Move the outfit to az and with the fiducial edge of the ruK 

 resting against a tine needle in the point 02, and coincident also with 

 the point ai t revolve the planchette until the light line hits the 

 tation ai. Great care in plumbing would be a waste of time. An 

 error of half the width of a 15 x 15 traverse board, in the light of a 

 mile, plotted to a scale of 2 inches on the map, would scarcely be 

 discernible. The instinctive tendency, however, is fortunately 

 toward greater accuracy of orientation and with care, when the last 

 station is occupied, the converging lines should meet in the same 

 needle hole. 



If it were necessary to locate some point as at R, the three- 

 point problem should be applied, as from the three nearest visible 

 stations, and the orientation conducted upon the most distant station 

 in the survey that is visible. 



The Mensula Tripod 



The most elaborate type of tripod and head mechanism is shown 

 in Fig. 22. It is usually associated with the thought of the Coast 

 Survey, although it is used by State and Municipal Departments for 

 the more careful field mapping. We have choosen to designate 

 this model as the Mensula Tripod in honor of J. Pratorius, who 

 so named the plane table when he invented it in 1590. 



In our model, the circle of rotation is 12 in. in diameter and 

 contains a ball-bearing race of 125,5-mm phosphor bronze spheres. 

 Otherwise the construction is all brass and weighs 28 Ibs. without 

 the planchette which is not shown. This weight would at first 

 seem excessive, but for work in windy country some weight will be 

 required to offset wind pressure on the planchette. Some topog- 

 raphers hang a canvass sack by three hooks from the tripod legs and 

 fill the same with stones to accomplish this purpose. 



Experience has shown that constant wear on the leveling screws 

 induces a certain amount of lost motion which would cause annoy- 

 ance in accurate orientation. We have met this situation by 

 supporting the threaded shanks in a split and tapered bushing which 

 can be adjusted by two check nuts, above and below, as indicated 

 in Fig. 22. 



The details in the larger topographical surveys, which have 

 been based upon a strong framework of manipulation, may be 

 accurately controlled and located by this more substantial construc- 

 tion. Where topographic mapping is carried on simultaneously 

 with plane-table triangulation, the availability of vertical control 

 should be considered in the selection of the initial station. Verti- 

 cal angle elevations have relative values dependant upon the size of 

 the angle measured and the distance between points. 



41 



