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Some examples of the most commonly used functional model forms of 
representing terrain elevation data as surfaces are: linear 
function (Allam, 1978), bilinear function (Leberl, 1973; de 
Manson d'Autume, 1979), hyperbolic paraboloid function (Grist, 1972), 
bicubic polynomial function (Schut, 1976), and double Fourier Series 
function(Maxwell and Turpin, 1968). 
The main uses of FDEMs are found in computer-assisted engineering 
applications and in mapping. Doyle (1978) and Turner (1978) have 
highlighted some of these applications. 
The usefulness of these applications notwithstanding, the mathematical 
formulation of the appropriate FDEMs and their solutions have some 
problems to overcome. The author has identified some of these problems 
at different operational levels as shown in Table 1. 
Table 1 : Problems Associated with FDEMs 
LEVEL OF OPERATION 
ASSOCIATED PROBLEMS 
l. 
DATA 
CAPTURE 
(a) 
Type of terrain 
(b) 
Accuracy of data 
(c) 
Density of data 
(d) 
Pattern of data 
2. 
DATA 
PROCESSING 
(a) 
Type of FDEM 
(b) 
Model constraints 
(c) 
Accuracy of output data (specifications) 
(d) 
Computer Algorithm 
3. 
DATA 
OUTPUT 
(a) 
Type of output data 
(b) 
Accuracy output data 
(c) 
Application of output data 
Surface Fitting 
In digital elevation model (DEM), functional mode has some particular 
advantages over discrete form or graphical form. In functional 
mode, a digital computer that processes the data also stores the 
elements of the surface either cellwise or globalwise. These elements