Efiong-Fuller, Emmanuel O. 
  
The erosional features and landforms must be identified, defined and/or delimited on the terrain. The geodynamic processes 
involved must be investigated and the impact of each assessed. The delimited erosional features and landforms must then 
be described, measured, and presented in the appropriate format, which necessarily always includes maps. 
The resulting maps can be geomorphological maps, geomorphometric maps, or even specialized engineering geological 
maps depending on the purpose and scope ofthe mapping; or they may simply be called "Erosion Maps". Erosion mapping 
and monitoring have generally been done by ground survey methods; however a method based on the use of multi- 
temporal aerial photographs and numerical data processing is hereby proposed. The practical procedure includes geomor- 
phological interpretation of the sets of aerial photographs, the measurement of coordinates, and appropriate processing of 
the data so obtained. 
4.] Geomorphological Interpretation 
The purpose of the geomorphological interpretation is to identify and delineate the outline and form of the portion of the 
ravine under study, as well as associated geomorphological features. It also includes the description and codification of 
the features. 
The geomorphological interpretation was performed by visual image analysis using the Mirror Stereoscope, and with 
transparent overlays placed on the photographs for annotations. For each date three consecutive photographs covering 
the ravine area were selected. The principal point and conjugates on each photographs, and the observed features were 
marked on the transparent overlay and subsequently compiled for each ofthe years 1969, 1978 and 1988 (Figs. 6, 7, 8) 
4.2. Measurement of Coordinates 
The outline of the ravine obtained by geomorphological interpretation is highly irregular. To this irregular outline the best 
fitting polygonal shape was constructed. In effect the irregular outline of the ravine was converted into a series of 
intersecting straight lines and the coordinates of the points of intersection then measured in the photo-coordinate system. 
To establish the photo-coordiante system the principal point and conjugates marked on each photograph and transferred 
onto the transparent overlay were used. For each set of photographs the line joining the principal point and conjugate on 
the first photograph ofthe strip defined the flight line, and this was adopted as the x-coordinate axis. Another line through 
the same principal point and perpendicular to the flight line defined the y-coordiante axis. These two perpendicular lines, 
with the principal point of the first photograph as origin, constitute the coordinate system for measurement on the photo- 
graphs. Thus for the 1969 set of photographs the principal pois numbered 172 was the origin and the coordinate axes as 
indicated (Fig. 6). 
All points marking the outline ofthe ravine were then measured in the above coordinate system. Also measured were three 
control points A, B, and C with known ground coordinates. These control points wére used to compute the parameters 
necessary to transform the coordinates of all other points measured on the photograph to the ground coordinate system. 
The coordinates were measured directly to half a millimeter, and by estimation to a quarter of the millimetre using the 
transparent plastic ruler with lens magnifier, this serving as an improvized micro-rule. 
5. DATA PROCESSING 
Data processing necessarily involves performing a set or series of mathematical operations on data so as to obtain the 
required information. In the present study, data processing includes transformation of coordiantes, computation of areas, 
and computation of the rates of erosion. 
5.] Transformation of Coordinates 
The coordinates of all points of interest measured in the photocoordinate system must be transformed to the corresponding 
ground coordinates of these points in order to give them geodetic significance, and relate the points to their correct 
positions on the terrain. This was done by two dimensional conformal coordinate transformation . The advantage of the 
two dimensional conformal coordinate transformation is that true shapes, are preserved after transformation (Efiong-Fuller, 
1979, 16) and the mathematics involved simple enough to be performed with a pocket calculator. 
  
392 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.