for purposes of analysis and correlation into elements of topography, drainage 
plan and erosional form. The tones of grey are subdivided into values and 
textures related to land use and vegetation and tones related to materials. 
Topography is subdivided into first order topographic forms and second 
order forms. The first order forms are mountains, hills, ridges, plateaus, plains, 
basins, and valleys. 
The second order forms consist of surface configuration including relief and 
slope. The surface configuration is the trace of the surface considered without 
erosion and then with erosion. The basic second order forms are: undulating, 
rolling, crested, and blocky. Each type, with and without erosion, has certain 
general implications as to parent materials, underlying structure, erosional and 
depositional history and overall climatic regime. 
The element of relief and slope in a quantitative or qualitative sense is also 
a component of the overall topography. Relief is estimated or measured by 
simple photogrammetric measurements. Slopes are described as uniform, 
concave, or convex. Quantitative values of ranges in slope and rate-of-change 
in slope are measured or estimated. 
The drainage plan, or its absence, is a form that can easily be deduced from 
the aerial photographs. The types of drainage plans and their significance in 
airphoto interpretation are recorded in geologic literature and various airphoto 
interpretation articles [2, 3, 7]. 
It is important to emphasize in this context that on water eroded surfaces 
the dendritic and rectangular plans are the most important. They are individ 
ually defined to be used to differentiate drift from rock respectively. The 
dendritic system is free-flowing without any joint or bedding control. The 
rectangular system and all its variations is controlled by rock joints, faults, 
and/or bedding. 
The erosional forms are the minor features of the landscape produced by 
localized concentrations of erosive energy, whether wind or water. Erosional 
forms produced by eolian activity are blowouts and streaks. Fluvial activity 
will produce forms that are related to sheet, rill and gully processes. Their 
density of occurrence, cross-sectional shape and gradient are important indi 
cators of textures of materials and general cohesive properties of the materials. 
Whenever possible, quantitative measurements of range in depth, range in 
gradient, and range in density are determined and correlated with ground 
control. 
The tones of grey respond to variations in a wide range of factors primarily 
due to reflectance geometry (i.e., the relationship between light source, re 
flecting surface, and camera position). These elements of the total airphoto 
pattern refer to individual photographs. Tones may vary from photograph to 
photograph, but they are always relative for any time of year and under any 
photographic processing condition. 
In general, tonal and textural arrangements on aerial photographs can be 
correlated with land use, vegetation, parent material textures, surface material 
textures, and near surface or surface moisture conditions. Tonal relationships