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REPORT OF COMMISSION VII 37
less ground work in conjunction with the photo-interpretation. When a soils
mapper becomes familiar enough with the characteristics of certain soils, he
can find many features on photos which aid his work. Such characteristics in-
clude: erosion tendencies by gullying or slipping; tone of soil as seen on the
photo; occurrence of rock outcrops; slope and micro-relief; and, of course,
vegetation correlations.
Minerals inventories are made from aerial photos in an indirect manner,
just as soils inventories are. The minerals, of course, cannot be seen, but their
presence is deduced from the landforms, vegetation types and other clues which
are visible. Ground work for the establishment of correlations is required. The
recent discovery of iron ore in Venezuela (Lake, 1950) provides a good example.
On the aerial photos, the ferruginous quartzite hills were distinguishable from
the areas of igneous or metamorphic rock. Field parties were sent to explore the
Cerro Bolivar hill, because its photo appearance indicated possible ore de-
posits; half a billion tons of high-grade ore were located.
Exploratory surveys made for the purpose of locating oil and gas deposits
depend to a large extent upon photo-interpretation of geologic formations. Such
interpretation is highly specialized, requiring special skills and knowledge of
geology on the part of the interpreter. Mineral deposits which are known to
occur as a rule in certain contact zones might be located by observing such
contacts on photographs. Fault lines are often distinctly visible, and changes in
drainage pattern and type of vegetation can serve as clues to changes in geology.
Serpentine areas, such as that shown in Figure 2, stand out prominently from
surrounding sedimentary rock formations. The trained interpreter can im-
mediately recognize the modified drainage pattern of the serpentine, and can
verify the rock by noting the vegetation, which consists of shrubs and character-
istic small conifers (e.g. cypress), and by noting the dark tone of the soil. Areas
of peridotite—sometimes the source of diamonds and platinum—may be similar
in appearance to serpentine, but they may support different species of shrubs
and trees, which can lead to their detection from photo study alone.
There are many products, aside from the true minerals, which are extracted
from the earth's surface. Gravels, clays, peat, and fill material are some exam-
ples. The interpreter who is familiar with ground conditions in a given locality
can detect deposits, or at least locate areas which would justify ground checking
for such materials. Again, it is recognition of land forms and analysis of topogra-
phy, vegetation, soil tone, and other characteristics which lead the interpreter
to his final conclusions.
5. WATER AND SNOW INVENTORIES
At present relatively little is being done with aerial photos in connection with
water and snow inventories. Figure 3 illustrates the usefulness of oblique photos
for delineating watersheds and estimating their conditions. Water in the form
of lakes and rivers is readily seen on aerial photographs, and such bodies can
be shown as features of standard maps prepared from aerial photographs. The
presence or absence of water can be of considerable importance in planning
locations for industrial sites such as sawmills, farm or ranch headquarters,
recreation areas, or temporary camps. The interpreter may experience some
difficulty in distinguishing between year-long and intermittent streams, but
helpful correlations may be found by analysis of the riparian vegetation. De-
tection of small bodies of water is easier if infrared photography is used rather
than panchromatic, because water is characteristically jet black in tone on the
former type of film.
