HILLSLOPES - Fundamentals of Geomorphology - page 170

Landform elements

contour curvature. Further parameters go beyond local
geometry, placing the element in a wider landscape
setting - distance from the element to the crest, catch-
ment area per unit of contour length, dispersal area (the
land area down-slope from a short increment of contour).
Digital elevation models (DEMs) have largely superseded
the classic work on landform elements and their descrip-
tors. Topographic elements of a landscape can be com-
puted directly from a DEM and these are often classified
into primary (or first-order) and secondary (or second-
order) attributes (Moore et al . 1993). Primary attributes
are calculated directly from the digital elevation data and
the most commonly derived include slope and aspect
(Table 7.1). Secondary attributes combine primary
attributes and are 'indices that describe or characterise
the spatial variability of specific processes occurring in the
landscape' (Moore et al . 1993, 15); examples are irradi-
ance and a wetness index (Table 7.1). Such methods allow

From a geomorphological viewpoint, the ground sur-
face is composed of landform elements. Landform
elements are recognized as simply-curved geometric sur-
faces lacking inflections (complicated kinks) and are
considered in relation to upslope, downslope, and lat-
eral elements. Slope is essential in defining them.
Landscape elements go by a plethora of names - facets,
sites, land elements, terrain components, and facies. The
'site' (Linton 1951) was an elaboration of the 'facet'
(Wooldridge 1932), and involved altitude, extent, slope,
curvature, ruggedness, and relation to the water table.
The other terms appeared in the 1960s (see Speight
1974). Landform element is perhaps the best term, as
it seems suitably neutral.
Landform elements are described by local land-
surface geometry. Several parameters are derivatives of
altitude - slope angle, slope profile curvature, and

Table 7.1 Primary and secondary attributes that can be computed from DEMs

Attribute Definition Applications

Primary attributes
Altitude Height above mean sea level or local

reference point

Climate variables (e.g. pressure, temperature),
vegetation and soil patterns, material
volumes, cut-and-fill and visibility calculations,
potential energy determination

Slope Rate of change of elevation - gradient Steepness of topography, overland and

subsurface flow, resistance to uphill transport,
geomorphology, soil water content

Aspect Compass direction of steepest downhill

slope - azimuth of slope

Solar insolation and irradiance,
evapotranspiration

Profile curvature Rate of change of slope Flow acceleration, erosion and deposition

patterns and rate, soil and land evaluation
indices, terrain unit classification

Plan curvature Rate of change of aspect Converging and diverging flow, soil water

characteristics, terrain unit classification

Secondary attributes

A s
tan b where A s is specific catchment
and b is slope

Wetness Index ln = Index of moisture retention

Irradiance Amount of solar energy received per unit

area

Soil and vegetation studies, evapotranspiration

Source: Adapted from Huggett and Cheesman (2002, 20)