38 Forest Mensuration 



REMARK : Bulletin 36, page 125, states that volumes are now computed 

 by the Bureau either by averaging the volumes found for the sample 

 acres, thus obtaining the volume of a model acre as 



(wherein n equals the number of sample acres) ; or by summing up all 

 trees of each diameter class, by dividing each sum by the number of sam- 

 ple acres, and by thus finding for a model acre the average number of 

 trees for each diameter class. In both cases the volumes for each diam- 

 eter class are read from volume tables. 



Allowance for defects is made according to local experience, all trees 

 being calipered as if they were sound. 



PARAGRAPH LXI. 



THE GRIDIRONING METHOD. 



1. Work with compass (if a topographical map is required, also with 

 barometer or clinometer) and with several tapes or ropes. These ropes 

 are meant to denote the sides of a strip; within the strip the sectional 

 areas are taken with calipers or Biltmore sticks. 



2. The tapes move continuously with the caliper men, and there is 

 no stopping. The compass man keeps ahead of the measuring crew. One 

 of the outside "tapers" has the correct length desired for a section. His 

 tape must be run straight. The inner tapes may make snake lines. The 

 tally man uses a fresh tally sheet for each section. 



3. All strips lie parallel and are equidistant. The width of the strips 

 depends on the density of growth, smallest diameter calipered, available 

 help and accuracy required. 



4. The distance between two parallel strips depends upon accuracy re- 

 quired, width of strip and variety of configurations. 



5. Each strip is divided into sections of equal length. The tally sheet 

 gives for each section the diameters (with bark) of the trees in that sec- 

 tion ; further, remarks on the run and altitudes of ridges and creeks 

 traversed, on roads, settlements, existing surveyor's marks, forest fires, 

 forest pasture, previous lumbering and regeneration. The number of 

 seedlings in a section might be approximately given under the same head. 



