320 FORAGE ESTIMATES 



the gridiron method. Elevations are recorded by means of the 

 aneroid barometer or by stadia calculations. 



The traverse-sketching method is not popular because the 

 sketching is not very accurate. It is, however, rapid and inex- 

 pensive and serves the purposes of the ordinary grazing recon- 

 naissance, except, possibly, where the forage crop is to be used 

 much more intensively than on most range lands of average 

 productivity. If an accurate topographic map is desired, the 

 telescopic alidade is substituted for the compass; the stadia 

 takes the place of the chain; and instead of the 4-inch section 

 plat the regulation planetable is used. 



The triangulation method is best adapted to rugged mountain 

 lands relatively free from vegetation that obstructs the line of 

 sight. The map work is based upon primary stations, secondary 

 stations, and topographic sketch points. The primary triangu- 

 lation stations are located by means of transit or theodolite; the 

 secondary control points, either by planetable or telescopic 

 alidade; and the sketch points, by the topographic Abney level 

 and the open-sight alidade. Vertical control is computed by 

 means of vertical angles; horizontal control, by horizontal 

 angles. When the sketch points are located and their elevations 

 determined, the topographic and cultural features are sketched. 

 Then one is prepared to proceed with the typing. 



The accuracy of the topographic map obtained by triangu- 

 lation makes it possible to sketch in with reasonable exactness the 

 type boundary Hnes. When the type boundary lines are located, 

 the examiner proceeds to record for each type such data as are 

 pertinent to its use as forage. 



Office Work. — In order to insure accuracy in transcribing the 

 field-map data the camp is provided with the necessary equip- 

 ment of a draftsman. All the field notes are transcribed in uni- 

 form and workable order on township plats, with a scale of 4 

 inches to the mile, or on a camp map with a scale of 2 inches to 

 the mile, the types being shown in colors. The forage acres ^ in 

 each type and each unit are then calculated and tabulated. The 

 final compilations are done in regularly established offices where 

 it is possible to record the data in finished form. For ofiice use 

 ' For definition of "forage acre" see p. 325. 



