A review of landform classification methods · A review of landform classification methods Marzieh...
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A review of landform classification methods
Marzieh Mokarram1• Dinesh Sathyamoorthy2
Received: 16 May 2018 / Revised: 13 August 2018 / Accepted: 14 August 2018 / Published online: 17 August 2018
� Korean Spatial Information Society 2018
Abstract The study of landform can be used to predict
specific solid rock and moisture conditions that exist in that
landform. Landforms are of significance in engineering
because they influence the quality and type of grading, pre-
determine the drainage requirements, and the soil or rock
conditions. They have added significance because there are
an infinite number of duplicates of each type. Given that
each of the landform units have different characteristics
(soil, slope, elevation, vegetation etc.), classification of
landforms is importance. Therefore, landforms are widely
recognized as natural objects that partition the earth’s
surface into essential spatial entities. Landform entities
differ from one another in terms of specification such as
shape, size, orientation, relief and contextual position.
They also differ in terms of the physical processes that
were involved in their formation and that continue to
operate within them at the present time. This paper aims at
providing a review of the landform characteristics and
classification. Various definitions and attributes of land-
forms which focus on aspects of morphometry, geomor-
phometric context, terrain positions and scale are discussed
as well. In addition, the methods developed for the land-
form classification are assessed.
Keywords Landform � Geomorphometric � Classification
1 Introduction
As there is a wide variety of landforms on the earth, it is
useful to classify the earth surface complexity to easily
identify and manage them. Hence, the description of the
landform characteristics has received significant attention
from the earth scientists for decades. Landform is a specific
geomorphic feature on the surface of the earth, ranging
from different scale features such as plains, plateaus and
mountains to hills, valleys or alluvial fans.
There is no unique and universal definition in the views,
applications and definitions of the landform classification
worldwide. Scientists have brought this concept into con-
servation planning by identifying land facets [1], abiotic
units [2], geodiversity [3], geophysical stages [4], and ge-
omorphometric units [3]. Therefore, researchers decided to
cooperate with users in different contexts and solve this
problem [5]. As a land surface morphology is based on the
landforms structures, it forms the basis for geomorphology
maps and other earth sciences [6].
Air-photo interpretation and field studies are traditional
methods of identifying landforms that are limited by the
efficiency of an individual performing the interpretation
[7]. Therefore, the processes of identifying and determin-
ing landforms were initially difficult and time consuming.
Until recently, there have been published researches on the
general geomorphometry regarding principles of measuring
a shape and surface. These studies can be applied to
landforms with some modifications to materialize these
methods. For example, a set of 15 generic landforms based
on geometric features, such as altitude, slope and curvature,
were defined by Dikau [8]. However, there still is a place
& Marzieh Mokarram
Dinesh Sathyamoorthy
1 Department of Rangeland and Watershed, Agriculture
College and Natural Resources of Darab, Shiraz University,
Shiraz 71946-84471, Iran
2 Science and Technology Research Institute for Defence
(STRIDE), Ministry of Defence, Kuala Lumpur, Malaysia
123
Spat. Inf. Res. (2018) 26(6):647–660
https://doi.org/10.1007/s41324-018-0209-8
for developing methods and general types of classification
models which represent topography better. For example
MacMillan et al. [9] (segmenting landforms using fuzzy
method), Burrough et al. [10] (using fuzzy k-means for
high resolution landform classification), Schmidt and
Hewitt [11] (Fuzzy land element classification from
DTMs), Saadat et al. [12] (landform classification using
10-m resolution digital elevation model (DEM)), Jasie-
wicz, and Stepinski [13] (using DEM with high resolution),
Jasiewicz et al. [14] (using DEM and GeoPAT toolbox)
and Zawawi et al. [15] (using DTM with a spatial scale of
10 9 10 m) used automated methods of landform classi-
fication that improve the previous methods with specific
scale, scope, and spatial resolution. These researches of the
landform classification used digital elevation models
(DEMs) and new algorithms, such as the artificial neural
network (ANN) and self-organizing maps (SOM).
This paper is aimed at providing a review of landform
characteristics and classifications in different scale. The
various definitions and characteristics of landforms,
focusing on aspects of morphometry, geomorphometric
context, terrain positions and scale, will be discussed. In
addition, the methods developed for landform classification
will be assessed.
2 Landforms
Characteristics and recognizable shapes of landforms
define the physical features of the earth’s surface [16]. A
subjective semantic definition of landforms in the field of
specific geomorphometry is a terrain unit created by natural
processes [17]. However, many geomorphologists prefer a
definition which includes the recognition of artificial
landforms, such as quarries and waste heaps [18].
Dimensions such as length, width and height, and the sta-
tistical frequency of principal geomorphic attributes are
used to distinguish a landform type [18].
In addition, landform types have been referred to as
relief forms with meso-form associations [19] and land-
form patterns [20, 21]. In fact, plains, hills, mountains and
valleys which can be observed from multiple scales are
examples of landform types. These names are used for
larger landscapes in geography. A landform element is a
sub-part of a landform type that is hierarchical to landform
type.
2.1 Landform definitions
Landforms are defined as specific geomorphic features on
the surface of the earth, ranging from large-scale features
such as plains and mountain to minor features such as
individual hills and valleys. In addition, studies of
landforms involve two major interrelated conceptual
frameworks; functional and historical [21]. Processes
operative in the fields of geomorphology, hydrology,
ecology, forest, soil, pedology, geology, cartography and
others can be defined by landforms [15, 19, 22]. For
example, the separate and distinct soil characteristics,
topography, rock materials, and groundwater conditions
can be presented for each landform [23].
Some researchers view landform units as terrain features
created by natural processes with dependable information
concerning their own structure [24]. Landforms are also
defined by their surface shapes and location (can be per-
ceived as a small peak) in relation to other landforms,
underlying geologic materials and soil attributes [25].
Evans [26] described the size and space of landforms
clustered around the space available at characteristic
scales. The surface shape of landforms has been consis-
tently used to interpret or infer hill slope forming pro-
cesses, such as erosion, denudation, accumulation and
deposition, and other geomorphic processes. In addition,
Pike et al. [27] suggested that a landform has morphologic
features such as a watershed, which is interrelated to a part
of the land surface formed by specific geomorphological
process. Each landform may be combined of several
landform elements which have relatively stable morpho-
metric properties [28].
Landform units have been used as basic landform
descriptors in vegetation mapping [29], soil mapping [30]
and landscape ecology [31]. Terms such as ridge, hollow
and foot slope are used in many mapping legends to define
essential terrain units at the hill slope scale [7]. However,
the rationale behind these land units is only weakly defined
and differs in different legends.
Various studies have suggested notations and descrip-
tions of land elements [30]. For example, Milne et al. [31]
defined 77 land units at different scales, while Speight [30]
described 70 different landform elements by using the
attributes of slope, morphological type, dimensions and
geomorphological activity.
2.2 Landform characteristics
2.2.1 Morphometry
Simple morphometric features, such as saddles, peaks,
channels, ridges and planes, are identified based on the
predefined rules in geomorphometry, [32, 33]. Bolongaro-
Crevennaa et al. [34] showed that the morphometric fea-
tures proposed by Wood [35] can characterize each DEM
element but they cannot fully describe a group of more
complex forms of actual landforms. They demonstrated
that a strict relationship exists between morphometric
features and geomorphologic processes. To elaborate their
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648 M. Mokarram, D. Sathyamoorthy
demonstration, they suggested that the morphometric
parameters can be used to characterize selected numeric
elementary forms associated with landforms.
Terrain form is categorized into ‘general geomor-
phometry’ and ‘specific geomorphometry’ [36]. General
geomorphometry involves the quantitative analysis of
landforms characteristics which are applicable to any
continuous surface. Specific geomorphometry is restricted
to specific concepts which may not be continuous for the
whole surface.
For general geomorphometry, the best known classifi-
cation design was proposed by Peucker and Douglas [37] in
six classes; ridge, peak, pass, channel, pit and planar
(Fig. 1; Table 1). This classification is based on a 3 9 3
window for evaluating DEMs. This design has been
employed by many authors [32, 34, 38].
Dikau [7] proposed a classification design for 15 classes
based on possible combinations of a land slope and a
curvature. For tangential curvature (slope area) these
classes can only define sloping elements of a surface
(Fig. 2) and do not have any class features such as chan-
nels, ridges and peaks. While for maximum curvature (flat
area) the classes of features consist of peak, ridge, plain,
saddle, channel and pit. As it includes, all possible forms
combinations are very appropriate for general geomor-
phometric classification. It has also been employed by
many recent authors [11, 39].
A more expanded design that defines a complete system
of the landform classification concerning the previous
classifications of Gauss [40] and Troeh [41] was presented
by Shary et al. [42] (Fig. 3) that is based on tangential
profile, mean difference and total Gaussian curvatures.
According to the scheme, form facets are defined with their
homogeneous ‘slope’ and ‘aspect’, whereas form elements
are described by their curvature and slope. This
scheme might remain theoretical rather than applicable as
it is not practically effective to extract these components
from DEMs and label them by using a generic nomencla-
ture in the landform classification context. Moreover, a set
of useful DEMs must be applicable to be related to land-
form classes. Then, a threshold is needed to judge if the
slope of an image element is satisfactory to be classified as
a sloping element or if the extent to which an element is
convex enough to be classified into a class or not (for
example, peak).
2.2.2 Geomorphometric context
Geomorphometry is a science devoted directly to quanti-
tative analysis of the earth’s surface. Nowadays, it is highly
regarded as a separate discipline. It provides qualitative
and quantitative descriptions and measurements of land-
forms [43]. An essential principle of geomorphometry
asserts that there exists a relationship between a relief form
and the numerical parameters used to describe it. Likewise,
a relief form relates to the processes related to its genesis
and evolution. One aim of geomorphologists working with
landform models is to obtain the best representation of a
physical reality [34].
The interaction between form and process is the center
of geomorphology [44] and form characteristics are con-
sidered as key components of geomorphological systems
[45]. A different method was built based on physiographic
aspects of land surfaces by Christian and Stewart [46].
Geomorphometric characteristics have been measured
manually for decades [47] with some methods which
involve derivation making from topographic maps that was
time consuming and labor intensive. To this end, Evans
[36] defined an integrated system of geomorphometry.
Since then, important progress has been achieved in
improving DTM accuracies [10–12], and developing new
algorithms and software to calculate terrain derivatives
[48].
Fig. 1 Six landform classes
proposed by Peucker and
Douglas [37]. a Ridge,
b channel, c plane, d peak,
e pass and f pit
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A review of landform classification methods 649
2.2.3 Terrain positions
Terrain positions depict geographic meanings and spatial
processes. They are usually labeled as ridge tops, shoul-
ders, back slopes, foot slopes, etc. Locations with the same
morphometric properties might belong to different slope
positions and are associated with different geomorphic
processes [49].
The system proposed by Gauss [40] described four field-
invariant geometric forms defined by total Gaussian signs
and mean curvatures (Fig. 4). This system, as expressed by
Troeh [41, 50], partitioned land surfaces into four gravity-
specific classes in order to determine two relative accu-
mulation mechanisms based on the consideration of tan-
gential and profile curvatures signs (Fig. 5).
Dalrymple et al. [51] proposed a nine-unit slope model,
while Conacher and Dalrymple [52] described each unit by
taking the best advantages of position, slope, profile cur-
vature and actual processes (Table 2). In addition, the four-
Table 1 Definitions of the xix landform classes proposed by Peucker and Douglas [37]
Feature name Description
Peak Point that lies on a local convexity in all directions (all neighbors lower)
Ridge Point that lies on a local convexity that is orthogonal to a line with no convexity/concavity
Pass Point that lies on a local convexity that is orthogonal to a local concavity
Plane Points that do not lie on any surface concavity or convexity
Channel Point that lies in a local concavity that is orthogonal to a line with no concavity/convexity
Pit Point that lies in a local concavity in all directions (all neighbors higher)
Fig. 2 The 15 form elements proposed by Dikau [8]. a Nose, b shoulder slope, c hollow shoulder, d super, e planar slope, f hollow, g super foot,
h foot slope, i hollow foot, j peak, k ridge, l plain, m saddle, n channel and o pit
Fig. 3 The landform classification scheme proposed by Shary et al.
[42]. a Total Gaussian curvature, b difference curvature, c total
Gaussian curvature, d mean curvature, e profile curvature and
f tangential curvature
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650 M. Mokarram, D. Sathyamoorthy
unit slope model proposed by Wood [53] (Fig. 6) delin-
eates slope profiles that represent typical examples of
classifying landscape into relative positions.
Ruhe and Walker [54] separated hill slopes into five
segments; summit, shoulder, back slope, foot slope, toe
slope and alluvium by using a slope length and width
(Fig. 7). They also recognized geomorphic units of head,
nose and side slopes which add divergent, convergent and
linear possibilities of plan curvature to their
scheme (Fig. 7).
Pennock et al. [55] used data published by Young [56]
to categorize nine 3D landform elements by applying
measures of plan, profile curvatures and slope (Fig. 8). The
Fig. 4 Illustration of the four landform classes defined by Gauss [40]
based on total Gaussian and mean curvature [42]. a C-depressions,
b C-hills, d C-saddles and e C-saddles
Fig. 5 Illustration of the four landform classes defined by Troeh
[41, 50]. a Transit 1, b relative deflection zones, c relative accumu-
lation zones and d transit 2
Table 2 Landform units defined by Conacher and Dalrymple [52]
Land-surface unit Characteristics
1 Interfluve Predominant pedomorphologic processes caused by vertical (up and down) soil–water movements; 0�–1� slopegradient
2 Seepage slope Upland area where responses to mechanical and chemical eluviation by lateral subsurface soil–water movements
predominate
3 Convex creep slope Convex slope element where soil creep is the predominant process, producing lateral movement of soil materials
4 Fall face Areas with gradients greater than 45�, characterized by the process of fall and rockslide
5 Transportational
midslope
Inclined surfaces with 1�–45� gradients and responses to transport of large amounts of material downslope by flow,
slump, slide, erosion and cultivation
6 Colluvial footslope Concave areas with responses to colluvial redeposition from upslope
7 Alluvial toeslope Areas with responses to redeposition from upvalley alluvial materials; 0�–4� gradient8 Channel wall A channel wall distinguished by lateral corrosion by stream action
9 Channel bed A stream channel bed with transportation of material downvalley by stream action as the predominant process
Fig. 6 Slope models proposed by Wood [53]. a Upper convex
segment, b cliff face, c straight segment and d lower concave segment
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A review of landform classification methods 651
elements were described with slope rate change in both
vertical and horizontal planes. This scheme is also similar
to the Richter [57] and Dikau [8] schemes.
In another study, three hierarchical terrain entities
classes and terrain and soil components were identified by
Zinck and Valenzuela [58]. They attempted to automati-
cally classify landforms to entities which are approxi-
mately equivalent to one, and only occasionally both of
two main hierarchical levels of landform patterns or land-
form facets (Fig. 9).
A two-level descriptive procedure of systematic and
parametric landforms description into landform patterns
and elements was proposed by Speight [20, 30]. Table 3
shows the classification system of profiles across the terrain
divided into morphological types of landform elements as
classified by Speight [30]. Skidmore et al. [59] related a
slope position to implied processes of the hill slope and soil
formation (Fig. 10). Upper slope positions were regarded
as zones of active removal and transporting materials while
lower slope positions were clearly identified as zones of
deposition and accumulation of materials. The proposed
method of Dikau et al. [19, 60] follows Hammond method
in defining four classes of a gentle slopes proportion, six
classes of a relative relief and four classes of a profile type
(Table 4) which, when combined together, result in 96
landform sub-classes [61]. MacMillan et al. [62] follows
Pennock’s modified original rules to include the consider-
ation of the relative slope position measured to the total
elevation difference from channel to divide or pit to the
peak (Fig. 11).
Most of the above discussed methods combine the
environmental meanings of terrain shape and position [63]
and have been attractive in representing specific charac-
teristics of topography [64]. Some of the indices that are
used to classify terrains (landforms) into positions gener-
ated from DEMs are the topographic position index (TPI)
[65], local elevation [66] and relative hill slope position
[67].
2.2.4 Scale
Most classifications of landforms are specific to a particular
scale. Landforms have been widely defined to occur across
a hierarchy of scales which assert that a geomorphic system
must be viewed in its complex, hierarchical context
[7, 32, 68, 69]. Etzelmuller and Sulebak [32] who assert
that a geomorphic system must be viewed in its complex,
hierarchical context.
Some characterizations of landforms are basically scale-
free and define the same invariant spatial entities regardless
of the scale [42]. Most either implicitly or explicitly define
a hierarchy with different sizes of landforms occurring at
different scales. Dikau [70] who follows Kugler [71],
illustrated how different landforms have been conceptual-
ized to occur over different scales. This hierarchical con-
ceptualization includes relief units ranging from relatively
homogeneous form facets to more complex associations or
patterns which consist of assemblages of lower level forms.
Hierarchical conceptualization was considered to make
form units (e.g. form facets) easier to be defined at micro-
scales by inherent shape-based attributes [70].
Dikau [70] suggested that most efforts to classify land-
forms correspond to the micro (1–100 m) and meso
(100 m–10 km) scales. Most scales/scholars also typically
focus on levels of complexity on the offered range of form
facets to form associations.
Many authors have examined the effects of varying grid
resolution on the values and accuracies of land-surface
characteristics and objects derived from elevation data sets
[72]. Ideally, each application scale in the wide range of
Fig. 7 Geomorphic units proposed by Ruhe and Walker [54].
a Summit, b shoulder, c backslope, d foot slope, e toe slope and
f alluvium
Fig. 8 Geomorphic units proposed by Pennock et al. [55]. a Diver-
gent, b planar, c convergent, d shoulder, e backslope and f footslope
Fig. 9 Illustration of conceptual differences between repeating
landform types and facets [42]. a Lower relief (undulating) and
b high relief (undulating)
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652 M. Mokarram, D. Sathyamoorthy
use refers to multiple concepts; thus, it is a vague term.
Scales are attributed to the following [49] concepts:
(i) Cartographic scale: Depicted size of a feature on a
map relative to its actual size in the world;
(ii) Generalization: Level of abstraction;
(iii) Extent of the area;
(iv) Phenomenon scale: Depicted size at which human
or physical earth structures or processes exist
(v) Analysis scale: Size of the unit at which some
problem is analyzed
Scale is the window of perception, and hence, it has a direct
influence on the landform type on a specific location
[49, 73]. For instance, as shown in Fig. 12, at the finer scale
topography, a location can be perceived as a small peak,
but when the window is magnified, it becomes part of a
planar slope, and at a coarser scale, even a channel. If a
morphometric class is found to be persistent in the same
area of geographical space at different spatial resolutions, it
is defined as scale-independent [74].
A recent study for classification with different scales
was done by Kramm et al. [75]. The results showed that
increasing resolution should extract more details from the
study area. Generally, most experiences indicate that DEM
pixel resolutions between 5 and 30 m are most suitable for
landform classifications [76].
DEMs of the Shuttle Radar Topography Mission
(SRTM) and Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) (30 m pixel resolution)
and two DEMs (5 and 10 m pixel resolution) use for
landform classification commonly. Based on the presented
method by Dikau et al. [19], Weiss [77], Jasiewicz and
Stepinski [13], Dragu and Blaschke [39], landforms were
classified in four different ways for each digital elevation
model.
Table 3 Topographic position classes proposed by Speight [30]
Name Topographic position classes
Crest Area high in the landscape having positive plan and/or profile curvature
Depression (open,
closed)
Area low in the landscape having negative plan and/or profile curvature, closed: minimum local elevation; open: extends
at same or lower elevation
Flat Areas having a slope\ 3%
Slope Planar element with an average slope[ 1%, subclassified by relative position
Simple slope Adjacent below a crest or flat and adjacent above a flat or depression
Upper slope Adjacent below a crest or flat but not adjacent above a flat or depression
Mid slope Not adjacent below a crest or flat and not adjacent above a flat or depression
Lower slope Not adjacent below a crest or flat but adjacent above a flat or depression
Hillock Compound element where short slope elements meet at a narrow crest\ 40 m
Ridge Compound element where short slope elements meet at a narrow crest[ 40 m
Fig. 10 Illustration of the inferred relationship between slope
position and geomorphic processes inherent in the approach of
Skidmore et al. [59]. a Stable residual, b degraded transportational
and c aggraded depositional
Table 4 Classification criteria of the method proposed by Dikau et al. [60]
Distribution of gentle slopes Local relief Profile type
(A) More than 80% of the area is gently sloping (1) 0–30 m (a) More than 75% of gentle slope is lowland
(B) 50–80% of the area is gently sloping (2) 30–91 m (b) 50–75% of gentle slope is lowland
(C) 20–50% of the area is gently sloping (3) 91–152 m (c) 50–75% of gentle slope is upland
(D) Less than 20% of the area is gently sloping (4) 152–305 m
(5) 305–915 m
(6)[ 915 m
(d) More than 75% gentle slope is upland
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A review of landform classification methods 653
3 Landform classification
The landform classification has a wide range of application
domains, including mapping geomorphologic units [78],
mapping lithology [79], predicting soil properties [80], a
landscape ecology [81], vegetation mapping [82], precision
agriculture [83] and landslide mapping [84]. The landform
classification also constitutes a basic research topic in
geomorphometry [43]. Up until the 1950s, poor qualitative
descriptions of landforms had been issued due to the lack
of quantitative methods [85]. Traditionally, studies have
been performed by using a manual overlay process that
relies on hand-drawn maps on the transparent vellum.
Lately, GIS permits the geographic data merging by using
computers. The knowledge-based process that combines
existing geographic data to generate new information is
generally known as a cartographic modeling. The compo-
nent maps that show the geographic distribution of a single
category of parameters are known as geographic themes
[21].
Therefore, the landform classification is described as the
science of land definition which focuses on the extraction
of land-surface parameters and objects from DEMs and
DTMs [27]. Since GIS tools and digital elevation data
become easily accessible, more research subjects are in
geomorphometry and neighboring fields that focus on the
landform classification [5].
3.1 Brief history
For a long time, the landform mapping was based on the
visual expression of stereo aerial photographs and topo-
graphic maps supported with field surveys [47]. The visual
expression and landforms extraction was tedious; on the
other hand, the measurement of parameters from topo-
graphic maps was labor intensive and time consuming. In
addition, the classes of landforms relied heavily on the
expert’s experience and conceptual model [86] which is
materialized implicitly onto maps.
By appearing computers in the 1950s, the availability of
DEMs and derived data sets, such as slope, aspect, shaded
relief and hydrographical pattern, technology was used to
examine terrains [87]. Furthermore, the advent of GIS
technology and digital image processing techniques have
led to significant advances in the terrain parameterization
[32], specifically for the development of the automated
landform classification [10, 11]. There are many methods
in digital geomorphology to delineate watershed catch-
ments and sub-catchments with standard procedures [32].
With the development of science, different methods
were proposed for the extraction and classification of
landforms. Some of these methods were designed for
identifying certain features types like the linear or circular
forms [88], or specific forms such as mountains [89], hill
tops [90], landslides or strike ridges [91] or other features
[92]. Many approaches aim at characterizing hill slope
forms [10, 69] as it is widely pronounced in hydrological
studies. Concerning hydrological responses, the systematic
down slope variations are shown due to the water, soil and
nutrient movement controlled by surface forms along the
hill slope. Ruhe and Walker [54] identified geomorphic
units that divide hill slopes into five segments; summit,
shoulder, back slope, foot slope, toe slope, alluvium using
slope, and slope length and width. Landform elements can
Fig. 11 Illustration of landform elements extracted from land-surface
parameters for a 64 ha site in Alberta, Canada [9]. a Level crest,
b divergent shoulder, c upper depression, d backslope, e divergent
backslope, f convergent backslope, g terrace, h saddle, i midslope
depression, j footslope, k toeslope, l fan, m lower slope mound, n level
lower slope and o depression
Fig. 12 Changing morphometric classes with scale [73]. a Morpho-
metric class, b, c channel, d planar and e peak
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654 M. Mokarram, D. Sathyamoorthy
be extracted from DEMs by applying various approaches
such as the examples given in Table 5.
3.2 Classification approaches
3.2.1 Classification based on general geomorphometry
General geomorphometry created a basis for the quantita-
tive comparison of different landscapes, as well as suiting
methods of the surface analysis provided outside geomor-
phology [27]. Before the advent of computers, the surface
analysis was acutely difficult; however, recently, with
increasing processing capabilities and great availability of
DEMs, it has become widespread and has many applica-
tions in digital terrain modeling [43]. The landform clas-
sification based on the general geomorphometry deals with
elementary forms and is less common as compared to a
wide range of application domains of the specific geo-
morphometry [27]. General geomorphometry is relatively
universal; it passes problems of landform subjective defi-
nition to an extent. Subjective ideas and operator biases are
reduced drastically though they are not totally eliminated
[93]. These biases appear objective in case that they have
choice of the data source, resolution and algorithms for
interpolation, smoothing and derivative calculation.
One significant characteristic and at the same time
drawback of the landform classification based on the gen-
eral geomorphometry is that it lacks accuracy whereas the
specific geomorphometry is precise, however, the specific
geomorphometry can only be applied in a specific context
[32]. General and specific geomorphometries are not
completely separated because some specific landforms like
hill slopes and drainage networks are so widespread on the
earth that their specific geomorphometry gets a general
importance and because some techniques of general geo-
morphometry can be applied to specific landforms as well
[94].
Geomorphons were introduced by Stepinski and Jasie-
wicz [95], and Jasiewicz and Stepinski [13] to automati-
cally classify pixels based on the local gradients (reduced
to the sign -, 0, ?), and computed according to the line-of-
sight principle. This way, 498 different geomorphon results
are produced (6561 if the orientation is taken into account)
in which the terrain setting may be categorized. Camiz and
Poscolieri [96] described in different ways to classify the
geomorphometry structures of the terrain under study and
provide appropriate graphical representations.
3.2.2 Classification based on specific geomorphometry
Specific geomorphometry involves the detection of discrete
features such as drumlins, sand dunes, alluvial fans, land-
slides, etc. [27]. The definitions of these specific geomor-
phologic features require operational determinations and
precise thresholds of parameters to ensure their proper
characterization [97]. The specific geomorphometry relates
to domain use such as the ecological, surface hydrology,
mapping and climate studies since these fields extract
specific landforms relevant to their interest using terrain
parameters. This type of classification covers a very great
part of the landform classification studies.
3.3 Automated landform classification
Due to the taxonomic designs of landforms like their
provenances, compositions and functions, these features
are hard to map and quantify by applying manual methods
merely. The automatic classification of geomorphological
land units mainly focuses on morphometric parameters
[89]; this can describe the form of a land surface in relation
to landform formation processes. Automated methods of
segmenting landforms into landform elements or facets
have been defined by different authors [39].
Consequently, researchers have been developed routines
procedures of the automatic landform extraction and clas-
sification for a variety of applications. Barbanente et al.
[98] expanded routines procedures of automatically iden-
tifying ravines and cliffs which are not categorized features
that can be justifiably included in a general landscape
classification methodology. Noticeably, several research
groups have developed methodologies to extract terrain
features from DTMs [91]. Dikau [8] expanded a method to
identify convex scarps, straight front and concave foot
slopes, scarp forelands, cuesta scarps, valleys, small drai-
nage ways and crests.
The automated classification of landform elements that
use object-based image analysis (OBIA) was proposed by
Dragut and Blaschke [39]. The OBIA are classified as
landform elements that apply a relative classification model
and built on both surface shape and altitudinal position of
objects. The classification has nine classes: peaks and toe
slopes (defined by an altitudinal position or degree of
dominance), shoulders and negative contacts (defined by
profile curvatures), head, side and nose slopes (defined by
Table 5 Examples of landform classification
Methods Authors
Combination of morphometric parameters [8, 28]
Fuzzy logic, unsupervised classification [10]
Supervised classification [106]
Probabilistic clustering algorithms [92]
Multivariate descriptive statistics [5, 8]
Double ternary diagram classification [34]
123
A review of landform classification methods 655
plan curvatures) and steep slopes and flat/gentle slopes
(defined by slope gradients). Classes are defined by taking
the best advantages of flexible fuzzy membership func-
tions. The mentioned method is directly applicable to
precision farming in Alberta, Canada. The model uses
derivatives computed from DEMs and the fuzzy logic rules
to identify up to 15 morphologically defined landform
facets.
3.4 Fuzzy versus crisp classifications
Methods of the landform classification can be sub-divided
according to whether they generate continuous (fuzzy) or
they generate rigid (crisp) classifications [99]. Each of
these classes may be further sub-divided according to
whether the classification rules are based on an expert
judgment or they are based on a statistical analysis.
The fuzzy classification is referred to a continuous
classification which takes the natural class overlap into
account to allow individual spatial entities to have mem-
berships (partial belongings) in multiple classes, and to
allow the existence of overlapped classes which have
gradual transitions from one another [100]. The degree to
which an entity belongs to a given class is not expressed in
terms of Boolean set theory (0 or 1), but in the fuzzy logic,
this theory is set based on membership function. Crisp
methods are increasingly being replaced by the fuzzy
methods classification because they allow a class overlap
which is very common for natural groups in terrain phe-
nomena [49].
Morphometric classes derived from mixtures of these
derivatives are more informative and useful if fuzzy sets
are used. In the context of analyzing DEMs, any given cell
may in fact contain elements of a number of different
morphometric classes. This is represented by the degree of
membership or belonging (range from 0 to 1). A mem-
bership grade of 1 would be associated with a cell that
exactly meets the ‘ideal’ attribute values of a particular
morphometric class, while a value of 0 would show that the
cell has no similarity or membership to that morphometric
class.
There are two methods of defining the membership
values of the fuzzy sets: ‘the semantic import model’ and
‘the similarity relation model’ [9]. The similarity relation
model uses surface derivatives as an input to a multivariate
fuzzy classification with a training set to define its ‘class
central concepts’ and yields the class membership values
for each case for instance a raster cell [9, 10]. The semantic
import (SI) model permits a formal identification and
incorporation of imprecise and overlapping semantics used
to describe or classify data [9]. In the case of terrain
classification, the SI model permits experts to identify
conceptual landform elements and to make class definitions
by using imprecise semantics [11].
Fuzzy c-means is an unsupervised technique which
identifies natural clusters and input data groupings in
geographical space [101]. It uses same methods of identi-
fying clusters, but it assigns each data point with a mem-
bership to each cluster representing its degree of
membership or similarity to that cluster [10]. In addition,
fuzzy k-means those algorithms which provide uncertainty
measures for the delineated land elements [10]. These
techniques are based on the fact that the geomorphometric
signatures of different landform units are related to similar
terrain parameters. The only difference between fuzzy c-
and k-means is the clusters model. Fuzzy c-mean uses
reciprocal distance to compute fuzzy weights while fuzzy
k-means algorithm is applied to solve clustering problems
[99].
3.5 Other methods
There are also other different methods of the landform
classification. The support vector machine (SVM) is a
group of theoretically superior machine learning algo-
rithms. It was developed to be competitive with the best
available machine learning algorithms in classifying high-
dimensional data sets [102]. For example, Stepinski et al.
[103] used SVMs for a test site on Mars to produce the
most accurate results as compared to other conventional
techniques of classifying topographic objects. Mangai et al.
[102] used SVMs to classify landforms and to identify a
wide variety of landforms in the subcontinent of India. In
addition, the artificial neural network (ANN) employs
learning algorithms to supervise the landforms classifica-
tion; consequently, it is applied to the landform classifi-
cation studies with specific aims [104].
There are semi-automatic methods to classify morpho-
metric features (landform elements) in a wide variety of
areas that have been developed by utilizing self-organizing
maps (SOM) [105]. It is an unsupervised ANN algorithm
which is used to cluster or visualize high dimensional input
vectors into two dimensional output based on the existing
regularities and correlations among them [105]. SVMs
have been used in a wide variety of areas such as the
classification of remote sensing data [106], the information
visualization and the knowledge discovery [107].
Mokarram et al. [108, 109], Mokarram and Hojati [110],
Shaygan and Mokarram [111] were used new methods such
as fuzzy and neural network for landform classification.
123
656 M. Mokarram, D. Sathyamoorthy
4 Conclusion
This paper provided a background on the theoretical basis
of landform characteristics and classification. It is impor-
tant to distinguish between objective classification
approaches which are primarily based on the surface shape
consideration and are typically applicable across all scales,
and subjective classifications that are typically applicable
to a specific scale or range of scales that tend to use a
variety of local and regional land-surface parameters as
inputs of the classification process.
In the past, the landform classification was performed by
using manual overlay processes which relied on hand-
drawn maps on the transparent vellum. Currently, GIS
permits the integration of geographic data by taking the
best advantages of computers. Over time and with the
creation of new methods such fuzzy methods, SVM and
ANN, the landforms classification have become easier and
more accurate. By using defined classes, users can deter-
mine natural phenomena such as a climate change, the type
of land and even disease distribution based on the landform
class type.
Using new methods such as genetic algorithm, PSO,
subpixel and etc. should increase resolution and extract
more details from input data for landform classification. So
we suggest to use the new algorithm form other fields for
landform classification.
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