7364 



HYDROGRAPHIC MANUAL 



Page 680 



back dms. and dps. in some cases), and with these vakies as radii from the plotted 

 position of the station as a center, swing short pencil arcs in the proper directions. 



Draw pencil lines tangent to these arcs and 

 parallel to the latitude and longitude lines on 

 the sheet. The intersection of these lines 

 will give the position of the new datum. 

 Carefully scale the offsets A<^ and AX from 

 the old datum on the sheet. (Fig. 149 illust- 

 rates the application of this method to the 

 example given in 7362.) 



Follow the same procedure with the 

 other selected stations and compare the offset 

 values obtained in each case. If they do not 

 differ by a plottable amount, accept the 

 intersections as determined, otherwise use 

 a mean of the values. Any appreciable 

 The method of designating the new datum on 



bcckdp A FRONT, 1918 



)n New Datum 



^Old Datun 



Figure 149.— Change of datum of survey sheet — by 

 graphic method. 



differences should be investigated 



the sheet is the same as for the numerical method. 



7364. Lost or Unrecoverable Stations 



Stations are sometimes lost or have become unrecoverable and cannot therefore 

 be included in the triangulation to connect them with the new datum. Where a survey 

 sheet contains such stations only, then the relationship between the old and new datums 

 must be obtained from stations on adjoining sheets for which the new datum values 

 are available. A mean of the corrections derived from the adjoining sheets should be 

 used. 



The present plan of the Bureau is to include, in the published triangulation data 

 for the coastal states, a list of lost stations with their geographic positions (unadjusted) 

 on the North American datum of 1927. While such values are not to be used for ex- 

 tending triangulatiou , they are adequate for coordinating old and new surveys. These 

 volumes should be consulted before applying datum corrections to a sheet. 



737. Projection Constructed After Survey 



It becomes necessary occasionally to execute a survey and complete the smooth 

 plotting prior to the determination of the geographic positions of the control points, as 

 where the local triangulation has not been connected to the main net of triangulation 

 or where astronomic observations have not been made. In such cases the control 

 points are plotted on the boat sheet and the smooth sheet either by angles or distances. 

 Many of the early surveys of the Bureau were made in this manner. (See 392 and 393.) 



In constructing a projection on a survey sheet that has no projection, the distortion 

 of the sheet is one of the important elements to consider and upon the accuracy of its 

 determination will depend the accuracy of the projection. Paper does not always 

 distort uniformly and the shrinkage or expansion should be determined in both a north- 

 south and an east-west direction and a factor applied to all measurements to be laid 

 down on the sheet (see 7114). It should be remembered, however, that the smaller the 

 distance to be plotted the less will be the error of distortion so that in laying down 

 projection lines from plotted triangulation stations, corrections may be avoided by 

 selecting stations close to the lines to be constructed. 



