446 Forestry Quarterly 



maintained in the plane table triangulation. On account of the 

 distance between these stations, it would generally be necessary 

 to plat their location on a small scale map, which would be changed 

 to the larger field scale as soon as several control points were 

 located on the area to be mapped. The exact latitude and longi- 

 tude of the U. S. Geological Survey triangulation stations and 

 bench marks may be obtained from the Geological Survey, and 

 by these values platted on the field sheet. These stations are so 

 located that they are especially adapted to triangulation work and 

 also give a basis for tying in the vertical control. It will, in some 

 cases, be preferable to obtain by triangulation with a transit, a 

 few primary control stations over the area. This method would 

 be of greatest desirability where the area to be covered was very 

 large and the control points a considerable distance away. The 

 resulting triangles could be balanced and the control stations more 

 accurately platted on the final plane table sheet than would be 

 possible by plane table triangulation, which, on account of the 

 great length of sights, would have to be done first on a small 

 scale and enlarged for the final sheet. 



There are often extending into a forest, valleys containing 

 areas surveyed by the land office. The corners in such a survey 

 can be used in starting triangulation work, if the survey is ac- 

 curate. The points may be platted generally on the 2-inch-to-the 

 mile scale, and a long base used, so that a location to a sufficiently 

 high degree of accuracy is possible. The table may be oriented by 

 sights at other visible survey corners or those made visible by 

 signals. There is in Forest Service work considerable need to 

 refer to land office subdivisions, and this method of originating the 

 triangulation system allows a very accurate extension of the sub- 

 divisions over the map in unsurveyed territory. Sometimes, in 

 old surveys made under contract, the locations in the field will 

 not coincide with the platted location, and if several years old, it 

 is difficult to find the corners. A check as to the accuracy can 

 be made by sights to several corners to check the platting, and if 

 they are found to be much in error a retracement may be 

 necessary. 



Where there is no survey system near the area which can be 

 used to start from, or one which it is impractical to use, it will 

 be necessary to measure a special base. In most cases it would 

 be located in the valley bottom, but a flat ridge top can be used 



