84 A MANUAL OF TOPOGRAPHIC METHODS. 



In case one line of sight upon the required station has been obtained, 

 that sight line may be utilized in making the location as follows by resec- 

 tion: Having leveled the table, place the alidade upon this sight line 

 already drawn, with the telescope pointing toward the object from which 

 the sight was taken. Then turn the table in azimuth until the telescope 

 falls upon this point, and clamp it. The table is now oriented, but the posi- 

 tion of the present station is unknown further than that it is known to be upon 

 this line. Then select sonic station whose direction makes a wide angle 

 with this line, and move the alidade until the cross wire falls upon this 

 selected station, while the ruler at the same time is upon the representation 

 of the station upon the map. The ruler will then cross the sight line at 

 the point desired. By way of check, repeat the process with another sta- 

 tion or located point. For this purpose a point in suitable direction is 

 valuable in proportion to its proximity. 



Using the instrument as described above, the topographer locates from 

 them all possible points. Then visiting in turn such of them as he finds 

 necessary, perhaps a dozen or twenty, he locates by intersection points all 

 over the sheet in as great number and as well distributed as possible, and 

 with special reference to the needs of the traverse men, who will come after 

 him and whose work will be located by means of his determinations. All 

 this work must be done with the utmost nicety and precision. The setting 

 of the alidade upon the station must bisect the needle hole by which it is 

 marked and the lines of direction must be drawn with a sharp-pointed pencil. 



The necessity for precision will be recognized when it is understood 

 that any error introduced in the early part of the plane-table triangulation 

 will be not only perpetuated, but increased many times over as the work 

 progresses, and as soon as an error becomes appreciable it produces difficul- 

 ties and uncertainties in making locations, which may lead to embarrassing 

 delays, and ultimately require that all the work be repeated. 



MEASUREMENT OF ALTITUDES. 



While making horizontal locations of points with the plane table, their 

 heights must also be measured, relative to that of the point occupied. This 

 is done by means of the vertical arc of the alidade and the level upon the 



