The PANAMA Basal Area Angle Gauge is a modification of the 33" long gauge described in Article by Mr. L.R. Grosenbaugh, Journal of Forestry Vol. 50, No. 1, Jan. 1952.  In reducing the length, we have tried to make it more convenient to carry --- and use.

In the PANAMA Angle Gauge the cruiser looks through the tube instead of over the top.  In reducing the overall length of the gauge we have maintained the plot radius factor or angle of 104.18 minutes.  In the forward end of the tube we have the two upright lines for paralleling the sides of the tree.  We have opened the parallel lines near the top so that the cruiser can more quickly come to a decision on borderline trees.  We have also added a slight arch in the exact top center for "centering" the tree.  The widening of the slot at the top for admitting light on either side and the arch at the top will greatly help in overcoming the objection to a shorter gauge.

Plotless timber cruising.  Shaded area represent 104.18 minute angles optically established by the instrument described above.  Circles represent cross-sections (at breast height) of trees viewed from the sampling point.  Cruiser merely stands at the sampling point (analogous to a plot center), counts every tree whose d.b.h appears larger than the angle or aperture, and disregards every tree whose d.b.h appears smaller.  All visible from the sampling point must be counted or rejected.  The count of trees, multiplied by 10, gives an estimate of basal area per acre.  In the diagram, only two trees are counted, so the basal area estimate is 20 square feet per acre.  Reliable estimates require more than one sample, of course.

Basal Area Estimates

Assured of an instrument, the cruiser should decide on the pattern of sampling points (analogous to plot centers) that he wishes to employ on the area to be cruised.  He must then visit each sampling point (or at least unbiased point in its vicinity), look in every direction through his instrument, and them count the number of trees whose d.b.h's appear larger than the crosspiece of the aperture. The principle is illustrated in figure 1.  The eyepiece (or vertex) of the angle-gauge should pivot on the sampling point until the count is completed, except that it may be temporarily moved sideways perpendicular to the line of sight to clear nearby brush or trees likely to mask other qualifying trees.  After a little practice, the cruiser will find he can gauge all borderline trees by eye alone.

Suppose that the cruiser has tallied a total of 240 qualifying trees at 30 unbiased sampling points on the area.

Estimated Basal area per acre

= (10)

Number of tallied trees

= (10)

240 = 80 sq. ft
Number of sampling points 30

Fixed Type, Weight 2 oz.