Why Not Photography?

Photography has its weaknesses, too. The major disadvantages of using photography to assess threats over time include: photography usually has to be converted from hardcopy (analog) to a digital format, it has non-map geometry, and it has a relatively small footprint.

All historical photography (meaning all photography before the current cycle of acquisition) is analog. Thus, one of the first steps required to get this imagery into a GIS is to scan each photo. This is a time-consuming operation affected by scanner speed, scan resolution, and the number of photographs. Time and cost of this step must be considered for each project.

The same geometry that allows for stereoscopic viewing is often viewed as a disadvantage when trying to get our photo-derived information into a GIS. Photography has single-point-perspective geometry. This means that a vertical photograph may appear map-like, but, features at differing elevations are displaced relative to one another, and the scale changes with each change in elevation. Time and money are required to orthocorrect the photography (make the digital photo imagery accurately conform to a map coordinate system and projection).

A common trade-off with the various remote sensing systems is between footprint (area covered) and spatial resolution—photography is no exception. In general, the larger the area that is covered, the coarser the resolution. This holds true when comparing photography of different scales, and it is generally true when comparing photography to most satellite imagery. Compared with satellite imagery, photography is usually more resolute but covers a much smaller area. Given the time and cost required for scanning and orthocorrection of historical photography, the small footprint constrains the applicability of using photography to relatively small areas or to sampling over very large areas for detecting, assessing, and monitoring changes.