2285 HYDROGRAPHIC MANUAL PaGE 88 



The offset traverse method may be expected to result in accumulated errors no 

 greater than 1 meter per mile. Stations located by this method shall be marked and 

 described as in 2281. 



2285. Three-Point Fixes 



Infrequently on combined operations and occasionally on air photographic surveys, 

 three-point fixes may be used for locating supplemental control stations in which one or 

 more of the stations observed are intersection points, such as spires, tanks, etc. The 

 observations shall consist of at least one set of 3D and 3R with a 10-second repeating 

 theodolite or two positions with a second-order direction theodolite. 



The computation of the three-point problem should be made on Form 655, as 

 described in detail on pages 98 to 100 in Special Publication No. 145, Manual of Second- 

 and Third-Order Triangulation and Traverse. Because there is no check on the posi- 

 tion, it is considered a topographic station and shall be marked and described as 

 explained in 2281. 



If observations are made on a fourth station so as to give a check, and all four 

 stations are third-order or better, the supplemental station shall be considered of third- 

 order accuracy, provided the observations consist of at least one set of 6D and 6R with 

 a 10-second repeating theodolite or four positions with a second-order direction theod- 

 olite. Standard triangulation station-mark disks shall then be used for marking. 



23. TOPOGRAPHY 



A topographic survey is the determination of the positions, on the earth's surface, 

 of the natural features and the culture of a locality and the delineation of them by 

 means of conventional signs and symbols on a plane surface called a topographic map. 

 Such a map shows both the horizontal distances between the features and their eleva- 

 tions above" a certain level called the datum plane. The representation of the variations 

 in elevation is called relief. 



Although the depths in the water area are the most important data on a nautical 

 chart, the topographic features of a coast are only second in importance because, in 

 sight of land, the mariner is cliiefly guided by, and determines his position from, the aids 

 to navigation and the shore landmarks which he can identify. Contours or form lines 

 must be iacluded so that the topographic relief of the area will permit the mariner to 

 identify quicldy and with certainty the rivers, valleys, hills, mountains, etc. A chart 

 should include the topographic details of the adjacent shore and all the salient features 

 of landmark value that are visible from a ship at sea. 



Complete instructions and specifications for topographic surveying are contained 

 in Special Publication No. 144, Topograpliic Manual. Except as modified in the 

 project instructions and in this Manual, those requirements shall be strictly followed 

 by hydrographic parties engaged in topographic surveying. Only the briefest dis- 

 cussion of the subject can be included in this Manual, which is particularly concerned 

 with the establishment of control for hydrographic siurveys, and which describes some 

 special applications and methods of planetable and ah' photographic surveys which are 

 peculiarly adapted to hydrographic surveys. 



231. General Statement 



Topographic control is necessary to supplement the triangulation control for use 

 in launch and visually controlled ship hydrography. 



