MAP SYMBOLS: EQUAL-APPEARING INTERVALS FOR PRINTED SCREENS1

Abstract

A common problem in map design is the one of selecting screens with which to show a scale of values. The screens should, as closely as possible, give a visual impression commensurate with the values they represent. A typical example would be a rainfall map with steps of 10, 20, 40, and 80 inches. How dark should each screen be to give the truest visual impression of these values relative to each other? The purpose of this study was to determine what values printed screens should have to give even-appearing visual steps from white to full color. Tone symbol tests were made with black, and, in order to see if colored screens behaved the same as black ones, for the colors red, orange, yellow, green, blue, and brown.2 Tests were also given to see if the pattern of the screen would affect its visual evaluation, and tests were given to see what would be the effect of changes in the end points of the screen series. The first problem in preparing these experiments was to draw screens that would print a known percent of the total area with ink. It was desirable to have screens with a wide range of tone. Two patterns were used, parallel lines and dots arranged in a triangular grid. These screens were drawn with the values shown in Table 1, the value representing the percent of the paper that is covered with the