Page 91 control and signal building 234 



clamping device designed in the Bureau is used to hold the sheet rigidly in place while 

 in use. The edges of the Bristol board extend approximately one-half inch beyond the 

 aluminum and are hermetically sealed to exclude any moisture which might destroy 

 the permanent bond between the paper and the aluminum. The Bristol boards on 

 the two sides of the aluminum must be of identical material and quality to preclude 

 a differential expansion or contraction which might warp the sheet. Ordinarily only 

 one side of the sheet is used for the planetable survey although both are sometimes used. 



The Bristol board of which the mounted sheets are made has a smooth hard 

 surface which is exceptionally suitable for drafting with either pencil or pen and ink. 



The absence of any noticeable distortion under any climatic conditions or over 

 long periods of time makes the use of this sheet a joy to the topographer. A sheet 

 tested in Alaska in 1932 showed no measureable dimensional change. Under the same 

 conditions a Wliatman cloth-mounted sheet underwent a percentage change from plus 

 0.50 percent to minus 0.82 percent. 



In 1934 an aluminum-mounted sheet, on which the topographic survey was practically complete, dropped overboard in 8H fathoms 

 of water. The sheet was retrieved by the topographer by being pierced by nails fastened to the lower end of a 30-pound sounding lead, 

 four nail holes being made in the sheet before it was recovered. It was in the water about 45 minutes. After recovery it was dried 

 carefully, and when the projection was checked it showed practically no measurable change in length and but 0.02 to 0.04 percent con- 

 traction in width. Today, in the archives of the Washington OflBce, this sheet shows no stains nor sign of disintegration and no addi- 

 tional change has occurred in its dimensions. 



Actual performance in the field, together with a number of special tests, has proved beyond question that the problem of distortion 

 has been solved by the aluminum-mounted sheet. In 1934 a topographer located a number of beacons by planetable on an aluminum- 

 mounted sheet, scale 1:10,000, and the distances between these were scaled from the topographic sheet at the time. Subsequently, 

 these identical beacons were located by triangulation and a comparison of the computed geodetic distances with the scaled distances 

 disclosed an average difference of only 0.7 meter, the largest difference being 1.6 meters in a distance of 2340 meters. The distances 

 varied from 1164 to 5675 meters. 



Other advantages of the aluminum-mounted sheet are that the wind cannot blow 

 between the sheet and the planetable board to disturb the topographer as was the case 

 with the Whatman sheet, and the sheet can be used under adverse climatic conditions, 

 such as exposure to heavy mist or light rain, which made the use of the Whatman sheet 

 impracticable. 



It has been proved that a considerably higher accuracy can be attained with the 

 use of the aluminum-mounted sheets and almost every topographer who has used them 

 has expressed pleasure and satisfaction with them. The absence of distortion makes 

 it a pleasure to use the three-point problem without the tedious procedure of adjusting 

 the position for distortion, and careful centering and orientation practically assure 

 that cuts taken to an object will intersect in a point. 



234. Hydrographic Signals 



In the absence of air photographic surveys, the necessary hydrographic control 

 may be located accurately and economically by means of the planetable. 



The locating of various hydrographic signals, shoreline, and off-lying detail is of 

 primary importance, as inshore hydrography cannot be done without a large number 

 of temporary signals necessary for sextant fixes close inshore and elsewhere. 



When topographic and hydrographic parties are operating in the same immediate 

 area, from the same ship or shore base, the topographer shall daily make a tracing of 

 his results for transfer to the boat sheet, thus providing the hydrographer with the 

 positions of all newly located signals for use in his next day's work. 



Since the topographic survey usually immediately precedes and locates control for 

 the hydrogi-aphic survey, the topographer must have an intimate knowledge of the 

 requirements of the hydrographer in order to provide him with stations at desirable 

 locations and frequency. The topographer should plan the stations from the point of 



