Slope compensating angle gauge

The "Cruiser's Crutch" is a slope correcting basal area angle gauge to be used in variable plot sampling for determining basal area in square feet per acre. It incorporates the critical angle for the 10, 20, 30 and 40 basal area factors. This gauge enables the forester or timber cruiser to make the same basal area measurements that would normally require a critical angle gauge for each factor chosen and at the same time compensates for slopes of from zero to ninety percent. The user selects the factor desired and then holds the instrument a fixed distance from his eye, as determined by the larger balls on the attached charin. The larger ball nearest the angle gauge is for 40 factor measurements; the next larger ball, 30 factor; the third, 20 factor and the last for ten factor measurements. (Other basal area factors between 10 and 40 can be easily added if desired.) User then rotates 360.degree. around a point, keeping his eye over the point at all times and the gauge perpendicular to the slope. Any tree that is large enough to subtend the fixed critical angle of the gauge is considered "in" and is tallied. All trees tallied around the circle are then multiplied by the factor number chosen; the result being equal to basal area per acre.

This invention relates to a gauge used in the forestry industry to take an 
inventory of timber in a given forested area. It enables the user, called 
a "cruiser" in the industry, to rapidly and expeditiously inventory the 
basal area of timber over extensive areas of standing timber in either 
level or mountainous regions. The gauge is accordingly called a "Cruiser's 
Crutch" by users in the field, and it has acquired a specialized use under 
a relatively new concept in forest inventory procedures as used in the 
United States. It is the purpose of the present gauge to accurately 
determine what is called a basal area (BA) for each area surveyed. Since 
the theory of variable plot cruising is quite involved, a detailed 
discussion here would be impractical as not directly concerning the 
present invention. Suffice it to say that this theory is widely accepted 
and extensively used in the forestry industry, and that it enters into the 
picture here mainly as background evidence of the specialized utility of 
the invention. What the gauge does is make possible the speedy 
determination of the basal area per acre, a factor which is important to 
the application of this theory in forest inventory procedures. For a 
better understanding of the above-mentioned theory, reference is hereby 
made to a copyrighted (1971) book entitled VARIABLE PROBABILITY SAMPLING, 
which contains a chapter entitled "Variable Plot Cruising" written by J. 
R. Dilworth and J. F. Bell, and published in 1975 by the Oregon State 
University Bookstores Inc., Corvallis, Oreg. 
The closest known pertinent prior art is: 
Merritt U.S. Pat. No. 1,455,347 May 15, 1923 
von Buchau (Ger) 30,422 Feb. 9, 1885 
Variable plot cruising is accordingly admittedly old in the art, but thus 
far the use therefor has been dependent on delicate and expensive sight 
gauges to determine the basal area. The variable plot or point method 
requires that the cruiser use some type of instrument which will permit 
him to ocularly project a predetermined angle. The most common of such 
instruments employed are horizontal angle gauges, or those using one or 
more wedge prisms, which are not only expensive, but unable to withstand 
the rough usage which the instrument is subjected as the cruiser climbs 
around over the rough terrain where much of the timber exists. 
Furthermore, they are useful for only horizontal observations, and 
consequently not suitable for use on slopes. 
It is accordingly an object of the invention to provide a sight angle-gauge 
which is not only inexpensive to make, but of sufficiently rugged 
construction to withstand the rough usage to which it will be subjected in 
normal use. 
A further object of the invention is to provide a sight angle-gauge that 
automatically compensates for the percentage of slope of the terrain where 
the device is being used.

Considering FIG. 1, 1 designates the sight scale piece which is calibrated 
as at 2 in degrees of slope varying from 0 to 90%. This scale piece is 
preferably made of a transparent plastic (such as "Lexan"), which is not 
easily broken or permanently deformed, and which is given a taper that is 
a trigonometric function of the slope of the terrain where the device is 
to be used. Thus the width of the sight scale piece, at any given slope 
calibration point, will correspond to the slope of the terrain under 
consideration. Attached to the middle of the gauge scale piece is a 
flexible cord or thong (preferably a small ball-chain), having thereon a 
plurality of knots or enlarged indicator beads 4 for quickly determining 
the distance the scale piece is to be held from the observer's eye. These 
enlarged indicators constitute the Basal Area Factor or (BAF). This is a 
factor arbitrarily chosen by the cruiser who is familiar with the theory 
of variable plot cruising and used by him in later computations. The 
farther the sight scale piece is held from the eye, the smaller will be 
the angle subtended by the scale piece itself, as well as the object 
observed. Further discussion of the spacing of these indicator beads will 
be found hereinafter in the discussion of the operation of the device. 
OPERATION 
To use the "Cruiser's Clutch," select the appropriate basal area factor 
(BAF) desired, and determine the slope to the nearest 5%. With your eye 
over the sample point, hold the appropriate distance bead under your eye, 
lightly touching the cheek. Extend the scale away from the eye until the 
cord is taut. With the scale piece held perpendicular to the slope and 
your eye over the observation point, line up the tree to be measured with 
the appropriate slope percent line on the gauge. Any tree that is large 
enough at dbh (diameter at breast height--usually 4.5 feet above the 
ground) to subtend the fixed critical angle of the gauge is considered 
"in," and is tallied. Then rotate 360.degree. about the sample point and 
repeat the procedure for all trees that might be tally trees. The number 
of trees tallied times the basal area factor used is the basal area for 
that sample point.