CHAPTER 2 LITERATURE REVIEW
2.8 GIS and Its Roles in Landslide Hazard Assessment
Geographic Information System (GIS) has been a useful tool for very broad applications such as in natural resources, environmental studies, etc. GIS also takes part in mitigation efforts in minimizing loss of lives and damage of infrastructures due to natural hazard threats such, flood and landslides, by providing hazard maps.
GIS has been proven to be helpful in LHA as indicated by the abundance of literatures on LHA utilizing GIS tool. Below is the description of the roles of GIS in LHA and brief review of GIS concept and the associated matters.
2.8.1. GIS roles in Landslide Hazard Assessment
Dealing with landslide hazard assessment of medium scale means handling and processing a large amount of spatial data with wide coverage. Without a powerful tool, this work will be cumbersome. GIS offers a powerful tool more than handling and processing data. As defined by Burrough and McDonell [214], GIS a powerful set of tools for collecting, storing, retrieving at will, transforming and displaying spatial data from the real world. Using GIS, modeling and predicting areas potential to landsliding can be made possible.
GIS was first used in landslide hazard assessment (LHA) in the early of 80‘s as reported by Carrara, et al. [59]. Earlier works used GIS as a tool to display data and results (final map) in an interesting way. During 1980‘s, GIS was used by few investigators for assessment and prediction of landslide hazard. It became extensively used as the popularity of statistical methods in LHA increased as described by Aleotti and Chowdhury [116] and Van Westen, et al. [49]. Recently, GIS has been proven to be a valuable tool for acquiring, storing data and manipulating data. Morphology factors such as slope, slope aspect, curvature, elevation, etc. can be easily derived
from DEM (Digital Elevation Model) easily using GIS. GIS has become more powerful when it able to handle and undertake image processing works as offered by stand alone image processing software. GIS is also capable in handling spatial data with different reference systems. For use in LHA, GIS encompasses the following components as suggested by Van Westen [108]: data collection, data entry, data management, and data modeling. Roles of GIS in every step of LHA are illustrated in Fig. 2.17. GIS stores required spatial data such as geology and LULC for LHA including remote sensing imageries into a database during data collection phase.
Attributing spatial data and generation of DEM are of GIS roles in the second phase.
During the third phase, GIS takes part in formation of landslide database and spatial database required for LHA such as hazard factors, elements at risk, and landslide triggering factors. The last phase of GIS role is data modeling. GIS contributes in finalizing LHA by providing landslide hazard/risk map, calculation of risk and vulnerability, etc.
2.8.2. Concept of GIS
Detail information regarding to principle of GIS can be referred to Burrough and McDonell [214]. When explicitly mentioning the hardware used in GIS, Rose [215]
defined that GIS is as a computer system capable of assembling, storing, manipulating, analyzing and displaying geographically referenced information, i.e.
data identified according to their locations. Besides the hardware, GIS components include GIS computer software, operating personal, geographic data, and data database management including functions to perform data analysis.
In real world, geographic information is represented by location such as location of Bench Mark, petrol station, landslide locations. etc.; attributes such as street name, land cover type, etc.; and spatial relationships such as sharing center line of river and states border, etc. To put such geographic information in a computer for further analysis, GIS is required. There are three steps to go from real world geographic objects to those on a computer: 1) representation of geographic objects, 2) relating attributes to geographic representation, and 3) spatial relationship between geographic representations.
Fig. 2.17 GIS roles in phases of LHA Source: Van Westen [108]
In GIS, there are two types of data models/formats as representation of real world objects in computer namely vector and raster data model (Fig. 2.18). In vector data model, real world geographic objects are represented by spatial features i.e. point (e.g. electric pole, control point, etc.), line (e.g. center line of river and road, geology lineament), and area (e.g. building, island, etc.). The coordinates of points and the points forming line/area can be obtained from measurement e.g. using GPS, picking up from topographical map, etc., and stored in GIS. Vector data model is good for representing accurate position of objects and is helpful when used for defining spatial relationship between objects. Raster data model uses regular grid of evenly size cell to represent real world objects in computer. Each represents a portion of area on earth. For example, a grid may represent 10 x 10m of area on earth and could be assigned a value of geology type, soil type, elevation, etc. Examples of raster data are slope, slope aspect, remote sensing imageries. Raster data is suitable for representing
continuously changing attributes such as elevation, reflectance, temperature, etc.
However, raster data is not suitable when dealing with accurate measurement on GIS.
Fig. 2.18 Vector and raster data model Source: Rose [215]
The next step for bringing real world geographic objects into a computer is by linking attributes of geographic representation. Attributes are non-spatial data associated with geographic representation. For example, one needs to input geology type on geology layer or street name on street layer. Tables are prepared to store the attributes associated with the objects. GIS with capability of database management and manipulation is able to link between spatial and attribute data and allow analysis and query of both data. The last step is building spatial relationship between geographic representations. Spatial relationships of individual object are length, area, shape, and perimeter. Meanwhile, spatial relationship among 2 objects or more could be distance, direction, and topology. Topology is simply defined as the spatial relationships between adjacent or neighboring features. For example, end of two roads may share the same endpoint; a river and the adjacent land cover may share boundaries or segments of the boundary, etc. Spatial distribution of objects described how a particular object is spread out on the map. For example, the spatial distribution of forest may across various type of geology.
2.8.3. GIS Software
There are non-commercial/open source and commercial GIS softwares. GRASS (Geographic Resources Analysis Support System) is an example of non-commercial software while ArcGIS, the software currently used in this work, is commercial software. Like any other GIS softwares in general, this software is capable for undertaking all previously mentioned GIS works as described in ESRI [63]. For use in LHA, the software offers facilities to cover all required processes i.e. data collection, data entry, data management, and data modeling. In data collection phase, a required landslide contributing factor, e.g. geology map, can be made available by digitizing a scanned geology map. A geocoding toolbox can be used to tie down the digitized map to the desired projection system. In addition, the software can perform image processing of satellite images to produce LULC, one of important landslide contributing factor. In the second phase, the software facilitates attributing of spatial data. The next phase, the software can be used to derive spatial data from DEM such as slope, slope aspect, curvature, etc. from DEM. The main function of the software in this phase is database management, to prepare all contributing factors, including attributing weightage values, to be ready for final stage. The final phase is executing the landslide hazard assessment model in order to produce a final landslide hazard map.
Talking more detail about ArcGIS software, the software has two powerful toolboxes for LHA purpose namely 3D Analyst and Spatial Analyst. The first is useful for deriving spatial data categorized as 3 dimensional (3D) surfaces such as TIN (Triangulated Irregular Network). This 3D surface is a main data used by Spatial Analyst to derived data such as slope, slope aspect, curvature, and elevation. Spatial analyst offered spatial analysis tools required for modeling landslide hazard such as extraction menu for evaluating the relationship between past landslide occurrences and causative factors and overlay tools using which the final landslide hazard maps are constructed.