In projecting a map first select a convenient measuring scale for setting off the dimensions given in the tables, or if no scale is at hand one may be constructed. Measuring scales are made, however, bearing divisions for miles and kilometers and finer subdivisions of 6 to 100 parts. They include the ratios of 1:31,250, 1:31,680, 1:48,000, 1:62,500, 1:63,360, 1:125,000, 1:250,000, 1:500,000, 1:1,000,000, and others. On a map drawn on the scale of 1 to 63,360, for example, 1 inch would represent 1 mile; on a map drawn on the scale of 1 to 1,000,000, 1 millimeter would represent 1 kilometer, and so on. It will be seen that the use of a scale that shows in ratios, such as those just given, the actual distance on the ground as compared with the unit representing the same distance on the map will reduce the possibility of error.
The method of projecting a map, illustrated in the accompanying diagram ([fig. 5]), is as follows: First draw a straight vertical line (A) through the middle of the sheet to represent the central meridian of the map and a line (B) at the lower end of this line exactly at right angles to it to represent the bottom of the map. Then set off on the line showing the central meridian the distances between parallels given in Table 6 on page 36 of "Geographic tables and formulas" (Bull. 650). It should be noted that the figures in these tables give the distance, in meters and statute miles, of 1° on a meridian measured 30' each way from a point where the meridian is intersected by a parallel. The exact distances between parallels as measured on the ground are given in the Coast and Geodetic Survey tables, or they may be computed from Table 6 of "Geographic tables and formulas" by adding the sum of the figures given for any two latitudes 1° apart and dividing by 2.
The distance between parallels that are 2° apart, as shown in the diagram, may be computed from Table 6 of "Geographic tables and formulas," as follows:
| Meters. | Meters. | |
| 1° of latitude on 37th parallel | = 100,975.1 ÷ 2 = | 55,487.5 |
| 1° of latitude on 36th parallel | = | 110,956.2 |
| 1° of latitude on 35th parallel | = 110,937.6 ÷ 2 = | 55,468.8 |
| True distance from 35° to 37° latitude | = | 221,912.5 |
The distances given in the diagram were obtained by adding the figures given in the Coast and Geodetic Survey tables, which yield the same results. Other tables in Bulletin 650 give the true distances in inches on maps of certain standard scales.
Through the points thus obtained on the central meridian draw lines at right angles to the vertical line. Along these horizontal lines lay off the dimensions in the column headed X, Table 6 (pp. 39-47) of "Geographic tables and formulas" as required for each individual map—in the diagram every alternate degree. Draw vertical lines at these points and set off the distance Y in the same table in a similar manner, and the points so found will be the points of intersection of the respective meridians and parallels. Figures are given on the diagram for the thirty-fifth parallel only.
DETAILS OF BASE MAPS.
Anyone who attempts to draw a base map must, first of all, know how each feature or part of the map should be represented. Most of the conventional symbols for features shown on base maps are well established and should invariably be used; for instance, a line composed of alternate long and short dashes (not dashes and dots) represents a county boundary, and a line or two parallel lines across which short lines are drawn at regular intervals represents a railroad. If he finds that two or more symbols have been widely used to represent the same feature the draftsman should select the one that is best suited to the map in hand. The correct forms of the conventional symbols or features to be used in preparing miscellaneous maps are shown in [Plate IV], but the size and weight of each line or symbol must depend on the size and character of the map.
TRANSFERRING OF COPYING.
TRACING.