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5.1.4 Mapping and GISBy definition, landslide hazard will be determined by where the potential for landslides occurs, and similarlyrisk will be estimated based on the location of people or assets in geographic proximity to such hazard. Thispoints to the extreme significance of mapping in any deployment and implementation of a landslide hazardmitigation strategy. Essentially, the successful management of landslide hazard will be predicated on knowingwhere such hazards are likely to occur. It is only within a spatial framework that landslide hazard can beoptimally understood and dealt with. Map products are therefore a very important part of landslide hazardmanagement, and the availability of map outputs are of huge benefit to government departments, state agencies,local authorities, engineers, and the public in general.GISThe development of Geographic Information System (GIS) technology has greatly facilitated spatial analysisand the creation of useful map outputs. Although the term GIS is often thought of as being a particular type ofsoftware, GIS is more correctly defined as a system of computer software, hardware and data, and personnel,to help manipulate, analyze and present information that is tied to a spatial location. GIS is a systematicmethod to visualize, manipulate, analyze, and display spatial data. Simply put, a GIS combines layers ofinformation about a place to give a better understanding of that place.VegetationGeologySoilsDrainageSusceptibilityAssessmentFig. 5.2 The concept of overlay analysis in GISFig. 5.2 is a simple graphic representation of how spatial data layers can be combined in an overlay analysisthus yielding new information about a location. In this figure data layers such as vegetation, geology, soils anddrainage networks are shown in overlay fashion, correctly located with respect to each other. These layers canbe analysed in association with the occurrence of landslide events to develop a model of the co-occurrence oflandslide causative factors. The output can be used either statistically or deterministically to develop a landslidesusceptibility map.The use of GIS is extremely important in both investigating and helping to establish the spatial relationshipsbetween causative factors and landslide events, and also in preparing map products of susceptibility, hazardand risk. For the first purpose, the development of spatial relationships, the GIS acts as an integrating frameworkfor the analysis. By providing a management system for the variety of input spatial datasets and the tools forinvestigating their interrelationships, the GIS can greatly improve the efficiency of such analysis. Table 5.1 listssome of the datasets relevant to landslide investigation that are available in Ireland.36