Page 667 the smooth sheet 732 



not only harmful to the paper, but results in a curve set in the paper, which makes 

 plotting- and inking unnecessarily difficult. If the smooth sheet must be rolled at any 

 time, it should be rolled loosely to a diameter of not less than 6 inches with the cloth- 

 backed side of the paper on the outside. 



Under no circumstances should the smooth sheet be allowed to extend over the edge 

 of the table during plotting. This subjects the sheet to permanent creasing or bending 

 which may result in weakening the paper to such an extent that it eventually cracks or 

 breaks. 



732. The Projection 



The graphic record of a hydrographic survey is a smooth sheet, which in effect is 

 a map of the area surveyed. A map is a flat-surface representation, at a given scale, of 

 some part of the curved surface of the earth. On a map the features of the earth's 

 surface are represented with reference to each other according to some system which 

 will simulate their relation in nature. In order to accomplish this best, a projection 

 is used — ^that is, an orderly arrangement of the terrestrial meridians and parallels on 

 a plane surface. 



No matter how small a part of the earth's surface is under consideration it is 

 still curved, and theoretically cannot be represented on a plane with perfect accuracy. 

 As the area increases, the distortion or the departure from nature becomes greater when 

 an attempt is made to show it on a plane surface. It follows consequently that the 

 meridians and parallels cannot be shown on a plane exactly as they are on the earth. 

 Projections can be constructed in which some one or more desirable properties are 

 mathematically exact but only at the expense of other properties. Therefore, every 

 form of projection is at best a compromise. For practical purposes, however, it is found 

 that as the area considered is reduced it eyentually reaches a size so small that it can be 

 represented at a given scale on a plane surface without any plottable error. 



The subject of projections is treated extensively in Special Publication No. 68, 

 Elements of Map Projection. 



7321. The Polyconic Projection 



Various projections have been adopted by different countries and different survey- 

 ing agencies within each country on which to plot the results of their surveys. The 

 Coast and Geodetic Survey has adopted for all surveys a projection, known as the Coast 

 and Geodetic Survey polyconic projection. All surveys must be plotted on this projec- 

 tion. 



This polyconic projection was devised by Ferdinand Hassler, the first Superintend- 

 ent of the Coast and Geodetic Survey. It is extensively used for surveys of compara- 

 tively small areas, such as those covered by the hydrographic and topographic surveys 

 of the Bureau, because it effects a satisfactory compromise with all of the most desirable 

 properties of map projections, because of its ease of construction, and because a general 

 table for its use has been calculated for the entire spheroid. The projection data are 

 based on Clarke's reference spheroid of 1866 and are contained in Special Publication 

 No. 5, Tables for a Polyconic Projection of Maps and Lengths of Terrestrial Arcs of 

 Meridian and Parallels. This publication is familiarly known by its short title, "The 

 Polyconic Projection Tables." 



The polyconic projection is the development of an area by means of successive 

 cones tangent to the surface at successive parallels. The central meridian of any 



