ard
ates
Photography for stand data collection
Trials have been made to photograph sample plots from smaller aircraft
with 70 mm cameras. Scales used were 1:2 000 - 1:4 000. In order to
sample 150-200 trees and to cover the variability within stands several
plots per stand have to be photographed. This resulted in a lot of photo-
graphs to keep track of and a lot of models to treat. An other problem
is the scale determination of the stereo models. This could however
be solved with the special radar altimeter /4/ developed in Canada.
Division into stands and area determinations must however be made in
photographs in smaller scales.
Thus it was not found practical to work with 70-mm photography over
sample plots for stand data collection. Instead conventional photography
in strips with ordinare mapping cameras (23x23 cm image format) is
preferred. This offers many advantages:
+ division into stands and area determination of stands can be made
+ fewer models to treat
* ordinary photogrammetric instruments and methods can be applied
(e.g. aerial triangulation for scale determination if necessary)
* several stand per model
Measurements and collection of stand data
A measuring system for collecting stand data from aerial photographs
has been developed. The system comprises a Wild Aviograph B8 equipped
with digitizers. To this is connected an HP 9821 desk-top programmable
calculator. The calculator is used for numerical relative and absolute
orientation of the models, for area and slope determination and for
collection of stand data. Programs for aerial triangulation have also
been developed.
Measurement of stand data in the photographs is made from a systematic
sample of plots. The sample plot is defined by a circular measuring mark
in the instrument. All trees within a plot are measured. Settings on the
tops of the trees are made first. Species identification and coordinates
are stored in the calculator. Measurements are then carried out on the
ground. For every tree, the calculator computes the location of the
nearest ground measurement and the tree height is determined. When the
ground is not visible, a digital terrain model is used to calculate
the ground elevations. Tree heights and species identification are
stored on cassette tape. The area of the sample plot is also deter-
mined, since this varies owing to differences in elevation of the terrain.
When all the plots in a stand have been measured, the calculator computes
the stand data. The particulars include the mean height weighed by basal
area, the mean dbh weighed by basal area, the basal area, and the volume
and number of trees per hectare. The distribution of the trees
according to height class and diameter class is also obtained. The
Standard errors in estimates are also calculated.
An accuracy study
A study on the accuracy of the method was made. Measurements were made of
29 stands with an average area of about 15 hectares. In every stand
the height of 150-200 trees were measured in the photographs. About
15 sample plots per stand were needed. Third degree regression functions
were used to obtain the dbh from the tree heights. Crown diameters were