Page 32 - Fundamentals of Geomorphology

WHAT IS GEOMORPHOLOGY? 15

work on landform elements and their descriptors as DEMs are, therefore, a subset of DTMs. Topographic
prosecuted by the morphological mappers. elements of a landscape can be computed directly from a
DEM (p. 170). Further details of DEMs and their appli-
Geomorphometry cations are given in several recent books (e.g. Wilson and
Gallant 2000; Huggett and Cheesman 2002).
A branch of geomorphology – landform morphometry
or geomorphometry – studies quantitatively the form
of the land surface. Geomorphometry in the modern PROCESS
era is traceable to the work of Alexander von Humboldt
and Carl Ritter in the early and mid-nineteenth cen- Geomorphic systems
tury (see Pike 1999). It had a strong post-war tradition Process geomorphologists commonly adopt a systems
in North America and the UK, and it has been ‘rein- approach to their subject. To illustrate what this
vented’ with the advent of remotely sensed images and approach entails, take the example of a hillslope system.
Geographical Information Systems (GIS) software. The A hillslope extends from an interfluve crest, along a val-
contributions of geomorphometry to geomorphology ley side, to a sloping valley floor. It is a system insofar as
and cognate fields are legion. Geomorphometry is an it consists of things (rock waste, organic matter, and so
important component of terrain analysis and surface forth) arranged in a particular way. The arrangement is
modelling. Its specific applications include measuring the seemingly meaningful, rather than haphazard, because it
morphometry of continental ice surfaces, characterizing is explicable in terms of physical processes (Figure 1.7).
glacial troughs, mapping sea-floor terrain types, guiding The ‘things’ of which a hillslope is composed may be
missiles, assessing soil erosion, analysing wildfire prop- described by such variables as particle size, soil moisture
agation, and mapping ecoregions (Pike 1995, 1999). content, vegetation cover, and slope angle. These vari-
It also contributes to engineering, transportation, public ables, and many others, interact to form a regular and
works, and military operations. connected whole: a hillslope, and the mantle of debris
on it, records a propensity towards reciprocal adjustment
Digital elevation models among a complex set of variables. The complex set of
variables include rock type, which influences weathering
The resurgence of geomorphometry since the 1970s is rates, the geotechnical properties of the soil, and rates
in large measure due to two developments. First is the of infiltration; climate, which influences slope hydrology
light-speed development and use of GIS, which allow and so the routing of water over and through the hills-
input, storage, and manipulation of digital data repre- lope mantle; tectonic activity, which may alter baselevel;
senting spatial and aspatial features of the Earth’s surface. and the geometry of the hillslope, which, acting mainly
Second is the development of Electronic Distance through slope angle and distance from the divide, influ-
Measurement (EDM) in surveying and, more recently, ences the rates of processes such as landsliding, creep,
the Global Positioning System (GPS), which made the solifluction, and wash. Change in any of the variables
very time-consuming process of making large-scale maps will tend to cause a readjustment of hillslope form and
much quicker and more fun. The spatial form of surface process.
topography is modelled in several ways. Digital repre-
sentations are referred to as either Digital Elevation Isolated, open, and closed systems
Models (DEMs)or Digital Terrain Models (DTMs).
A DEM is ‘an ordered array of numbers that represent Systems of all kinds are open, closed, or isolated accord-
the spatial distribution of elevations above some arbitrary ing to how they interact, or do not interact, with their
datum in a landscape’ (Moore et al. 1991, 4). DTMs are surroundings (Huggett 1985, 5–7).Traditionally, an iso-
‘ordered arrays of numbers that represent the spatial dis- lated system is a system that is completely cut off from
tribution of terrain attributes’ (Moore et al. 1991, 4). its surroundings and that cannot therefore import or