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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.

3.6 Orthophoto manipulation and image analysis

At 1:5,000 resolution, the components of the stone circles are understandably poorly resolved, even when DPW processing is completed, but enhancement of the images using standard filters can dramatically improve both image quality and definition. Filtering is a standard image processing tool that essentially overlays a standard mathematical matrix to each pixel in the image or to groups of pixels to enhance specific features. Although it is possible to carry out this process using the DPW, it is also possible to use some standard graphics and image analysis packages (e.g. Adobe Photoshop, ENVI, HLImage ++, NIH Image (freeware Mac), Image Tool (freeware PC)) to obtain the same effect. There is little difference in the end product from these packages, although some (including the DPW) allow you to define your own matrix to overlay on the images, therefore giving more control on the final image produced. Fewer enhancements are needed on the 1:2000 scale images, as most image processing packages can readily distinguish between the stone and the base topography at this scale. Image analysis was carried out on a desktop system-based PC software package (HL Image++) to demonstrate the ease of transfer of information from the DPW and the results possible on desktop systems. The purpose of image analysis in this project was to provide a second and more readily transferable method of characterizing the stone dimensions and morphology to researchers using more routinely available systems. The two-dimensional image that is generated in the orthophoto generation stage of the DPW analysis is calibrated very accurately and the individual co-ordinates of the stones can be extracted on the DPW very rapidly. Image analysis packages can use this accurately calibrated image to produce a binary image of the stones themselves, without the associated terrain, which can be used in ground-based site surveying and examination. Furthermore, standard image analysis packages can carry out object analysis, extracting each stone and measuring a series of parameters that can be stored in a flat 2D database for correlation purposes with similar datasets generated at later times. This opens a significant number of avenues for routine data storage, comparison and processing to the remote researcher (i.e. one who does not have access to the DPW) and for the graphic representation of this data on intranets, WWW sites and integration into databases. In summary, for the purposes of spatial measurement and analysis of the individual stones the two processes of DTM generation and 2D image analysis provide two products:

  • True 3D representative model of the stones and terrain. The full details of the model can be rapidly extracted using the DPW and subsequently generated DTM's subtracted from the base model for accurate topographic change measurement.
  • 2D object analysis that rapidly demonstrates the change in overall 2D shape and size of the stones. This process can be carried out on a desktop computer system. The advantage in this output is that the end-user does not need the highly specialized DPW to carry out meaningful measurement and dimensional analysis on the stones.

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