Page 669 



THE SMOOTH SHEET 

 7323. The Projection Lines 



7323 



The projection intervals between the meridians and parallels to be shown on a 

 survey sheet depend on the scale and shall be according to the following table: 



Table 24. — Projection line intervals for various scales 



87^ 



50' 



32° 15' 



Figure 145.— Referencing: 

 projection lines. 



The projection is constructed in pencil. It must be verified and all of the control 

 stations plotted by dms. and dps. (see 7411) and checked while the lines are in pencil. 

 Only then are the projection lines to be inked as fine solid black lines 0.15 mm in width. 

 The lines should extend entirely across the sheet. No borders are to be placed on 

 smooth sheets. 



The numbers representing the latitude and longitude shall be drawn in black ink 

 at the ends of each parallel and meridian. They should be 3 mm in height and should 

 be made in accordance with figure 145, preferably with a mechanical 

 lettering set. Those representing degrees shall be repeated every 

 fifth minute on scales of 1:20,000 and larger; every tenth minute on 

 a scale of 1:40,000; and every thirtieth minute on smaller scales. 

 For scales larger than 1:8,000, where the entire area falls within a 

 1 -degree interval, the degree number must be shown at least once 

 on the sheet, preferably near the center, the other lines being identi- 

 fied by the minutes and seconds only. The numbers must be fol- 

 lowed by the respective degree, minute, and second symbols. Degree 

 symbols should be 1.0 mm in diameter, and minute and second symbols 1.0 mm long, 

 all centered on line with the top of the numbers. The numbers and symbols must- 

 be checked. 



7324. Construction of a Polyconic Projection 



The construction of a polyconic projection is a comparatively simple problem, but 

 extreme accuracy and care are necessary. The following instruments are needed: 

 meter bar, beam compass, straightedge, dividers, and a quarter-meter scale (see 4821). 

 All of the elements are found in Special Publication No. 5, "The Polyconic Projection 

 Tables." The area to be embraced and the orientation of the sheet and the scale are 

 found in the layout of the project (see 136); the interval between projection lines to 

 be drawn depends on the scale (see 7323) . 



From, the layout locate the intersection of the most central meridian and parallel 

 which are to be shown, and the direction of the central meridian. The accuracy with 

 which this needs to be done depends on how near the edge of the sheet the work will 

 extend. If extreme accuracy is required it is generally necessary to lay off carefully 

 the limits of the sheet on a published chart of the area, thereby locating the central 

 intersection and orientation. In some cases even preliminary mathematical computa- 



