16 



Old sheets on which no recent datum has been indicated can sometimes 

 be adjusted by the use of stations which are common to another sheet of a 

 later or earlier period or between the topographic and hydro graphic survey 

 of the same period on which a datum is given. 



Photostat or bromide copies of field sheets can be obtained. Both 

 methods of reproduction are subject to distortion and are difficult to use 

 for precise purposes. Protective coverings of silk, acetate, etc., have 

 been applied to some of the originals and these sheets do not copy well. 

 Many lines were inked so finely they are lost in reproduction and inasmuch 

 as these are the base to which corrections for datum are applied, their 

 ommission is an important factor. 



The importance of getting the surveys properly adjusted can be seen 

 by observing Figure 1; section (a) shows a section of a survey which has 

 been superposed without adjustment, section (b) shows the same area when 

 the adjustment has been applied. 



The use of old editions of published charts as a substitute for the 

 original field sheets in compiling changes in shore lines and depth curves 

 is to be discouraged unless it is done by someone thoroughly familiar with 

 the methods is use in the various periods. The projection shown on such a 

 chart will be on the datum currently in use at the time of publication. 

 Several governmental departments other than the Coast Survey furnish data 

 that are used to revise charts, particularly at inlets and harbor entrances 

 where from a navigational standpoint the changes are sufficiently important 

 to warrant a new edition being published. Except for the local revised 

 areas the information shown on the new edition is based on the previous 

 general surveys of the area and does not reflect conditions as of the date 

 of publication. The Coast and Geodetic Survey keeps a record of surveys by 

 other agencies that are used to revise charts and while it is possible to 

 break the edition down to show the date, source, and extent of all revisions, 

 it is only on rare occasions that this is justified. Working from the field 

 sheets is much to be preferred. 



In the use of historic hydrographic surveys, an evaluation of the depth 

 curves should be made based on the methods used in the different periods. 

 The earlier surveys were made using sailing ships and handpulled boats. 

 The depth curves are of necessity generalized and do not, in most cases, 

 delineate the bottom conditions sufficiently for relative comparison with 

 modern surveys. 



It is also well to keep in mind that surveys by the Coast and Geodetic 

 Survey are made primarily for use by mariners. It is the practice of 

 cartographers to include representative soundings in the placement of the 

 depth curves, whereas the engineer usually draws the depth curves through 

 the representative sounding. Ythen engineer surveys are used in conjunction 

 with navigational surveys an erroneous movement of the depth curves may be 

 indicated by reason of the methods used. 



