233 HYDROGRAPHIC MANUAL PaGE 90 



merits shall be used that 90 percent of these control stations will be within 0.5 mm of 

 correct geographic position, and no station shall be in error by more than 0.8 mm. 



Except as otherwise provided in the project instructions, topographic surveys 

 (either planetable or air photographic) to locate control stations for hydrography shall 

 be on a scale equal to or larger than the largest-scale hydrographic survey to which 

 they apply. 



Planetable traverses, and graphic triangulation by planetable or by means of 

 air photographs, shall start from and end at control stations of third order or higher. 

 Exceptionally, minor control may be extended for short distances up relatively unim- 

 portant waterways without ending at a control station of third order or higher, where 

 compliance with the rule would be uneconomic. In such case extraordinary pre- 

 cautions must be taken to ensure the accuracy of the work; the figures in any graphic 

 triangulation must be strong, and the length of sights in any traverse must be neither 

 excessively long nor short, with check distances measured at every traverse set-up. 



To attain the required accuracy in planetable and air photographic surveys, the 

 following precautions are essential: 



(a) Projection lines shall be fine and shall check within 0.15 mm. 



(b) Basic control stations shall be plotted and checked within 0.15 mm, with reference to the 

 nearest projection lines. 



(c) Control stations (basic and minor) shall be identified on air photographs within 0.2 mm. 



(d) Triangles of error at minor control points located shall never exceed 1.0 mm, regardless of 

 the method of location. 



(e) Closing errors of planetable traverses, prior to adjustment, shall not exceed 0.4 mm per mile, 

 at the scale of the sheet; and in no case shall the total closing error, which may be adjusted, exceed 

 2.0 mm at the scale of the sheet. 



Closing traverse errors within the limits prescribed in (e) may be adjusted graphi- 

 cally on the sheet. Traverses resulting in larger closing errors shall be partly or 

 entirely rerun to obtain the prescribed accuracy. 



Most planetable traverses are short, since basic control stations are usually only 

 2 miles apart and never more than 5 miles apart (see 2231). Aluminum-mounted sheets 

 (see 233), which are used for practically all planetable surveys, eliminate errors due to 

 distortion. This amount of control and the elimination of distortion make compliance 

 with the requirements relatively easy; in fact most traverse closure errors should not 

 exceed one-half the allowable error. 



For planetable surveys, the Descriptive Report shall contain in tabular form a 

 list of all traverses, with their lengths and closures before adjustment, and the names 

 of the control stations at the ends of each traverse. 



233. Aluminum-Mounted Topographic Sheets 



The distortion of the topographic sheet has been one of the most serious handicaps 

 in planetable surveying. Prior to 1932 cloth-mounted Whatman sheets (see 7111) 

 were used almost exclusively in the Coast and Geodetic Survey for topographic surveys. 

 An aluminum-mounted topographic sheet, designed especially for topographic work, 

 is now being used for practically all planetable surveys. Unless special instructions 

 are issued to the contrary, these aluminum-mounted sheets shall be used for all future 

 planetable topographic and graphic control surveys. 



The aluminum-mounted sheet is constructed of highest quality Bristol board 

 mounted on both sides of a flat aluminum sheet 0.04 inch thick. The size commonly 

 used is 24 by 31 inches, which is identical with the size of the planetable board. A metal 



