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   Data Source
   Ground Survey
   Image Prep.
   Digital Terrain Model
   Orthophoto manipulation





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The Application of Digital Photogrammetric Techniques and Aerial Photography to the Preservation of Archaeological Detail.

3.5 Generation of the Digital Terrain Model, Orthophoto and Perspective Scene for the Copney Site using 1:10000 scale imagery

The DPW's Automatic Terrain Extraction Feature was used to produce a digital terrain model for the Copney section of the images. The digital terrain models may be generated at a range of accuracies, controlled by the selection of post spacing by the user. A dense post spacing (say 1m) will produce a very detailed and accurate three dimensional model of the terrain. The DTM forms the basis of all further processing of the image. It is critical, therefore that this be as accurate a representation of the terrain as possible. Once generated, the DTM is readily storable for further comparison with successively generated DTM's of the same area to allow determination of change in the area through time. Furthermore, once the DTM is generated, Socet Set allows the user to generate a range of visually dramatic and accurate representations of the site, as well as generating extremely accurate base maps of the site. Some of the features available to the researcher at this point are:

  • Contoured base map generation. It is possible to generate contoured maps of the site, or it's underlying terrain, with very accurately places contours (the spacing of the contours is controlled by the spacing of the post spacing on the DTM).
  • Orthophoto generation. An orthophoto represents the image that would be observed on the area if the observer were looking at the site from an infinite distance away. In a normal aerial image, the position immediately beneath the camera focal point is undistorted, but as we travel across the image away from the camera's focal point, the image becomes increasingly distorted. Over a large area, the effects of this distortion can dramatically reduce the accuracy of measurements made on the image. Orthorectification removes this distortion and generates a true image on which we can base spatial measurements. Additionally, the process removes some of the shadows cast by the stones that can cause problems during further image processing. The orthophoto is generated not from the image, but from the DTM. There is less of an effect visible in the small area we are dealing with but the process is essential if spatial measurements and positions are to be accurately constrained.
  • Perspective Scene generation. Socet Set allows the generation of Perspective Scenes from any angle or height in the area of interest. Further, it is a relatively simple process to add structural features to the DTM using the feature extraction and editing facilities of the DPW. For example,
    • the suggested elevated walkway to allow visitor access to the site can be added to the DTM and viewed using the Perspective Scene feature to decide on architectural design and engineering considerations
    • a full or partial screen over the site can be added to show the visual impact of such a structure on the surrounding terrain etc.

The Perspective Scene module can also be used to generate fly-throughs of the area along specific flight paths and heights to give visitors who cannot access the site some feel for its structure and beauty. Perhaps the most interesting possibility from this technique is the generation of a 3D model that can be added to web sites for remote access to the site. The basis of 3D reconstruction is the provision of images from specific heights and angles of the intended area to be modeled. The Perspective Scene module can provide all of these images from a single set of aerial photographs. Export of images from the system is a simple procedure and can be carried out in most main image formats.

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