Scanner resolution and cell size

[Rich Shepard]

  If this is not the proper forum for this question, please point me to the
appropriate mail list.

  My scanner has an optical resolution of 600 dpi x 1200 dpi and a plate
size of 8.5 inches x 14 inches. When I scan standard contact prints of
aerial photographs (9 inches x 9 inches), I capture an area 8.5 inches x 9
inches. Now, I want to calculate the cell size at different resolutions.

  First assumption: the cells are rectangular, not square.

  Second assumption: the maximum horizontal resolution is 600 dpi while the
maximum vertical resoltion is 1200 dpi.

  Third assumption: I cannot make square pixels/cells directly from the
scanner, but perhaps can in the software.

  Since the standard scale of aerial photos (at least in the western US) is
1 inch = 2000 feet (1:24,000), the horizontal extent of a scanned photograph
is 1700 feet x 1800 feet.

  If I set scanner resolution to 500 dpi I'm setting the horizontal pixel
density, and I will have a cell width of 4 feet (approximately 1.2 meters.)
But, the vertical pixel density will be 1000 dpi which works out to a cell
height of 2 feet, or approximately 0.6 meters.

  Although I have scanned aerial photos before for use as underlays to the
vector information, I've not scanned them for use as raster data (e.g.,
vegetation classification). Are my assumptions and calculations correct?
What do other folks do when using scanned aerial photos to define cell size?

TIA,

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
              Making environmentally-responsible mining happen. (SM)         
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 + 1 503-667-4517 (voice) | + 1 503-667-8863 (fax) | rshepard at appl-ecosys.com