736 HYDROGRAPHIC MANUAL PaGE 678 



the positions. The purpose of inking is to give permanence to selected arcs, so as to 

 facilitate future identification of the positions. The smooth-sheet plotter should bear 

 this in mind when selecting the arcs to be inked. In general, a spacing of about 3 inches 

 between inked arcs will be satisfactory. Only those portions of the arcs in the sounded 

 area should be inked. 



736. Changing Datum of Survey Sheet 



To correlate the results of two surveys, it is first necessary to bring the two to the 

 same geographic datum (see 2171). This makes it possible to compare corresponding 

 areas by means of the projection lines on the sheets. 



Practically all of the recent surveys of the Bureau in continental United States are 

 based on the North American datum of 1927, but many prior surveys are still on 

 the North American datum or some independent datum. To change the datum of a 

 survey sheet the usual practice is to apply corrections to the projection on the sheet. 

 When copies of old surveys are furnished to field parties, the latest datum is usually 

 shown on the sheet. The procedure given below describes how this may be accom- 

 plished under various conditions. Where the datum on a survey sheet has to be cor- 

 rected in the field, the data available will generally determine the method to be used. 



There are two methods of applying a datum correction to a survey sheet, a numeri- 

 cal one (7362) and a graphic one (7363). 



7361. Distortion Factor 



Before datum corrections can be applied, the distortion factor of the sheet must 

 be determined in both a north-south and an east-west direction. This is done by 

 comparing the scaled distances between projection lines on the sheet with the corre- 

 sponding values given in Special Publication No. 5, "Polyconic Projection Tables" 

 (see 7324). The distortion factor is determined from the relation: 



Tabular value— Scaled value 



Tabular value 



= ± Distortion factor 



Several distances in each direction should be measured in order to obtain a mean factor 

 and to ensure against errors m the original projection. The distortion factor should 

 be applied to every distance that is to be plotted on the sheet. To reduce measure- 

 ments on the distorted sheet to true values the following relation should be used: 



Tabular value-Scaled value ^ ^ ^^^^^^^^-^^ ^^^^^^ 

 Scaled value 



7362. Numerical Method 



In the numerical method three widely distributed triangulation stations on the 

 sheet are selected, whose geographic positions on the N.A. 1927 datum are available. 

 Identify these stations on the datum of the projection in the old registers in the Surveys 

 Branch of the Chart Division, or in any of the publications of the Bureau, and check 

 their geographic positions with the positions plotted on the sheet. The mean of the 

 differences between the values on the two datums is the correction to be applied to the 

 projection on the sheet. The differences for the tlnee stations should nearly equal each 

 other. If a wide variance is found, an investigation should be made for possible errors 

 in the computations or for failure to identify common stations on the two datums. 



