, for example) should be given on all except the more simple kinds of maps, and the date of publication should also appear just below the scale or scales. The single-line bar scale should be used only on small or simple maps. The length of the bar scale must depend on the size of the map and the space available. Those shown in [figure 7] were made over blue prints from scales used by the Survey.
To make a bar scale for a map of unknown scale that shows only a single meridian and parallel, or for a map on which no meridians or parallels are shown, first ascertain the distance between two points shown on the map by reference to other authentic maps. If, for example, the distance between two such points is 16.315 miles draw a horizontal line (a in [fig. 8]) representing this distance on the map, and at its end, at right angles to it, draw another line (b) actually measuring 16.315 units of any convenient denomination. Draw a straight line (c) diagonally between the ends of lines a and b. Then set off on line b any convenient number of the units selected, say 5 or 10, and project from the points set off lines exactly parallel with line c to line a. The distance and the number of the units thus marked on line a will indicate the number of miles covered by that distance on the map, as shown in [figure 8].
Figure 7.—Designs for bar scales.
Figure 8.—Method of making a bar scale for a map of unknown scale.