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PHOTOGRAMMETRIC ENGINEERING
stances (climatical or soil conditions), or can
be caused by human interference (traces of
shifting cultivation, logging, grass fires, etc.).
The identification of types is mostly based
upon the following principles:
1) General aspect of the vegetational cover
to be differentiated in closed forest, open
forest, woodland forest, bushes, scrub
and grass land. Further also vegetation
types caused by shifting cultivation, as
occupied agricultural fields, old and
young secondary forest.
2) Possible subdifferentiations based upon
particular aspects of the crown canopy
caused by local differences in tone and
texture of the individual trees. The
identification of the individual tree spe-
cies in the tropical forests is generally
impossible on the photographs caused
by the very heterogeneous species com-
position of these forests. However, the
typical distinctive pattern based upon a
combination of features brought out in
stereo of tree height, crown size and ap-
pearance, density of stocking and,
finally, variations in tone and texture,
car be an indication for a particular
forest type. An analogous statement was
also made by de Rosayro based upon
his experience in photo-interpretation
in the tropical rain forests of Ceylon (4).
D. CONSTRUCTION OF THE TYPEMAP
The forest type boundaries indicated on the
photographs are afterwards transferred to the
map. For this purpose ready-made plani-
metric maps could be used at an equivalent
map-scale. By superimposing topographic
features on the photograph, like drainage
patterns, good discernible forest roads or
other features, over the corresponding ones in
the map, the type boundaries can be traced
without any difficulties, especially in flat ter-
rain. If no good maps are available, first a
triangulation of pass-points had to be made;
in most cases the slotted-templet method will
be quite sufficient; for this a certain amount of
ground-control must be available. If there are
occasional triangulation stones in the region
to be mapped, the situation of these pillars
can be pinpricked upon the photographs dur-
ing the preceding field work.
Maps are mostly made at photo-scale or a
scale, which is a little bit smaller than that of
the original photographs. The last method is
always preferable, as possible mapping errors
are accordingly reduced. A reduction in the
scale from the photograph to the original map
10
of more than two times is not advisable as,
under such circumstances, the application of
the normal transfer methods is much more
difficult. If small-scale maps are desired—to
get a better overall impression of the prevail-
ing vegetative conditions—then such maps
preferably should be made afterwards by
compiling the mapping results of the original
maps together upon a smaller scale—a tech-
nique, which would be sometimes applied
in Land Use Mapping projects. In such a case
it will be advisable to avoid making the indi-
vidual type areas too small, in order that the
final map will not be too complicated and
difficult to read.
E. EQUIPMENT TO BE USED FOR TRANSFER OF
DETAIL
When considering which kind of plotting
instrument would be advisable for forest
mapping work in tropical areas, the general
rule will hold, that the simplest solution will
generally be the best one. In this respect with-
out doubt, the most appropriate instrument
for transfer of detail will be the Sketchmaster
and in particular the construction made by
Zeiss factory, which according to the exper-
ieace at the ITC, is preferable to the same
type of instrument made by other factories.
Projection instruments have the advantage
that the work to be done is not so straining on
the eyes compared with the Sketchmaster,
and that reductions or enlargements with the
first type of instruments can be made more
than two times. However, these instruments
are not advisable in the tropics as the work
could be done only in air-conditioned rooms.
The Zeiss Stereopret, produced recently,
proved to be no improvement when compared
with the instruments mentioned above.
Actually these simplified plotting instru-
ments can be used only if the terrain is rather
flat. For hilly terrain with moderate differ-
ences in elevation, the limitation of the trans-
fer within the central part of the photographs
can give reasonable results. For mountainous
terrain with important elevation differences
in the same model, other, and more compli-
cated plotting instruments are required.
Under such circumstances, the Radial Line
Plotter will perhaps be preferable as the most
practical. The other instruments, sometimes
specially recommended under such circum-
stances, like the Stereotope, are generally too
complicated for type mapping work. Further
the application must be based upon more de-
tailed information regarding the knowledge of
the elevation of the pass-points in every
model. Also the application of the Multiscope