A procedure is described for estimating timber volume from high-precision measurement of the cross-sectional area of a canopy profile on medium-scale vertical aerial photographs. Timber volume data were obtained from 75 data points in a study area containing several forest types, and canopy profile areas were measured with a stereoplotter at the corresponding points on the aerial photographs. Film density values were also measured along each profile using a scanning microdensitometer. Canopy profile area was found to be independent of the direction of the profile relative to the flight line of the photography. The relation between timber volume and profile area was found to be highly significant, semilogarithmic, and species dependent, with regression R2 values ranging from 0.67 to 0.79. The area under a curve obtained by plotting film density values is not sufficiently correlated with timber volume to be a significant independent variable, either alone or with profile area. However, film density information was found to be of significant value in correcting the profile areas for canopy microopenings too small to be measured with a stereoplotter. With the area of microopenings included as a separate independent variable, regression R2 values range from 0.82 to 0.88.
134. Study of film density variation at fild size on cardiac cine angiograhy
