286 



HYDROGRAPHIC MANUAL 



both the geographic position and geographic, 

 datum are listed. 



For a more detailed discussion on datums, 

 the spheriods adopted for survey and chart 

 projections, and the earth's geoidal surface, 

 refer to the discussion, "Geographic Datums 

 of the Coast and Geodetic Survey," by A. L. 

 Shalowitz, published in the Field Engineers 

 Bulletin No. 12, December 1938, U.S. Coast 

 and Geodetic Survey. 



6-103 Datum ticks. — Before comparing 

 surveys of various years the geographic da- 

 tums must be correlated. It f' all .aver be 

 assumed that an unlabelled projection on a 

 survey is on a certain datum. The difference 

 in datums, if any, shall always be deter- 

 mined before comparing surveys or trans- 

 ferring data. 



The U.S. Standard or North American 

 Datum is shown by a complete projection in 

 color on many of the older surveys. Many 

 others show only one or two marked pro- 

 jection intersections (ticks) on the above 

 datum or the NA Datum of 1927 (Fig. 86). 

 These conditions reflect the specific needs 

 and time available for application of the 

 datum correction. If the prior survey does 

 not show the NA 1927 datum, this datum 

 shall be established. At least three widely 

 separate triangulation stations on the sheet 

 are selected, whose geographic positions on 

 the NA 1927 Datum are available. Two 



26' 



AFRONT. 1918 



124°10' 



N. A, 1927 Datum ^Jm,i7,°^ ^' 

 6/9/41 F.B.E. 

 -B.R. 



Old Datum-'^ 



124° 10' 



Figure 86. — Change of datum of survey sheet- 



methods can be used for determining the 

 datum differences. 



In the first, the geographic position data 

 used in plotting the original survey are de- 

 termined from old registers of the Nautical 

 Chart Branch, or records or publications of 

 the Bureau. The mean of the differences be- 

 tween values on the two datums is the cor- 

 rection to be applied to the projection on 

 the survey. The differences for the three 

 stations should nearly equal each other. If a 

 wide variance is found, an investigation 

 should be made for possible errors in compu- 

 tations or for failure to identify common 

 stations on the two datums. The position of 

 the NA 1927 Datum relative to the original 

 projection can be determined by back plot- 

 ting the NA 1927 values from one of the 

 triangulation stations or by following the 

 rule that if the latitude (N) and longitude 

 (W) on the old datum are greater than the 

 corresponding values on the new datum, the 

 new projection will be north and west, re- 

 spectively, of the old projection. If the old 

 values are smaller than the new ones, it will 

 be the south and east. 



The second method is applicable where the 

 geographic position data used in plotting the 

 selected triangulation stations on the prior 

 survey are not available. A graphic method 

 of determining the datum differences is used. 

 Arcs based on the NA 1927 values of latitude 

 and longitude are swung from the selected 

 triangulation stations shown on the survey 

 and tangents to the arcs are drawn parallel 

 to the projection. The datum differences are 

 scaled in meters between the tangents to the 

 arcs and the original projection. An average 

 of the differences is used in plotting the 

 datum tick. 



In applying datum ticks to old surveys ex- 

 treme care shall be taken in determining the 

 proper relation of the NA 1927 datum to the 

 original projection. Distortion in the origi- 

 nal projection shall be measured and prop- 

 erly applied to all distances measured or 

 plotted. Plotting of the datum tick in col- 

 ored ink shall be verified by another person. 

 Both shall initial and date the plotting be- 

 side the tick. The tick shall be identified by 



