10huong..gis in city planning - tieng anh
TRANSCRIPT
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GIS-based geo-environmental evaluation for urban land-useplanning: a case study
F.C. Daia, C.F. Leeb,*, X.H. Zhangc
a Institute of Geographical Sciences and Natural Resources, Chinese Academy of Sciences, Beijing 100101, People's Republic of ChinabDepartment of Civil and Structural Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong
c Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China
Received 2 August 2000; accepted for publication 8 February 2001
Abstract
A geo-environmental evaluation for urban land-use planning often requires a large amount of spatial information.
Geographic information systems (GIS) are capable of managing large amounts of spatially related information, providing
the ability to integrate multiple layers of information and to derive additional information. A GIS-aid to the geo-environmental
evaluation for urban land-use planning is illustrated for the urban area of Lanzhou City and its vicinity in Northwest China. This
evaluation incorporates topography, surcial and bedrock geology, groundwater conditions, and historic geologic hazards.
Urban land-use is categorized according to the types of land-use and projects planned, such as high-rise building, multi-storey
building, low-rise building, waste disposal, and natural conservation. Multi-criteria analysis is performed to evaluate devel-
opment suitability of the geo-environment for each category, according to appropriately measured and weighted factors. Asuitability map for each category is developed using an algorithm that combines factors in weighted linear combinations. It is
demonstrated that the GIS methodology has high functionality for geo-environmental assessment. q 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Urban geology; Land-use planning; Analytical hierarchy process (AHP); Geographical information systems (GIS); Suitability
assessment
1. Introduction
In China, cities are growing in importance, and
urban areas are expanding rapidly, primarily because
the population of the nation is increasing and propor-tionally more people are congregating in urban areas.
The census records show that the number of cities in
China increased rapidly from 193 in 1978 to 300 in
1984, to 450 in 1989 and to 622 in 1994. Cities are
growing not only in number but also in size as well.
There were 13 cities with a population of over one
million in 1978, 19 in 1984, and 30 in 1989. The
percentage of population living in cities increased
from 14.4% in 1982 to 28.6% in 1994. With a further
enhancement of the open-up door and economicreform policies, an acceleration of population growth
is anticipated. It is predicted that the percentage of
population living in cities will reach 3436% in
2000, 4447% in 2010 and about 60% in 2020 (Liu,
1997).
The rapidly changing pattern of urban growth has
given rise to new problems for urban planning and
redevelopment in China. The expansion of the various
basic urban facilities, especially water supply,
Engineering Geology 61 (2001) 257271
0013-7952/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0013-7952(0 1)00028-X
www.elsevier.com/locate/enggeo
* Corresponding author. Tel.: 1852-2559-5337/2859-2645; fax:
1852-2858-0611.
E-mail address: [email protected] (C.F. Lee).
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sewerage and sewage disposal, and transportation,
constitutes the foremost municipal problems in most
cities. Other problems include the mitigation of
natural hazards, which are to a certain degree caused,
or at least enhanced, by human activities. Urbaniza-
tion often needs the acquisition of appropriate sites for
engineering construction. A major objective of urban
land-use planning is to evaluate the advantages and
disadvantages of one use of land parcels as compared
to another, so as to yield the most benecial use of
land parcels and the conservation of fundamental
natural resources. Problems of urban land-use that
are related to the geo-environment ultimately involve
every aspect of civil engineering through their
impacts on the design, construction and maintenance
of specic engineering works. Some of theseproblems, such as earthquakes and landslides, occur
as natural hazards inherent in the geo-environment.
Other problems, such as groundwater pollution,
could pose actual or potential threats in the case of
poorly planned engineering works. Still others may
have to do with the economics of land-use or devel-
opment. Therefore, the geo-environment must be duly
taken into account when planning and carrying out
remedial measures that are designed to protect the
environment.
Technologically, geographical information systems(GIS) provide a powerful tool for geo-environmental
evaluation in support of urban land-use planning. An
important feature of a GIS is the ability to generate
new information by integrating the existing diverse
datasets sharing a compatible spatial referencing
system (Goodchild, 1993). Although GIS technology
has been widely used to assess natural geologic
hazards (e.g. Carrara et al., 1991; Wang and Unwin,
1992; Atkinson and Massari, 1998; Mejia-Navarro
and Garcia, 1996), groundwater vulnerability assess-
ment (Hiscock et al., 1995; Halliday and Wolfe, 1991)
and site selection for waste disposal (Irigaray et al.,1994; Carver, 1991), studies which address geo-envir-
onmental evaluation for urban land-use planning have
been relatively limited. The purpose of the presenta-
tion of geo-environmental evaluation for urban land-
use planning in the form of maps is ideally suited to
management by a GIS, in which multiple layers of
information can be integrated in different combina-
tions. This can also avoid the existing difculties of
combining numerous spatially related parameters
involved in geo-environmental evaluations, thereby
providing a relatively easy tool.
In this paper, a GIS-aid to geo-environmental
evaluation for urban land-use planning is used for
the urban area of Lanzhou City and its vicinity in
northwestern China. This evaluation incorporates the
following information: topography, geology, ground-
water conditions, and geologic hazards. Multi-criteria
analysis is performed to evaluate development suit-
ability of the geo-environment for various land-use
categories, including high-rise building, multi-storey
building, low-rise building, waste disposal, and natural
conservation, according to appropriately measured and
weighted factors. Suitability map for each category is
developed using an algorithm, which combines factors
in weighted linear combinations.
2. Description of the study area
The urban area and vicinity of Lanzhou city, the
capital of Gansu Province and the second largest city
in northwestern China, is selected as the study area in
this paper (Fig. 1). The study area, with an area of
about 370 km2 and situated on the upper reaches of
the Yellow River, is extremely varied in topography,
relief, population density, and relevant geological andgeomorphologic processes (Fig. 2). Historically,
urban population growth has been conned primarily
to the lowlands or low slope areas in the Yellow River
valley basin. However, in recent years, development
has spread rapidly upslope and also into small narrow
valley areas, where slope stability and debris ow
problems have become increasingly common. The
ofcial records show that at least six signicant dama-
ging episodes of debris ows have occurred in the
study area since the 1950's. The largest events with
documented records occurred on 14 August, 1951, 8
June, 1966, 20 June, 1964 and 7 August, 1978, respec-
tively. For example, the 1964 debris ow, caused by a
rainstorm during which 150 mm of rainfall precipi-
tated within 4 h, resulted in 43 deaths and 166 injuries.
On an average, there is more than one debris ow of
major magnitude every 10 years. In addition, there
were over 10 large-sized old landslides in the study
area, particularly concentrating in the southeast part
of the area. Most of these old landslides are in a
dormant state, or in an intermittent creep state, but
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are ready to be destabilized by heavy rainstorms, seis-
mic activity, and/or human activities. The scarcity of
stable lands for urban development has exposed an
increasingly large population to geological risk.
Geomorphologically, the study area lies in the tran-
sition zone between the Loess Plateau and the
QinghaiTibetan Plateau. Since the loess was depos-
ited as a drape over a hilly palaeo-landscape, the land-
forms of the Loess Plateau in the study area are
dominated by ridge and rounded hill. The surcial
loess, the Malan loess formation of upper Pleistocene,
overlying the Lishi loess formation of middle Pleisto-
cene and the Wucheng loess formation of lower Pleis-
tocene, is a sensitive soil deposit. When it is in a dry
F.C. Dai et al. / Engineering Geology 61 (2001) 257271 259
Fig. 2. Topography and geomorphologic elements of the study area.
Fig. 1. Location of Lanzhou city.
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and undisturbed state, its shear strength is high and
nearly vertical walls of up to 20 m height are common
(Wang and Unwin, 1992). As a result of the semi-arid
climate, most of the thick loess deposits have low
natural humidity of the order of 812%. However,
the metastable structure of the Malan loess is prone
to collapse upon an increase in moisture content
during events of rainfall. Deeply incised gullies
exist in the loess-covered area, frequently with sink-
holes and pipe systems at the valley heads, and gullies
generally incised down to the bedrock. Mass move-
ments frequently occur on the steep slopes of such
deeply incised valleys. Debris ow events occurred
as a result of mass movements and the precipitation
characteristics of the area. About eight terraces have
been developed in the Yellow River basin in the Lanz-hou City vicinity. However, heavily incised, the fth
to eighth terraces have been modied into hilly areas
by crustal uplifting and surcial erosion, and are thus
classied as rounded hill forms on the Loess Plateau.
Bedrock outcrop in the study area comprises Precam-
brian schist and gneiss, lower Cretaceous sandstone
interbedded with claystone and conglomerate, Tertiary
sandstone interbedded with conglomerate and silty
claystone. In addition, some intrusive granitic rocks
sparsely outcrop in the northeast part of the study area.
From the viewpoint of geotectonics and neo-tecton-ism, the study area is characterized by crustal uplifting
and streams down-cutting. Historic records show that
about ninestrong earthquakes haveoccurredin Lanzhou
city and its surrounding regions since the year 193 bc,
causing extensive damage. The most devastating event
with a magnitude of 7.0 occurred in 1125 ad. The last
strong earthquake, the Haiyuan earthquake, whose
epicenter was about 150 km away from the city,
occurred in 1920 with a magnitude of 8.5. The geologic
faults can be grouped into three classes: NNW, NWW
and NE trending (Fig. 2); they are considered to be
relatively inactivebased on the available seismotectonicinformation. Seismic risk analysis shows that the poten-
tial seismic intensity with a 10% probability of excee-
dance over a period of 50 years is eight on the Chinese
MCS intensity scale (Sun and Wang, 1993).
3. Procedures and methodology
The aforementioned geological and geomorpho-
logical information is considered to be pertinent in
dening the general geo-environmental characteris-
tics of the study area. The thematic maps were digi-
tized using the PC Arc/Info GIS software, and then
transferred to a desktop ArcView GIS environment. In
the ArcView GIS, a raster grid cell of 20 20 m 2 was
generated. Each cell is considered as a homogenous
unit for any given factor. All inuential factors were
standardized and weighted, and then combined for
each urban land-use categories, respectively (Fig. 3).
3.1. Data collection and processing
The inputs to a GIS include remote-sensed data
from satellites or aircrafts, existing digitized data-
bases of maps, and information from tables andreports. The common characteristic is that each type
of data input describes the attributes of recognizable
point, linear or areal geographical features. Details of
the features are usually stored in either vector or raster
formats.
The selection of data sources should be inuenced
by their accuracy and resolution, together with the
nature of the problem to be investigated (Hiscock et
al., 1995). The 1:50,000 topographical maps (20 m
interval) covering the study area were purchased
from local survey authority, and digitized manuallyinto a computer. The slope and elevation maps were
developed from the digital elevation model (DEM)
data generated from the digitized contour lines. Sur-
cial and bedrock geology, groundwater conditions,
and distributions of landslides and debris ows were
obtained from the Hydrogeology and Engineering
Geology Team of the Gansu Provincial Bureau of
Geology and Mineral Resources (1988), and supple-
mented with eld observations. During the eldwork,
observations were made of the landslide type, scarp
and possible causes including the nature of the mate-
rials involved and hydrogeology at all landslide sites.
For debris ow gullies, the steepness of the terrain,
evidence of past activity, erosion and hydrological
features were noted. Lithology of bearing layer and
liquefaction potential were determined by geomor-
phologic features and site investigations for buildings
and structures, supplied by the Lanzhou Institute of
Urban Design and Construction. The available infor-
mation shows that the corrosive potential of ground-
water is dominated by high SO422 content, and
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crystallizational corrosion is thus considered to be the
most possible hazard to structural foundations. The
corrosive potential is classied as very low, low,
medium, and high based on the ranges of SO422 content
of ,500, 5001500, 15002500 and .2500 mg/l,
respectively. The above-mentioned base maps were
compiled at a scale of 1:50,000 for the study area.These vector base maps were then transferred to the
desktop ArcView GIS, and rasterized for subsequent
analyses. The raster grid cell denition was selected as
20 20 m2 resolution, which ensured that small
geomorphic features or most of the detailed slope
units would be mapped. This also permitted a closer
approximation of a spatially continuous description of
the geomorphic features. In the ArcView software
environment, several base raster maps could then be
generated, especially the distance calculations between
cells as required. These included distances from land-
slides, debris ow gullies, and geologic faults. Becausethe landslide problem in the study area is characterized
by a reactivation of old landslides due to a change in
groundwater condition, and/or human activities, land-
slide hazard is thus accounted for by setback from
historic landslides in this study.
3.2. Urban land-use categories
Urban land-use evaluation aims at providing a
scientic basis for urban land-use planning and rede-
velopment as well as site selection for engineering
works based on the actual geo-environmental char-
acteristics, so as to achieve maximum socio-
economic benets at a minimum environmental
cost (Shi, 1993). Site selection should take into
consideration both site conditions and infrastructures.Different land-use categories have different physical
requirements. It must be admitted that it is extremely
difcult to make an inventory of and classify all
types of land-uses due to their diversity and complex-
ity. Urban land-use categories need to be selected
very carefully, so that they are representative. On a
regional scale the categorization needs to reect and
should be formulated by planning expertise. Shi
(1993) classied the types of urban land-uses into
high-rise building, multi-storey building, low-rise
building, and natural conservation. Dai et al. (1994)
categorized the types into high-rise building,common civil industrial building, one-storey build-
ing, construction material exploitation, waste dispo-
sal, and park. Considering the possible impacts of
various uses of land parcels on the geo-environment
and having consulted the local urban planning
authority, we categorized the types of urban land-
use into ve categories: high-rise building (residen-
tial building with $10 oors or commercial and
institutional building that is higher than 24 m),
F.C. Dai et al. / Engineering Geology 61 (2001) 257271 261
Interpret urban development policyDefine land use categoriesSet areal boundary
Establishment of spatial databaseVectorize mapsEstablish attribute databaseRasterize vector mapsCalculate distance parameters
Selection of appropriate factors
Standardization of factors
Computation of weights of factorsEstablish a pairwise matrixCalculate factor weights
Collection and collation of data
Multi-criteria evaluationEvaluate suitability for each categoryCreate single-category suitability mapVectorize suitability maps
Result analysis
Fig. 3. Flow-chart for GIS-based geo-environmental evaluation for urban land-use planning.
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multi-storey building, low-rise building, waste dispo-
sal, and natural conservation.
3.3. Factors for suitability evaluation
In this section, the various factors for determining
the suitability of land parcels for each land-use cate-
gory are provided. It should be noted that this selec-
tion is not exhaustive, and that only those salient
factors for which information is of great signicance
were considered (Table 1). Flooding is excluded in
this study because articial levees have been built
along the Yellow River channel, and no historic
record on ooding is available. The local planning
authority also conrms that ooding is not a salient
problem after construction of articial levees.
Four factor groups comprising 10 separate sets of
geo-environmental attributes were accounted for the
high-rise building and multi-storey building cate-
gories. Topography forms an important determinant
of suitability assessment for both categories. Eleva-
tion is considered because high areas suffer from inac-
cessibility and lack of basic urban facilities, such as
transportation, water supply, and sewage and sewer-
age disposal, both at present and in the near future.
Slope is even more important while considering the
ease of engineering construction and susceptibility to
landsliding. The likelihood for construction problems
to be encountered was accounted for by consideringground and groundwater conditions. Lithology of the
bearing layer determines its bearing capacity and
compressibility characteristics. It should be recog-
nized that the lithology of bearing layer most suitable
for development was determined empirically by
considering the geology within a depth of about
10 m for the high-rise building category and 5 m for
the multi-storey building category. Groundwater,
including the depth to groundwater table, the corro-
sive potential, and possible groundwater rise, may
pose actual or potential threats to engineering
construction and maintenance, and thus must betaken into consideration. Geologic hazards are an
important geo-environmental consideration in land-
use planning. Liquefaction potential may be consid-
ered for suitability evaluation of both categories due
to its potential damage. Distance parameters were
employed to control building allocation. The distance
to landslides and debris ow is an important consid-
eration in ensuring the safety of engineering construc-
tion and maintenance work. In addition, the distance
F.C. Dai et al. / Engineering Geology 61 (2001) 257271262
Table 1
Selection of factors for suitability evaluation
Factors Urban land-use categories
High-rise building Multi-storey building Low-rise building Waste disposal Natural conservation
Topography
Slope K K K K K
Elevation K K K K K
Ground conditions
Surcial geology K K
Formation combination K
Lithology of bearing layer K K
Groundwater
Depth to groundwater table K K K
Corrosive potential of
groundwater
K K K
Groundwater rise K K K K
Geologic hazards
Distance to debris ow K K K K K
Distance to landsliding K K K K K
Liquefaction potential K K K K
Distance to fault K K K K
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to a fault is considered because the bearing capacity
and compressibility characteristics of fractured soils
and rocks may not meet the demands for suitable
ground conditions.
Suitability for the low-rise building category was
determined from eight factors. Similar to high-rise
and multi-storey buildings, topographic factors, both
elevation and slope are critical for the same reason.
Liquefaction potential and distance to landsliding and
debris ow are vital for the safety of low-rise build-
ings. Surcial geology is inuential in the economic
construction of this category. Groundwater rise may
pose a potential threat because the overlying surcial
loess deposits are prone to collapse upon wetting. The
distance to faults is also considered based on the
assumption that building on a fault is to be avoided.For the waste disposal category a critical concern is
the long-term geomorphic stability of the disposal
site. Site location in a geomorphologically stable
area is imperative in preventing the failure of retain-
ing structures and in protecting surface and ground-
water quality (Rockaway and Smith, 1994).
Therefore, liquefaction potential and distance to land-
sliding and debris ow were accounted for in this
suitability assessment. Elevation was considered
because high areas suffer from inaccessibility and
lack of transportation. Groundwater vulnerability isimportant in site selection for waste disposal, and
can be dened as a function of: (a) the accessibility
of the saturated zone; and (b) the attenuation capacity
of the strata overlying the saturated zone (Hiscock et
al., 1995). In this study, the nature of surcial geology
and formation combinations, and the distance to faults
accounted for the attenuation capacity of the strata
overlying the saturated zone; and groundwater condi-
tions comprising depth to groundwater table and
groundwater rise were considered to represent the
accessibility of the saturated zone. The loess with a
saturated permeability of 1.6 10273.0 1027 m/s(Li, 1994; Fu, 1994) is considered relatively imperme-
able, compared to other types of surcial deposits.
Corrosive potential of groundwater was accounted
for because it might have some inuence on waste
disposal sealing.
Only four factors were considered for the natural
conservation category. All land with high topographi-
cal location and steep slopes were rated high for
conservation. In addition, distance to landsliding and
to debris ow was adopted to promote natural conser-
vation. The smaller the distance, the higher the rate
that was awarded.
3.4. Standardization of factor measurements
In the geo-environmental evaluation process, a
primary step is to ensure a standardized measurement
system across all factors considered. Since most
images still hold cell values for the original map
codes, these have to be standardized to a uniform
suitability rating scale in this case between 0 and
4 for ease of analysis. Assigning values to specic
factors amounts to the making of decision rules in
the shape of thresholds for each factor. As a general
guideline, a positive correlation between the value
awarded and suitability is employed. These integer
numbers ranging from 0 to 4 were assigned to very
low, low, medium, high, and very high classes,
respectively. Table 2 shows the class boundaries and
standardized measurements employed for each factor.
It should be noted that various statistical and empiri-
cal guidelines from the related national codes and
literature were used to determine the boundary values
for the various land-use categories. For distance to
landsliding and debris ow, the determination of
class boundaries takes account of the possible conse-
quence, including the possible runout zone and theinstability of landslide scarp or debris ow channels.
This estimate was made based on eld observations.
3.5. Development of weights
A primary issue in the evaluation is to assign
weights to each factor separately. For each land-use
category, a set of relative weights for inuential
factors should be developed in advance so that it
can be used as input for suitability evaluation in the
next step. In this regard, the analytic hierarchy process
(AHP), a theory for dealing with complex technolo-
gical, economical, and socio-political problems
(Saaty, 1977; Saaty and Vargas, 1991), is an appro-
priate method for deriving the weight assigned to each
factor. Basically, AHP is a multi-objective, multi-
criteria decision-making approach that employs a
pair-wise comparison procedure to arrive at a scale
of preference among a set of alternatives. AHP gained
wide application in site selection and suitability
analysis (e.g. Banai-Kashani, 1989; Carver, 1991;
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Bantayan and Bishop, 1998), and regional planning
(e.g. Jankowski, 1989). It became popular following
its implementation in the Idrisi GIS software package
(e.g. Eastman et al., 1995; Van der Merwe, 1997). To
apply this approach, it is necessary to break down a
complex unstructured problem into its component
factors; arrange these factors in a hierarchic order;
assign numerical values to subjective judgements on
the relative importance of each factor; and synthesize
the judgements to determine the priorities to be
assigned to these factors (Saaty and Vargas, 1991).
In the construction of a pair-wise comparison matrix,
each factor is rated against every other factor by
assigning a relative dominant value between 1 and 9
to the intersecting cell (Table 3). When the factor on
the vertical axis is more important than the factor on
F.C. Dai et al. / Engineering Geology 61 (2001) 257271264
Table 2
Standardized potential rates (HB high-rise building; MB multi-storey building; LB low-rise building; WD waste disposal; NC
natural conservation)
Factors Category Potential rating
0 1 2 3 4
Slope (8) HB/WD . 12 812 58 25 , 2
MB . 15 1215 812 58 , 5
LB . 20 1520 1015 510 , 5
NC , 5 510 1015 1520 . 20
Elevation (m) HB/MB . 1660 16201660 15801620 15401580 , 1540
LB . 1700 16601700 16201660 15801620 , 1580
WD , 1540 . 1700 15401600 16401700 16001640
NC , 1600 16001700 17001800 18001900 . 1900
Depth to groundwater table (m) HB , 2 24 46 69 . 9
MB,
1 13 35 57.
7WD , 3 35 58 815 . 15
Corrosion potential of
groundwater
HB/MB/WD High Medium Low Very low
Distance to debris ow (m) HB , 80 80150 150200 200300 . 300
MB/WD , 40 4080 80150 150250 . 250
LB , 20 2050 50100 100150 . 150
NC . 500 300500 200300 100200 , 100
Distance to landsliding (m) HB/WD , 50 50100 100150 150250 . 250
MB , 30 3060 60100 100150 . 150
LB , 20 2050 5080 80120 . 120
NC . 400 300400 200300 100200 , 100
Distance to fault (m) HB/ WD , 40 4080 80120 120160 . 160
MD/LD , 30 3060 6090 90120 . 120
Surcial geology LB Collapsible soils Loess Sand, bedrock
WD Sand, bedrock Collapsible soils Loess
Formation combination WD Sand, bedrock,
sand underlain
by bedrock
Collapsible
soils
underlain
by sand
Collapsible
soils, loess
underlain by
bedrock
Collapsible soils Loess
Lithology of bearing layer HB/MB Collapsible soils Loess Bedrock, sand
Groundwater rise HB/MB/LB/WD Yes No
Liquefaction potential HB/MB/LB/WD High Low
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the horizontal axis, this value varies between 1 and 9.
Conversely, the value varies between the reciprocals
1/2 and 1/9. For example, four factors including slope
(A1), elevation (A2), the distances to debris ow (A3)
and to landsliding (A4), respectively, are consideredthe most appropriate factors for determining the suit-
ability of the land-use category of natural conserva-
tion in this study. The factors are compared against all
others with respect to the land-use category of natural
conservation. The pair-wise comparison matrix for
these four factors can be constructed as shown in
Table 4, where the main diagonal is always equal to
unity. It has been demonstrated that the eigenvector
corresponding to the largest eigenvalue of the matrix
provides the relative priorities of the factors, i.e. if a
factor is preferred to another, its eigenvector compo-nent is larger than that of the other (Saaty, 1977; Saaty
and Vargas, 1991). The components of the eigenvec-
tor sum to unity. Thus we obtain a vector of weights
which reects the relative importance of the various
factors from the matrix of paired comparisons. In this
case, the following weights for the four factors are
obtained from the matrix in Table 4: slope
0.5426, elevation 0.3211, distance to debris ow
0.0462, distance to landsliding 0.0901. Because
the complete pair-wise comparison matrix contains
many multiple paths by which the relative importance
of factors can be assessed, it is also possible to deter-mine the degree of consistency that has been used in
developing the judgements. In the construction of the
matrix of paired comparisons, the consistency of the
judgements should be revealed because this matrix is
a consistent matrix. For example, if A1 is preferred to
A2 and A2 to A3, then A1 must be more preferred to
A3. In AHP, an index of consistency, known as the
consistency ratio (CR), is used to indicate the prob-
ability that the matrix judgements were randomly
generated (Saaty, 1977)
CR CI=RI
where RI is the average of the resulting consistency
index depending on the order of the matrix given bySaaty (1977) and CI is the consistency index and can
be expressed as
RI lmax 2 n=n2 1
where lmax is the largest or principal eigenvalue of the
matrix and can be easily calculated from the matrix,
and n is the order of the matrix.
A consistency ratio of the order of 0.10 or less is a
reasonable level of consistency (Saaty, 1977). A
consistency ratio above 0.1 requires revisions of the
judgements in the matrix because of an inconsistenttreatment of particular factor ratings. In this case the
consistency ratio of the matrix of paired comparisons
between the four inuential factors in the suitability
assessment of natural conservation is 0.04, and is thus
acceptable. Once a satisfactory consistency ratio is
obtained, the resultant weights are applied. The
weights should add up to a sum of 1.0, as the linear
weighted combination calculation requires. A similar
process takes place in other land-use categories, as
shown in Table 5. Because the matrix is symmetrical,
only the lower triangular half actually needs to belled. The remaining cells are then simply the reci-
procals of the lower triangular half. In this study, an
external program was developed to implement the
AHP algorithm described above.
3.6. Geo-environmental evaluation
Multi-criteria evaluation is used to combine a set of
criteria to form a single suitability map according to a
specic category. In this study, factors are combined
F.C. Dai et al. / Engineering Geology 61 (2001) 257271 265
Table 3
Scale for comparisons (after Saaty and Vargas 1991)
1 Equal importance
3 Moderate p revalence of o ne over a nother
5 Strong or essential prevalence
7 Very strong or demonstrated prevalence
9 Extremely high prevalence
2, 4, 6, 8 Intermediate values
Reciprocals For inverse comparison
Table 4
An example of a pairwise comparison matrix for assessing the
weights of factors
Factors A1 A2 A3 A4 Weights
A1 1 2 9 7 0.5426
A2 1/2 1 6 5 0.3211
A3 1/9 1/6 1 1/3 0.0462
A4 1/7 1/5 3 1 0.0901
Consistency ratio: 0.04
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F.C. Dai et al. / Engineering Geology 61 (2001) 257271266
Table 5
Relative weighting of factors for each urban land-use category
Factors 1 2 3 4 5 6 7 8 9 10 Weights
Category 1: High rise building
(1) Slope 1 0.2623
(2) Elevation 1/3 1 0.1212
(3) Lithology of bearing layer 1/5 1/3 1 0.0611
(4) Depth to groundwater table 1/7 1/5 1/3 1 0.0290
(5) Corrosion of groundwater 1/3 1 2 3 1 0.0770
(6) Groundwater rise 1/5 1/3 1 2 3 1 0.0720
(7) Distance to debris ow 1/2 2 3 5 3 3 1 0.1640
(8) Distance to landsliding 1/2 2 3 5 3 3 1 1 0.1640
(9) Liquefaction potential 1/7 1/5 1/3 1 1/3 1/2 1/4 1/4 1 0.0322
(10) Distance to fault 1/9 1/7 1/5 1/2 1/5 1/4 1/7 1/7 1/3 1 0.0172
Consistency ratio: 0.04
Category 2: multi-storey
building
(1) Slope 1 0.1944
(2) Elevation 1/3 1 0.0696
(3) Lithology of bearing layer 1/5 1/2 1 0.0363
(4) Depth to groundwater table 1/7 1/4 1/2 1 0.0258
(5) Corrosion of groundwater 1/5 1/2 1 2 1 0.0352
(6) Groundwater rise 1/3 1 3 2 3 1 0.0768
(7) Distance to debris ow 1 3 5 7 5 3 1 0.1944
(8) Distance to landsliding 1 3 5 7 5 3 1 1 0.1944
(9) Liquefaction potential 1/2 2 3 4 3 1 1/2 1/2 1 0.1050
(10) Distance to fault 1/3 1 2 2 3 1 1/3 1/3 1/2 1 0.0681
Consistency ratio: 0.01
Category 3: low-rise building
(1) Slope 1 0.2858(2) Elevation 1/3 1 0.0963
(3) Surcial geology 1/6 1/2 1 0.0468
(4) Groundwater rise 1/9 1/7 1/3 1 0.0233
(5) Distance to debris ow 1/2 2 4 6 1 0.1690
(6) Distance to landsliding 1/2 2 4 6 1 1 0.1690
(7) Liquefaction potential 1/2 2 4 6 1 1 1 0.1690
(8) Distance to fault 1/6 1/2 1 2 1/5 1/5 1/5 1 0.0408
Consistency ratio: 0.01
Category 4: waste disposal
(1) Slope 1 0.1662
(2) Elevation 1/3 1 0.0697
(3) Surcial geology 1 3 1 0.1662
(4) Formation combination 1/3 1 1/3 1 0.0587(5) Depth to groundwater table 1 3 1 3 1 0.1662
(6) Corrosion of groundwater 1/5 1/3 1/5 1/3 1/5 1 0.0308
(7) Groundwater rise 1/2 1 1/2 2 1/2 2 1 0.0766
(8) Distance to debris ow 1/2 1 1/2 2 1/2 2 1 1 0.0855
(9) Distance to landsliding 1/3 1 1/3 1 1/3 3 2 1/2 1 0.0664
(10) Liquefaction potential 1/2 1 1/2 2 1/2 2 1 1 2 1 0.0855
(11) Distance to fault 1/5 1/3 1/5 1/3 1/5 1 1/3 1/3 1/2 1/
3
1
0.0282
Consistency ratio: 0.02
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in weighted linear combinations. With a weighted
linear combination, factors are combined by applying
a weight to each, followed by a summation of the
results to yield a suitability map (Eastman et al.,
1995), i.e.
S X
wi xi
where Sis the suitability, wi the weight of factor i, and
xi is the potential rating of factor i.
For each land-use category, the data layers of
factors that affect the suitability of land parcels for
this land-use category were then reclassied so that
they could be used as rating maps required in the
process of geo-environmental evaluation. The calcu-
lated weight values are then transferred to theArcView GIS, and weighted linear combination is
repeated for each category separately to create a suit-
ability map with a value range per cell matching that
of the standardized factor maps using a range 0 4 in
this case. For each suitability map, a ve equal inter-
val classication between the minimum and the maxi-
mum cell values calculated is employed in this study,
i.e. assigning the ve ranges in an increasing order to
very low, low, moderate, high, and very high, respec-
tively. The resultant raster maps were then vectorized.
Figs. 48 display the results for each of the categories
investigated here.
From Figs. 48, it can be seen that high-rise build-
ing category is clearly concentrated in the Yellow
River terraces and some relatively wide valleys,
while high topographical locations and steep slope
areas are avoided. Likewise, the multi-storey building
category concentrates in the basin and relatively high
topographical river terraces. Suitability for low-rise
building is high in the basin and high topographical
at lands. The potential for natural conservation is
highest along the mountains and narrow valleys. Suit-
ability for waste disposal is also satisfactorily distrib-
uted in these areas with thick Quaternary loess
deposits. Field checks conrmed that the evaluation
results are consistent with the actual situations.
4. Results and conclusions
To make the maximum benecial use of land for a
certain area, a planner should take into consideration
the actual geo-environment. This will allow the accu-
racy and implementation of basic information to be
improved and then applied in the planning process.
An important goal in geo-environmental evaluation
is to provide assistance to policy makers, planners anddevelopers in the optimal development of an area
while preserving the environment. The evaluation
results can assist planners in making decisions on
land-use alternatives for specic land parcels. These
are intended only to be a guide in determining the
general trends and spatial distribution of suitability
for the various possible types of developments.
The GIS methodology for macro- or micro-zona-
tion is capable of providing a degree of accuracy in
assessing the potential suitability of land parcels for
urban development. The most important advantages
of this methodology over manual map production in
geo-environmental evaluation for urban planning and
development purposes are: accessible methodology at
relatively low costs, ease of use of commonly avail-
able data with minimal cost, very short time for data
manipulation, the possibility to explore diverse
scenarios, potential to develop an optimum type of
land development, and ease of handling the graphic
output. Traditionally, geo-environmental evaluation
and mapping were laborious and time-consuming
F.C. Dai et al. / Engineering Geology 61 (2001) 257271 267
Table 5 (continued)
Factors 1 2 3 4 5 6 7 8 9 10 Weights
Category 5: naturalconservation
(1) Slope 1 0.5426
(2) Elevation 1/2 1 0.3211
(3) Distance to debris ow 1/9 1/6 1 0.0462
(4) Distance to landsliding 1/7 1/5 3 1 0.0901
Consistency ratio: 0.04
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F.C. Dai et al. / Engineering Geology 61 (2001) 257271268
Fig. 4. Suitability potential for the high-rise building category.
Fig. 5. Suitability potential for the multi-storey building category.
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F.C. Dai et al. / Engineering Geology 61 (2001) 257271 269
Fig. 6. Suitability potential for the low-rise building category.
Fig. 7. Suitability potential for the waste disposal category.
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tasks because of the large amount of time and effort
required for the manual handling and processing ofthe spatial data. A GIS software can be used to store,
analyse, and display all data required, and allows
these spatial data layers to be analysed as accurately
as needed when investigating spatially complex geo-
environmental potentials. The application of such a
GIS technology has demonstrated that most opera-
tions can be accomplished efciently and cost-effec-
tively. The functional capabilities of GIS software
support the development of spatial geo-environmental
evaluation for urban land-use planning purpose. The
study results presented herein have demonstrated the
great potential of GIS-based geo-environmentalevaluation for urban planning purpose. However, it
needs to be emphasized that the reliability of the
assessment results depends on a multitude of factors
ranging from the quality of the database to the intro-
duction of potential errors associated with data entry,
manipulation, and analysis within the GIS. Another
problem is that the weighting method employed in
this study can, although rationally defensible, be fairly
arbitrarily applied and depends entirely on the percep-
tions and priorities of the evaluators. In this case, a
knowledge of the local geology is critical to therationality of the weights applied. As mentioned by
Van der Merwe (1997), the modeling results are
highly sensitive to the weights applied, and altering
the weights assigned to the various factors will have
signicant effects on the results. The determination of
weights for the various factors is one of the most
important challenges, as frequently encountered in
conventional evaluation. In addition, it should be
noted that, in practice, urban land-use categories are
much more complicated, as compared to the categor-
ization employed in this study, and that similar studies
could be carried out for more comprehensive classi-cation of urban land-use types.
Acknowledgements
This study is nancially supported by the Jockey
Club Research and Information Centre for Landslip
Prevention and Land Development, the University of
Hong Kong. Special thanks have to be given to Mr
F.C. Dai et al. / Engineering Geology 61 (2001) 257271270
Fig. 8. Suitability potential for the natural conservation category.
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Dong Kangjia and Miss Wu Yaping for the valuable
information provided for this study. The rst author
would like to thank Prof. Huang Dingcheng for his
partial nancial support in carrying out the eld study.
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