An analysis of visually equal-stepped grey values for automated cartography.
Date
1970
Authors
Meinhardt, Aldy Walter
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Abstract
Automated mapping is a new frontier in cartography. Commercialized maps are for the most part still in the experimental stage of production because many standard cartographic procedures have yet to be mechanized. This study deals with the automation of areal symbols for the presentation of choropleth and isarithmic maps.
Areal symbols on an achromatic map are formed by a gradation of grey values ranging from white to black. To maximize map-reading efficiency, these grey values must be chosen carefully to allow an optimal number of values with optimal spacing between consecutive values. Thus, the selection of grey values is done in accordance with both psychophysical and cartographic rules.
Existing literature from the field of psychophysics is examined thoroughly for concepts that might apply to cartographic areal symbols. Because many past approaches to areal symbols in mapping have been based on experimental results from disciplines which have used non-cartographic symbols as stimuli, inconsistencies and unfounded assumptions have degraded the resulting cartographic grey scales.
This study investigates the relationships between physical measurement and psychological sensation which are applicable to cartographic areal symbols. A set of rules based on a cartographic parameter of "per cent ink", rather than the psychophysical standard of "per cent reflectance", is used as a basis for construction of an optimal grey scale series.
Before a series of grey values can be applied to a distribution map, the data must be generalized into a set of intervals that best portrays the distribution. Graphical and mathematical methods that deal with this difficult generalization process are discussed for both theoretical and practical applications.
Because the series of grey values derived in this study were drawn by the ink-line plotter, they reflect the mechanical-limitations of the plotter. Although the first grey series derived had correct grey values, the precision of the plotter produced a pronounced pattern to several of the tones. This necessitated development of several greys whose values deviated slight ly from the ideals, but were of a regular pattern. The final grey scale, therefore, is not ideal, but closely approaches the required per cent ink parameters.
An experiment was conducted to test the automated grey scale for equal visual sensation steps. The inconsistencies in the experimental results are reflected in the limitations of the ink-line plotter.
This study produces a workable approach for automated areal symbols, and derives a set of conditions for the construction of an equal-stepped grey scale applicable to cartography.