or minimize errors and anomalies in the models. This 
common concern leads to a variety of analytical and 
empirical approaches that are bound together by basic 
mathematical and physical properties, and a common desire 
to develop new production scenarios for digital terrain 
that better define the surfaces being modelled. 
A major problem in the discussion of digitaF terrain models 
is the definition of what is meant by the terminology used 
in the description of the model and associated accuracy 
statements. For purposes of this paper, a Digital Terrain 
Model (DTM) is defined as any numerical representation of 
a landform, not to be confused with a Digital Elevation 
Model (DEM), which only describes digital terrain elevations 
at regular or irregular intervals. Note that a DEM is one 
version of the general class of DTM's. Furthermore, this 
paper will concentrate on production of DTM's over large 
landmass areas (one degree cells and larger) because many 
significant problems arise that are not important in small 
area production, : 
CLASSES OF DIGITAL TERRAIN MODELS 
There are many ways in which DTM's may be classified; how- 
ever, DTM's may be logically discussed by classifying them 
in terms of type of source data used for DTM generation, 
the data collected .in DTM compilation, and the method of 
representation of the DTM as a product. Each of these 
classifications are important, because each class defines 
the requirements for the compilation equipment, the mathe- 
matics used in compilation, and the methodology for evalua- 
tion and usage of the DTM. 
Sources of DTM Information 
DTM's may be produced from two basic sources, physical 
measurement of the surface or mathematical derivation from 
remotely sensed images of the surface. This paper will 
constrain its discussion of physical measurement of the 
surface to cartographic DTM's, those produced from digitized 
contour sheets produced by conventional surface mapping. 
All DTM's mathematically derived from remotely sensed 
imagery will be designated as photogrammetric, consistent 
with a similar definition appearing in the Fourth Edition 
of the Manual of Photogrammetry. 
Cartographically produced DTM's require digitization of 
contour sheets either manually with the aid of a digitizer, 
or by automatic methods using a raster scanner with sub- 
sequent vectorization or by using a line following scanner. 
if a grid of elevations are required, then some interpola- 
tion algorithm must be used to generate points from the 
contour information.