98 NAUTICAL CHART MANUAL 



CONSTEUCTION OF A MERCATOR PROJECTION 



To construct a Mercator projection, draw a straight line for a central meridian and then 

 draw a central parallel perpendicular to it. Each should be as central to the sheet as the se- 

 lected intervals of latitude and longitude will permit. To insure greater accuracy on large 

 sheets, the longer line of the two should be drawn first, and the shorter line erected perpendicular 

 to it. 



Example 



Required a Mercator projection for a new chart of Corpus Christi Bay, Texas, extending 

 from latitude 27°38' to 27°55'30"; longitude 97°00' to 97°27'; scale of middle parallel to be 

 1:40,000; projection interval 5 minutes, with construction line subdivisions 1 minute 

 apart. For convenience all measurements are determined in millimeters. 



THE MBBIDIANS OF LONGITUDE 



The middle parallel being 27°47', the length of a minute of longitude is found, by reference 

 to the Coast and Geodetic Survey Special Publication No. 5, page 64, to be 1642.65 meters 

 (the value of 5' = 8213.3 divided by 5 = 1642.65). At the scale of the chart, one minute of longi- 

 tude equals 1642.65 divided by 40,000=0.0410665 meters or 41.067 miUimeters. Lines repre- 

 senting 5-minute intervals of longitude (5X41.067=205.335 millimeters), and 1-minute subdi- 

 visions for construction purposes (41.067 millimeters) may now be drawn. 



THE PARALLELS OF LATITUDE 



The distance between parallels of latitude is obtained from C&GS publication Mercator 

 Projection Tables, Clarke Spheriod of 1866 (1955), page 7, by multiplying the differences between 

 any two parallels of latitude by the value of 1 minute of longitude. For example, the value 

 of a minute of latitude between 27°38' and 27°39' is 1.12283X41.067 or 46.111 millimeters. 

 The spacings for all desired intervals may be obtained in the same way. 



The latitude and longitude values thus obtained should be entered on Form C&GS-1016 

 "Mercator Projection Data on N.A. 1927 Datum." After the necessary data have been tabu- 

 lated, the required projection lines are drawn parallel to the central meridian and central parallel 

 as indicated in the illustration. When the Coordinate Plotter or Ruling Machine is used, the 

 sum of the intermediate distances should be tabulated as shown on the sample of Form 1016. 



PROJECTION LINES (Fig. 1) 



Projection lines are full lines extending from neatline to neatline. They are broken for 

 soundings, lettering, and other features which otherwise would be obscured by having the 

 projection lines drawn through them. The projection line intervals in Figures 4 and 6 of the 

 Appendix should be used in all cases, as they have been selected for a convenient span with 

 dividers. 



