RULES AND PRACTICES 13 



PANTOGRAPH 



The formula for the pantograph setting to be used in making a reduction is as follows: 



bX960 



s= 



a 



a^distance between any two points as scaled from the 



sheet to be reduced. 

 b= distance between same two points as scaled on the 



drawing. 

 s=setting for each of the pantograph arms. 

 960 = factor of pantograph. 



Limit of reduction by the pantograph is about 1/20. If the setting is less than 45.0, which 

 is too small to be set on the pantograph arms, two reductions must be made. If the first 

 reduction is J^ or 1 to 4, 



the first setting will be Si = 960-^4 = 240 

 the final setting will be S2=sX4. 



II 



Specifications and Source Material 



CHART SPECIFICATIONS 



When a new chart or the reconstruction of an existing chart is assigned, Form C&GS- 

 8324, "Chart Specifications" containing the general specifications for the chart will be 

 furnished. It includes the title, limits, size, scale, kind of projection, plane of reference and 

 other detail. The specifications must be followed carefully unless an amendment is authorized 

 by the Chief of Division. 



EVALUATION OF CHARTING MATERIAL 



Charting material consists principally of topographic and hydrographic surveys made by 

 this Bureau supplemented by miscellaneous surveys made by other organizations. All material 

 must be examined critically, and particular attention must be directed to the actual date of 

 survey, geographic datum, depth unit, plane of reference, purpose and character of the survey, 

 and whether original or compiled. The latest information does not necessarily supersede all 

 earlier data. In unchanging areas, all well-controlled hydrographic surveys of other organi- 

 zations should be combined with the basic surveys of this Bureau. In localities under constant 

 and extensive change, only the latest information should be used. Shoal soundings over 

 obstructions or rocks must not be rejected without convincing proof that the dangers have 

 been removed. In regions where some areas are undergoing change while other areas remain 

 unchanged, partial rejection of the various surveys may be necessary. The proper acceptance, 

 rejection, and coordination of available information constitute the supreme test of the 

 cartographer's skill. 



