Escenario contributivo en el que se controla la exposición de los trabajadores (PROC7)

2.3.1 GIS – an Overview

GIS stands for Geographic Information Systems. Worded in this way, GIS sounds like a method to accomplish a task, but it is really more of a concept. GIS describes the idea of presenting geographic data spatially. These data, referred to as attributes, can include values of ground surface elevation, land coverage, locations of department stores, population densities, and just about any type of information that varies spatially. Attributes that describe a single data type are called layers. A GIS database usually contains many such layers, each describing the spatial variation of a particular attribute. The key feature of any GIS program is the graphical interface, which allows a user to actually see how a certain type of data varies across a geographic area by displaying attribute layers. In the same way, a user can study the relationship between multiple attributes by displaying several layers at once. Since these systems are very visually oriented, most GIS databases employ a map or other image file as their base layer. GIS programs also allow users to modify existing attribute layers, or create new layers by performing mathematical operations on the data sets, merging one of more attributes together based on common properties, and a host of other options.

All attribute layers in a GIS database are linked together by a common coordinate system, and map projection. Without this link of common geographic reference it would be impossible to consider the relationship between multiple attributes. The Environmental Systems Research Institute (ESRI), creators of the popular software ArcGIS, define a coordinate system as “a reference system used to measure horizontal and vertical distances on a planimetric map” (ESRI, 2003). Coordinate systems consist of a map projection, a reference spheroid and a datum (ESRI, 2003). According to ESRI, a map projection is “a mathematical model that transforms the locations of features on the Earth's surface to locations on a two-dimensional surface. This is required because we wish to represent data from a three dimensional earth (the reference spheroid) on a two dimensional map. The two most common projections are the Universal Transverse Mercator (UTM), common to national data sets, and the State Plane Coordinate System (SPC), used mainly in state local data sets (Emerick, 2003). ESRI defines datum as “a set of parameters and control points used to accurately define the three-dimensional shape of the

Earth” (ESRI, 2003). Most data sets use either the North American Datum of 1927 or 1983, referred to as NAD1927, and NAD1983, respectively. In an ideal world, all data sets would reference the same coordinate system. Since in reality this is far from the case, most GIS programs contain modules for re-projecting data sets to a different coordinate system.

GIS programs use two main types of data layers, raster files, and vector files. A raster files is essentially a large matrix of cells or pixels, with each pixel containing a value of the data that the raster file represents. Raster files normally represent images, or large spatially variable data sets such as the intensity of precipitation shown by a Doppler radar image. The size of each pixel in the raster file is a measure of the file resolution. Files with high resolution have a greater number of smaller pixels than files with low resolution. Vector files consist of a series of lines connected by points called nodes. Areas defined by an enclosure of lines and nodes, are called polygons. GIS programs use vector files to represent linear features, such as rivers, highways, or watershed boundaries.

As simple collections of pixels, lines and nodes, both raster and vector files are completely meaningless in the context of a GIS database without some sort of information describing what they represent, and how they should appear. This descriptive information is contained within a metadata file, which accompanies every raster and vector file. Metadata files contain information about the data projection, datum, maximum and minimum values, scale, data collection procedures, the author of the data set, and pretty much anything else that might be useful to know about the data set at some point in time. GIS programs automatically generate an accompanying metadata file with each new raster or vector file, and will not display a layer without one. Thus, GIS data files should be considered as data/metadata pairs rather than individual raster or vector files.

2.3.2 Use of GIS in Hydrologic Modeling

GIS programs are incredibly useful tools in hydrologic modeling, providing modelers with a quick and easy way to manipulate and display large data sets that describe the variation of such characteristics as soil type and land coverage within a watershed. Presently, there are literally thousands of raster and vector data sets describing all matter of subjects available for download on the web sites of many Federal Government, state and local agencies. Using these published

GIS raster files as input data for a model permits the consideration of spatial variability within a watershed at a level impossible to achieve by any other means within a reasonable time frame. This permits us to move away from the use of modeling approaches that involve lumping of watershed characteristics, in favor of distributed approaches, which are more representative of the natural conditions.