result shows that the method gives reasonable results, even if the surface model is inaccurate or incomplete. It is furthermore assessed that the method can be applied to large-scale true orthophoto projects."
}