----------------------------Original message----------------------------
 
REF:  Date:    Sat, 20 Aug 1994 17:48:25 EDT
      From:    "Brill, Margaret" <[log in to unmask]>
      Subject: DOQs
 
----------------------------Original message----------------------------
 
    I attended a seminar on Digital Orthophotography a couple of weeks ago,
sponsored by the NC Center for Geographic Information and Analysis.  I learned
that although NC was flown last year by NAPP, the $2m contribution which the
state must make in matching funds for the USGS cooperative program has not yet
been approved.  There is some opposition to the program on the grounds that
1:12,000 is not detailed enough coverage.  . . . [text deleted]
 
Margaret Brill
Duke University
Durham, NC
[log in to unmask]
 
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Margaret --
 
        Reference the detail in the USGS DOQ's.  The 1:12,000
scale that you refer to in you msg is just "loosely" connected to
the detail contained in the DOQ files.  Specifically, 1:12,000
scale is the printed scale for map or image products based on the
"quarter-quad" (3.75' or one-quarter of a standard 7.5') map
area.  The DOQ's are generated to cover a quarter-quad plus a bit
of "overedge."
 
        The standard DOQ digital image files have pixels repre-
senting 1 meter on the ground.  When those files are displayed on a
high-resolution (i.e., standard VGA or work-station resolution of
75 to 90 pixels per inch) image display at full resolution, the
displayed scale is in the range of 1:3,000.  In other words, there
is much more detail in the DOQ than would be expected in a
1:12,000-scale printed product.
 
        Try measuring the length of your display line on your
CRT; it probably measures about 9 inches if you have a 14-inch
(measured diagonally) CRT.  At a resolution of 640x480 pixels on
the display, the DOQ would display 640 meters in 9 inches,
which approximates a scale of 1:2,789.
 
        The upshot of all this of course is that the DOQ's can be
thought of as "1:3,000-scale" products from the standpoint of the
details they contain and the methods in which they generally will be
displayed.
 
        Hope this information is useful to interested parties.
 
                                        -- Clark
                                           [log in to unmask]
                                           opinion is personal