NUMBER OF DIAMETERS REQUIRED TO REPRESENT STEM PROFILES USING INTERPOLATED CUBIC-SPLINES

Interpolated cubic splines were used to estimate diameters and partial or total volumes for 285 trees from Pinus taeda L. plantations in sou thern Brazil. A range of 4 to 13 interpolated points was tested in an attempt to determine how many points are necessary to adequately repre sent stem taper. The various number of combinations with 13 or fewer p oints for interpolation were compared with a full spline of 16 points, which according to pertinent literature, estimates volume similarly t o the traditional volume formulas (Newton, Huber, Smalian). The result showed that 10 well-placed points are necessary to obtain reliable es timates of diameter and partial or total volume along the entire stem. If the top 15% of the stem is ignored, 5 points adequately estimate v olume, with an average residual of less than 2%. However, when fewer t han 7 points are used, placement of points is very important to avoid illogical oscillation inherent in the use of cubic splines.