The draftsman should determine in advance the reduction for each drawing or for each group of drawings, in order that he may use the same size of letters or the same kinds of type for the lettering on a series of drawings that require the same reduction. The reduction should preferably be marked in fractions (as "1/2 off," "1/4 off" or "reduce 1/2," "reduce 1/4"), and the choice of the same reduction for a group of drawings will not only insure greater uniformity in the drafting and in the reproduction but will permit the drawings to be reproduced more economically, for the engraver can photograph them in groups instead of each one separately. The draftsman should therefore note and consider (1) the special features shown in the author's originals; (2) whether or not these features have been plainly indicated and whether the originals are complete; (3) the size of the printed page of the volume in which the illustrations will appear and the reduction required for each drawing; and (4) the process by which each drawing should be reproduced. If an original is of doubtful or uncertain interpretation or appears to be incomplete the draftsman should confer with the author of the paper if he is within reach or should bring the matter to the attention of the chief of the branch; otherwise he may waste much time in making the drawing. PREPARATION OF MAPS. PROJECTION. The base maps furnished by authors (see pp. 13-14) are prepared in many different ways and in different degrees of refinement and of crudity, but the work of redrawing them for reproduction involves well-established and generally uniform principles. All maps except those of very extensive areas should be based on a map projection which will show with a minimum of distortion the effect of the curvature of the earth. The polyconic projection (see fig. 5) is used for most Government maps. In this projection the central meridian is a straight vertical line, and each parallel of latitude is developed independently of the others. The mathematical elements of map projection are given in tables published by the Geological Survey[6] and the Coast and Geodetic Survey.[7] Figure 5, however, illustrates the mechanical or constructional features of the polyconic projection and if used in connection with the published tables will probably be a sufficient guide for projecting a map on any desired scale. [Footnote 6: Gannett, S. S., Geographic tables and formulas, 4th ed.: U. S. Geol. Survey Bull. 650, 1916. See also Gannett, Henry, Manual of topographic methods: U. S. Geol. Survey Bull. 307, pp. 85-86, 1906.] [Footnote 7: Methods and results: Tables for the projection of maps and polyconic development; Appendix No. 6, Report for 1884; Tables for a polyconic projection of maps, based upon Clarke's reference spheroid of 1886; 3d ed., 1910.] [Illustration: Figure 5.--Diagram illustrating method of projecting a map.]