Page 679 



THE SMOOTH SHEET 



7363 



Great care must be taken to see that the correction is applied in the proper direc- 

 tion. The following rule will be found lielpfid in determining the direction: 



If the latitude (N.) and longitude (W.) on the old datum are greater than the corresponding 

 values on the new datum, then the new projection will be to the north and west, respectively, of the 

 old projection. If the old values are smaller than the new ones, it will be to the south and east. 



From this the direction of the correction for 

 other relationships can be readily determined. To 

 avoid errors of application in plotting, a sketch 

 should be made, showing the relationship of the 

 two datums to one of the selected triangulation 

 stations, with the corresponding latitudes and 

 longitudes indicated. 



The new datum should be marked by short 

 intersections at not less than two projection inter- 

 sections on the sheet and should be shown in 

 colored ink, preferably red. At one of the inter- 

 sections the name of the datum, the latitude and 

 longitude, the initials of the cartographer making 

 the correction and of the verifier, and the date 

 the correction was made, should be noted in colored ink. (See fig. 148.) 



Example: In figure 148, the latitudes and longitudes of A FRONT, 1918 on the old and new 

 datums are as follows: 



AFRONT, 1918 



N. A. 1927 Datum; 

 6/9/41 F.B.E. 

 ■ B, R. 



Figure 148.— Change of datum of survey sheet— by 

 numerical method. 



North Latitude 

 47° 25' 450 meters (old datum) 

 47° 25' 300 meters (N.A. 1927 datum) 

 150 meters difference 



West Longitude 

 124° 10' 900 meters (old datum) 

 124° 10' 750 meters (N.A. 1927 datum) 

 150 meters difference 



Applying the above rule, the new projection (N.A. 1927 datum) is found to be to the north and west 

 of the old projection. Therefore, the intersection of latitude 47° 25' N. and longitude 124° 10' W. 

 on the new datum is obtained by laying off the distances 150 meters (corrected for distortion) in a 

 north and in a west direction from the old datum. 



Rejerence station, on, the sheet. — If the latitude and longitude of one of the plotted 

 stations is recorded on the sheet (see 746), the' correction may be obtained by subtract- 

 ing from them the corresponding values on the N.A. 1927 datum. For field application, 

 where photographic copies of old surveys are used, the single correction thus obtained 

 is sufficiently accurate for use on the entire sheet, but in the Office where the original 

 sheet is available, additional stations should be used and a mean correction obtained 

 in accordance with the procedure outlined above. 



The numerical value shown on the sheet should always be checked against the 

 plotted position of the station before using it for determining a datum correction. 



7363. Graphic Method 



This method of correcting the datum on a survey sheet is applicable under three 

 conditions: (1) as an alternative to the numerical method, (2) where geographic 

 positions of the stations on the datum of the sheet can be found neither in the old 

 registers (see 7362) nor in any of the publications of the Bureau, and (3) where the 

 old registers are not available, as is usually the case in the field. 



Select three well-distributed stations on the sheet, the geographic positions of 

 which are available on the N.A. 1927 datum. Determine the dm. and dp. of each 

 station (see 7411) from the projection lines nearest to it (this will mean using the 



