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 expense. Sometimes only one map is available when a variety of plans must be presented. In such cases, it is not feasible to draw plans for new construction work, or alternative working schemes, on the map itself. Most draughtsmen put the map on a large drawing board and then draw the new constructions on separate sheets of tracing cloth. The tracing cloth is not very transparent, however, and it will not help greatly to get a project voted upon favorably if the plan must be presented to a board of directors on tracing-cloth drawings with the map itself only very dimly visible through the tracing cloth. A much more effective scheme is to use a sheet of almost transparent celluloid made with a rough surface that will take drawing ink fairly well. The different alternative plans may then be drawn on the celluloid in colored inks, and the different sheets of celluloid superimposed upon the map one by one as quickly as any executive committee or board considering the proposals may desire. The main difficulty with the celluloid method is that the inks will rub off if the celluloid is handled too much with moist hands. Ease of erasure has some advantages, however, for tentative schemes can be rubbed out or changed at will, simply by using a damp cloth. After the final adoption of the plan, tracing cloth can be laid over the combined map and celluloid sheet, and the important features of both may be traced for blue-printing or for general reference later. Where maps must have constant use, it is sometimes convenient to have them mounted between two sheets of clear celluloid. The celluloid-manufacturing companies are prepared to do this kind of mounting to order.

Engineering Record

Fig. 169. Drainage Area of the Canadian River, New Mexico

Areas of different kinds may be distinguished on maps by various classes of shading when color printing is not available

Contrasts in map areas as shown in Fig. 170, by superimposing one portion of the world on top of another portion, or by placing two portions side by side on the same scale, are of considerable value.