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Another special type of photography is terrestrial photography; horizontal 
or slightly inclined photography for photogrammetric or interpretation 
purposes (10,11,12). 
4.2 Type of information provided by photography 
Aerial photography can provide valuable information for engineering 
geological mapping: 
— Rock and soil units. Main information in photography on character and 
outline of soil and rock units is the indirect evidence obtained from 
the morphology of the earth's surface, which is excellently visible in 
stereo photography. The landscape and its morphology is the result of 
weathering, erosion and deposition processes which act upon the 
available geological materials of rock and overburden. 
The physical aspects such as strength, resistance to erosion, 
permeability, which play an important part in this interaction, are 
also important aspects in engineering geology. This logically leads to 
the conclusion that the surface form, position in the landscape, the 
relation with erosive and depositional processes may give important 
engineering geological information about the types of soil (grainsize 
distribution) and rock (rock mass strength, as a combination of material 
strength, weatherability and jointspacing) (13). 
Detailed analysis of the character and density of the drainage system 
may give information on permeability and erosion resistance of soils as 
well as bedding, joining, folding and faulting orientation, solution 
aspects and erodibility of rocks. 
- Hydrogeology. Detailed observation of the morphology, character and 
density of the drainage system and type and abundance of vegetation 
may give information on flooding, seepage, infiltration and water 
saturation. 
— Geomorphology. Quantitative information on slope form, Slope steepness, 
river gradient can be obtained by stereoscopic interpretation of 
aerial photographs combined with simple photogrammetric procedures (14). 
It also allows for the determination of the present stage of the 
geomorphological development and for an assessment of short term and 
long term inpact of futural landscape evolution on the engineering 
project.