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bit computer monitors exceeds the available range
of plotters. This, and other differences, requires
that thematic cartographers continue to identify
the correct output device when designing thematic
maps. The success or failure of the final map
depends on a faithful representation of the original
design. Using a more economical but less capable
plotter often does not allow a reader to correctly
interpret the hydrologic data and other spatial
information.
A number of recent technological innovations have
provided numerous economical plotters and on-
demand printers to scientists for visualization of
their data. Often these plotters and printers serve
to produce intermediate paper copy prior to
publishing the information using traditional or
digital lithography. This is because variations in
color occur due to different color models, lack of
device profiles, combination of plotter technology
with selected substrate, dot gain and other
differences between the plotting, imagesetting and
lithography technologies.
The continued advances in plotter and imagesetter
technology do not happen at the same rate as
advances in GIS or image interpretation sciences.
A basic procedure in map production is to
determine and understand what technologies will
be used to prini or image the final product. The
constant, and increasingly rapid, evolution in
plotting and printing technologies as well as the
mapping, remote sensing, and the
photogrammetric sciences requires continual
review and assessment of device availability and
planned procedures.
The cartography assessment project also has been
determining the process required to successfully
implement a GIS digital mapping system. Recent
tests have been conducted in the area of map
design as it relates to digital offset lithography
and print-on-demand systems. Several digital
lithography and print-on-demand tests included
some smaller format presses, in addition to
plotters, that allow imaging directly on the
printing drum. These test assessed document
management as well as correct visualization of
hydrologic data. Measurements of cost and quality
were made. Plotters offered costs of $2 to $30
per sheet for a 24 by 30 inch plot. Film negatives
and positives generally cost $25 or more per
square foot. Ink jet plots are generally the least
expensive plots, followed by electrostatic, dye-
sublimination, and film negatives, respectively.
The quality of each type of plot was generally in
alignment with the price -- the higher the cost the
75
better the quality. Ink-jet plot quality was
enhanced by using a better, but more costly,
substrate. The durability of the plot image should
be considered when selecting an output device.
The project is assessing the minimum resolution
standards that an imagesetter must meet to plot
digital orthophoto quadrangles and other raster
data. The data is often sampled to one meter
accuracy at 256 levels of gray or 8-bits of data
at a map scale of 1:12,000. A device resolution of
at least 2,395 dots per inch (94 lines per
millimeter) using a 150 line-per-inch screening
algorithm is necessary to correctly image 256
levels of gray. These numbers are based on the
generally used and accepted equation:
; : 2
device resolution
( ) "- 1
— shades of gray
line per inch
Device resolution is the maximum number of
addressable pixels per inch. For purposes of
this paper is will be considered that both
horizontal and vertical pixels per inch are
identical.
Line per inch is the number of printing dots per
inch. These dots are formed by imaging a
number of pixels in a given area to equal a
specified value . A fifty percent gray dot will
have one-half the pixels in a given area imaged
black with remaining one-half left white on
paper or clear on a film positive.
Many paper laser printers have the capability of
printing graphics at 300 or 600 pixels per inch.
Some newer laser printers have a device
resolution of 1,200 pixels per inch. It is noted
that some recent introductions have a claimed
1,200 or 1,800 pixel per inch “apparent” device
resolution. These may claim a higher visual level
of quality but may not produce visually accurate
results. Using the above equation and requesting a
150 line per inch screen for an map or image:
Device resolution Maximum gray levels
300 4
600 16
1200 64
Imagesetters can produce film ^ negatives or
positives varying in size from normal book
publications to large wall maps using resolutions of
up to 4,000 pixels per inch or greater. The final
use of the film also determines which way the
image reads relative to the film's emulsion. A film
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
