GRAVITATIONAL METHODS 397 



the top target. The small hand crank is then turned counter-clockwise until the first 

 mil scale line (white numbered line on the film) is brought into coincidence with the 

 screen index line. This interval is read under the front dial index, added to the mil 

 scale line value, and recorded in the notes under the H column. The projector is moved 

 still further in the same direction until the index line centers the bottom target ; then 

 the conversion factor and distance are both read under the rear horizontal dial index 

 and recorded in the F and Distance columns, respectively. Vertical angle measurements 

 are made in this manner to within 6 seconds of arc, and distances are read to the nearest 

 10 feet. The azimuth or magnetic direction scale is read directly on the screen to the 

 nearest 15 minutes of horizontal angle. 



Fig. 240. — Calculation sheet for photogranimetric mapping. 



Figure 240 shows a sample of engineer's notes which have been partially com- 

 pleted with office readings and calculations. The instrument man in the field fills only 

 the columns marked Shot and Station, and places a check in the BS or FS column. The 

 remainder of the columns are filled in with readings and calculations made by the 

 office computer. Since the instrument is adjusted to the number SO mil scale line, it is 

 necessary to subtract 50.0 from all H readings, as shown in the sample calculations. 

 The remainder is then multiplied by the factor in the F column and entered in the 

 Diff. E. column, with due regard to algebraic sign for this elevation difference. 



General Operational Procedure. — The instrument man levels his instrument, 

 sights on the rod, and snaps the picture. Where transportation and visibility do not limit 

 the speed of the survey to any great extent, it is possible to run between forty and eighty 

 stations set at half-mile intervals in one day of operation. The accuracy which can 

 be expected is given by the vertical closure formula in feet which applies to this method, 

 ±0.20Vniiles of traverse. Length of shots should not be over one-fourth mile. 



Experience has shown that there are three simple requirements for obtaining the 

 above accuracy with the camera. First, the instrument must be in good adjustment; 

 second, the instrument must be level at the instant the picture is taken ; and third, the 

 rodmen must be absolutely certain of exact take-off and tie-in points within each closed 

 traverse run. 



After the shots have been taken in the field, the film may be processed in the local 

 field office. The readings are taken from the negative by the field computer. All spur 

 lines are double-run on the film to check possible reading errors. 



