10Huong..GIS in City Planning - Tieng Anh

8/8/2019 10Huong..GIS in City Planning - Tieng Anh

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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

F.C. Dai et al. / Engineering Geology 61 (2001) 257271258


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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),


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


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


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


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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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


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


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


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



11/15

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


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



12/15


Fig. 4. Suitability potential for the high-rise building category.

Fig. 5. Suitability potential for the multi-storey building category.


13/15


Fig. 6. Suitability potential for the low-rise building category.

Fig. 7. Suitability potential for the waste disposal category.


14/15

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


Fig. 8. Suitability potential for the natural conservation category.


15/15

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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