Map pins and tacks of this sort placed upon the surface of a map can give a spot map with any desired diameter of spots, no matter what size of map is used or what the amount of photographic reduction may be. It is simply a question of selecting from the spherical heads, 8 or 10, and the flat heads 18, 22, and 24, to determine which size head is best suited to the size of the original map and to the size of dot desired in the finished illustration. Large celluloid-covered tacks 18, 22, and 24 are also valuable to show the location of main offices and different factories, or the locations of particularly important distributing points. The pins numbered 8, 10, 18, 22, or 24, are five different sizes of pins which may be used simultaneously on the same map to show different degrees of importance in the things represented.

When used for photographing to produce an illustration such as is shown in Fig. 196, lettered or numbered pins should have a black background so that the black circle outlining the tack head will show out in clear contrast against the map itself. This requires white figures on a black or red background. Pins having red letters on a black background cannot be used for photographing, as the red letters would photograph black and disappear entirely, leaving a solid black circle instead of a circle with figures. Since pins with a black background and white figures are not commonly obtainable, it may occasionally be necessary to use pins like No. 20 in Fig. 199 having black figures on a white background. Sometimes it may be feasible to draw an ink-line circle around each number which appears in the photograph so that the circles will be on the copy sent to the engraver who makes the zinc plate.