Page 95 control and signal building 237 



mined points separated as widely as possible, preferably near the margins of the blue- 

 print or map, and forming a well-shaped triangle, ordinarily will provide excellent 

 control. These points must be tied into the control net by triangulation or located 

 by the topographer at the' time of his planetable survey and identified both on the 

 map or blueprint and on the topographic sheet. Features which are to be recommended 

 as landmarks for the use of the navigator are important and shall be located by the 

 topographer independently from the plan. 



237. Graphic Control Surveys 



A graphic control survey is a skeleton survey made by planetable for the purpose 

 of (a) locating the hydrographic signals, (6) locating additional control for air photo- 

 graphic surveys and clarification of indefinite detail on the photographs, or (c) a com- 

 bination of the two. 



Since much of the topography on nautical charts is now obtained from air photo- 

 graphic surveys, graphic control surveys are frequently made in connection with the 

 hydrographic survey. These surveys are usually made on aluminum-mounted sheets 

 (see 233) which are not subject to distortion and on which the desired control can be 

 located most accurately. Any of the conventional planetable methods may be used. 



Such surveys ordinarily do not contain enough topographic detail to be considered 

 topographic surveys and they are not registered as such in the Washington Office. 



If a hydrographic survey is made in an area where an air photographic survey 

 is planned in the near future, the graphic control surveys consist almost exclusively 

 of the locations of the hydrographic signals. If the hydrographic survey is made 

 after the photographs are available, but before they have been compiled into plani- 

 metric maps, the graphic control surveys may contain, besides hydrographic signals, 

 additional points for controlling the photographs, as well as the delineation of certain 

 indefinite or indistinct features on the photographs. Among the latter are the high- 

 water line along gently sloping sand beaches, intricate waterfront detail, and rocks 

 nearly awash. It may be desirable in some cases to delineate the shoreline in critical 

 areas, as in narrow passages or where channels border the shore closely. Where fre- 

 quent changes in signals are necessary during the hydrographic survey, the shoreline 

 will often aid in identifying the signals — even a sketched shoreline may be useful in 

 this respect. 



Graphic control surveys should also include the location of all aids to navigation 

 within the area which have not been located by triangulation — and cannot be unmis- 

 takably identified on the photographs. 



238. Special Topographic Methods 



The use of the planetable and alidade has been found a most efficient and adaptable 

 method of making ground topographic surveys, but in special circumstances the use of a 

 substitute method will be found advantageous, especially in places where the principal 

 aim is to locate stations for the hydrographic control. An experienced topographer 

 will soon learn when and where the use of substitute methods is of advantage or can 

 be tolerated. Generally, any method may be used which will result in accuracy as 

 great as or greater than that obtainable by the usual planetable methods. A method 

 which may result in a lesser accuracy can be tolerated only where conditions practically 

 prohibit the more conventional methods. 



