7413 HYDROGRAPHIC MANUAL PaGE 686 



These scales may be used satisfactorily for plotting stations on scales of 1:10,000 

 and 1 : 20,000. They are not particularly useful for other scales. Their chief advantage 

 is in plotting on sheets that contain some distortion, and in checking previously plot- 

 ted positions. Tests in the Washington Office indicate that geographic positions can be 

 plotted with these scales with approximately the same accuracy as is attained by the 

 use of a beam compass or dividers, and can be scaled or checked with considerably 

 greater speed by one accustomed to their use. 



7413. Plotting by Transfer 



The stations located by topography are usually placed on the smooth sheet by 

 transfer from the topographic sheet. Ordinarily the topographic survey has imme- 

 diately preceded a hydrographic survey' and its scale has been determined by the de- 

 sired scale of the hydrographic survey. In such cases, the scales are identical and, if 

 there is no distortion in either sheet, the most expeditious method of transferring the 

 positions of the signals to the smooth sheet is by tracing paper. Tracing cloth should 

 not be used for this purpose since it is subject to larger dimensional changes than the 

 paper. 



The tracing paper is securely fastened over the topographic sheet and a needle, or other fine 

 point, is used to prick the exact positions of the topographic stations and the adjacent projection inter- 

 sections through the paper; the shoreline and topography are usually transferred at the same time 

 (see 7561). The stations and the intersections are temporarily identified by symbols or otherwise in 

 pencil. The tracing paper is then superposed on the smooth sheet with the corresponding projection- 

 line intersections in coincidence, and the positions of the topographic stations are pricked through the 

 holes in the tracing paper into the smooth sheet, and temporarily marked in pencil. 



If there is distortion in one or both of the sheets, the projection intersections will not all coincide. 

 In such a case the tracing paper must be adjusted for each quadrilateral so that the difference between 

 the two projections is proportioned for each station plotted. Sometimes the tracing has to be shifted 

 several times, as where stations are located in different parts of the quadrilateral. Even if there is no 

 apparent difference between the two projections the coincidence of the adjacent intersections should 

 be verified before the stations are pricked through. 



Where shoreline and other topographic detail are also to be transferred, it may be 

 found more expeditious to prick on blank tracing paper the projection intersections of 

 the sheet to which the transfer is to be made. These pricked intersections are then 

 adjusted, quadrilateral by quadrilateral, to the topographic survey from which the detail 

 is traced. This eliminates the awkwardness of having to shift constantly both the 

 tracing and the transfer paper when making the final transfer to the smooth sheet (see. 



7561). 



7414. With Proportional Dividers 



Control stations may be transferred from one sheet to another by proportional di- 

 viders, but this method should be used only where the transfer is from a comparatively 

 large-scale survey to a comparatively small-scale one. The principle of the propor- 

 tional dividers is explained in 4812. In using them for the transfer of stations, the scale 

 setting of the dividers must be used only for a preliminary adjustment. Before actual 

 use, a final setting must be made by trial and error on the two projections. In most 

 cases, one setting will suffice for both latitude and longitude measurements; but this 

 should be verified and if necessary a different setting used. Control stations trans- 

 ferred by this method should be plotted as explained in the second and third paragraphs 

 of 7411. 



