520 JOURXAL OF FORESTRY 



At present we have cadastral maps of all National Forests, mainly 

 compiled from General Land Office surveys, and though their accu- 

 racy and the amount of detail shown sometimes are not all that they 

 should be, yet in general these maps are sufficient for ordinary purposes. 



Topographic maps of perhaps one-half of the National Forest lands 

 already exist, mostly on rather small scale for administrative purposes. 

 These maps are utilized as far as possible as the basis for extensive tim- 

 ber reconnaissance, grazing reconnaissance, engineering plans and esti- 

 mates, and many other important National Forest activities. But the 

 more intensive timber reconnaissance, and certain other projects as well, 

 demand topographic maps of fairly large scale and showing much detail. 

 If not available, they must be made before the project is finished. For 

 engineering construction, particularly, the control also must be reliable, 

 or wasteful expenditures might easily result. 



In the standard Forest Service method of making an intensive tim- 

 ber reconnaissance two things are necessary: (i)to determine the exact 

 stand and yield on certain sample strips, and (2) to estimate the char- 

 acter of the forest cover on the entire area. From this an accurate esti- 

 mate of the stand and yield on the entire area is obtained. In practice, 

 the party goes through with compass and chain, running parallel strips 

 1/12 mile apart throughout the entire area and measuring or estimat- 

 ing all trees in each strip. In case no good topographic survey of the 

 area is in existence, a modified Abney level is used to determine the rela- 

 tive elevations every two chains along the strips, as well as prominent 

 topographic features between the strips. All these data are tied in with 

 points of known geographic location and elevation and the result is a 

 topographic map on whatever scale is desired, the accuracy of which 

 can be made much greater than that of maps ordinarily made with plane- 

 table and alidade, because of the vastly greater number of points to 

 which the control is carried. Thus the necessary, though secondary, 

 feature of the topographic base map is obtained practically as a by- 

 product of the labor of the timber estimators. 



Extensive timber reconnaissance and grazing reconnaissance are usu- 

 ally made by other methods, which do not lend themselves as readily as 

 the strip method to the systematic gathering of topographic data. Either 

 the base map used is the General Land Office survey, or extensions of it, 

 or else a topographic base is made with plane-table and alidade. The 

 contour, as well as the boundaries of the areas under forage, are deter- 

 mined, and the amount of forage on the area determined by inspection 

 of suitable units, such as sections or drainage basins. 



Having briefly outlined the manner of making aerial photographs and 



