negative generally requires the image be wrong-
reading when the emulsion is up on the film for
creating a printing plate but right-reading when
being used for creating an intermediate composite
with other negatives.
Using an imagesetter, devices able to image a file
to film and generally having a higher resolution,
for preparing originals for lithographic offset
printing a 150 line per inch screen is used for
comparison to the above example.
Device resolution Maximum gray levels
1270 72
1800 144
2395 256
Relatively new digital lithographic offset presses
are available for smaller-format printing with
resolutions of 1,270 and 2,540 pixels per inch.
It is possible to achieve a higher number of gray
levels by lowering the number of line per inch
screen used for plotting or printing. However,
this lower screening value results in an increas-
ingly coarser appearing image. A 120 line per
inch screen is used for comparison to the 150 line
per inch printing screens used in the above
examples.
Device resolution Maximum gray levels
1270 142
1800 225
1916 256
A can be seen, it is possible to maintain the
number of maximum gray levels with some
sacrifice of visual quality of the printed material
by lowering the lines per inch of the printing dot
screen.
Another factor that is required to be considered
when deciding on device resolutions, line per inch
screens and gray levels is the resolution of the
GIS, satellite, or photogrammetric raster data.
Data sampled and stored at one meter resolution
would require an extremely high resolution
plotting or imaging device to support complete
visualization of this data when plotted or imaged
at full scale. For instance, U. S. Geological
Survey DOQQ (digital orthophoto quarter-
quadrangle) data is captured at 1:12,000 with a
one-meter raster cell sampling rate. One meter?
at a map or image scale of 1:12,000 is equivalent
to an area of approximately 0.003 by 0.003
inches when plotted. The equivalent area of a
printing dot on a plotted or printed map using a
76
120 line per inch screen is 0.008 x 0.008 inches.
A visual loss in data accuracy is caused by one
printing dot taking the physical paper space of
more than four pixels of image map or raster
data. Therefore, the final plotted or printed
material is not a completely accurate representa-
tion of the data used for analysis if a fine screen
is not used for plotting the raster data.
Tremendous effort normally is expended in the
capture, processing, and storage of data. A high
level of effort and expense is also required for
plotting or printing images. It is suggested that
some data reduction or re-sampling be done before
requesting a plot of raster data. Re-sampling will
greatly reduce the processing time required to
create a plot file, reduce storage space require-
ments, and improve the probability of the image
being created correctly. Greater pixel-depth
improves color fidelity but also increase storage
requirements.
Many software packages also have a default
threshold for plot file resolutions. For example,
one commonly used GIS software plotting package
defaults all data to 500 pixels per inch. This often
lowers the resolution of the information without
the user being aware of the change in the data.
This is also a factor for vector data as the
precision of the locations of vertices and nodes is
rounded down. For raster data, table 1 shows a
comparison of printing dot sizes versus distance
on the ground at common U. S. Geological Survey
map scales.
The optimal bit-depth for data depends on the final
device used for presentation. Digital offset
presses, as well as slides and some monitors,
require 24-bit data. Images for offset presses
should be converted to CMYK (the four-process
colors for printing). Data can be 16-bit for
plotters since most plotting devices cannot image
24-bit data.
Maps and images often serve as a final visualiza-
tion and documentation of a GIS project. The
effort necessary to create a visually and
technically correct image of the data sets need not
be intimidating if the required questions and steps
are determined during the planning phases of the
project. Thematic cartographers experienced with
digital techniques may also be a source of support.
The initial effort may seem time-consuming but
becomes easier each plot or printed map.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
Table
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