Chapter III: Softcopy Photogrammetry and Desktop Mapping

42 downloads 771 Views 2MB Size Report
computer-based, softcopy photogrammetric mapping and GIS systems that operate in a ... Top quality scanners designed specifically for digital photogrammetry.
Using Scanned Aerial Photographs1 by Roy Welch and Thomas Jordan Center for Remote Sensing and Mapping Science (CRMS) University of Georgia - Athens, GA 30602

Introduction Aerial photographs are a major data source in the construction of geographic information system (GIS) databases. With the introduction of computer-based, softcopy photogrammetric mapping and GIS systems that operate in a desktop computing environment and are able to manipulate both vector and raster data, the demand for images in raster (digital) format has increased sharply (Welch, 1989, 1992; ASPRS, 1996). While the intent of this chapter is to provide guidelines related to the use of aerial photographs in digital format, it must be stressed that aerial photographs in hardcopy format may prove to be a more economical solution for many mapping and GIS database construction tasks.

Conversion of aerial photographs to raster format Aerial photographs recorded with standard mapping cameras must be scanned and converted to raster format before they can be used in digital image processing/mapping environments. Decisions made concerning the scanning process will determine, in large part, the geometric accuracy of the scanned photographs, the amount of information that can be extracted and the speed with which the images can be processed. The basic function of a scanner is to convert a hardcopy photograph to digital format (that is, softcopy format). Scanners are designed to capture image data in reflection (paper print) and/or transmission (film transparency) modes. Because film transparencies tend to have higher spatial resolutions and a greater range of gray values than paper prints, they are the preferred source material when converting aerial photographs to digital images. Low-cost, flatbed scanners (