Introduction to GIS Phil Guertin School of Natural Resources and the Environment dguertin@cals.arizona.edu Mapping GeoSpatial Technologies Traditional Survey Global Positioning Systems (GPS) Remote Sensing Spatial Data Management and Analysis Geographic Information Systems One of the fastest growing industries in the world
GIS What does it mean? Geographic (geospatial) Place-based, georeferenced, location is quantitatively defined Information Attributes, data on spatial features not related to location S Systems (technology) Science (knowledge) Studies (study on how GI is used) Geographic Information System Organized collection of Hardware Software Data People Procedures Network People Network Software Data Procedures Hardware
Hardware Platform Stand-alone Applications (Desktop, Laptop) ESRI ArcGIS 10.x ArcMap ArcCatalog ArcToolbox Server-based Applications (Internet - Browser) Google Maps, Earth (Direct Read) Bing Maps (Direct Read) Google Maps
Georeferencing Is essential in GIS, since all information must be linked to the Earth s surface The method of georeferencing must be: Unique, linking information to exactly one location Shared, so different users understand the meaning of a georeference Persistent through time, so today s georeferences are still meaningful tomorrow Georeferences as Measurements Most georeferences are metric They define location using measures of distance from fixed places E.g., distance from the Equator or from the Greenwich Meridian (x,y coordinates) Others are based on ordering E.g. street addresses in most parts of the world order houses along streets Others are only nominal Placenames do not involve ordering or measuring Usually today everything will be represented as a metric
Creating Scientific Coordinates Coordinates have three properties: 1. Origin 2. Axis (easting and northing) 3. Units of measurement Two Classes Geographic (spherical, nonlinear) Projection (Planar, linear) Latitude change in the y direction Northing Longitude change in the x direction Easting Traditional negative in the southern and western hemispheres
Projections There are many reasons for wanting to project the Earth s surface onto a plane, rather than deal with the curved surface The paper used to output GIS maps is flat Flat maps are scanned and digitized to create GIS databases Rasters are flat, it s impossible to create a raster on a curved surface The Earth has to be projected to see all of it at once It s much easier to measure distance on a plane A GIS database is typical a set of thematic layers that represent different information types. Information can be stored using different data structures depend on their characteristics.
Rasters and Vectors How to represent phenomena conceived as fields or discrete objects? Two basic data structures. Raster Divide the world into square cells, rectangle matrix Register the corners to the Earth Represent discrete objects as collections of one or more cells Represent fields by assigning attribute values to cells More commonly used to represent fields than discrete objects Rasters Most rasters are uniform tessellation of square cells, but raster elements do not need to be square, nor must a raster be uniform.
Continuous Information Each cell has a value that represents the average elevation for the cell. Discrete Types Legend Mixed conifer Douglas fir Oak savannah Grassland Raster representation. Each color represents a different value of a nominalscale field denoting land cover class.
Characteristics of Rasters Pixel size The size of the cell or picture element, defining the level of spatial detail All variation within pixels is lost Assignment scheme The value of a cell may be an average over the cell, or a total within the cell, or the commonest value in the cell It may also be the value found at the cell s central point Vector Data Used to represent points, lines, and areas All are represented using coordinates One per point Areas as polygons Straight lines between points, connecting back to the start Point locations recorded as coordinates Lines as polylines Straight lines between points
Vector Data Points 1 2 3 4 Point Number X,Y Coordinates 1 2,4 2 3,2 3 5,3 4 6,2 Lines 1 Line Number X,Y Coordinates 1 1,5 3,6 6,5 7,6 2 1,1 3,3 6,1 7,3 2 Polygons 1 2 Polygon Number X,Y Coordinates 1 1,5 3,6 4,4 2,2 1,5 2 6,6 8,7 8,3 7,3 6,6 An area (red line) and its approximation by a polygon (dashed blue line)
Raster vs. Vector Smallest addressable unit in Raster is a cell and in Vector is a point. Vector can represent the true dimensions of a object, Raster will always distort an object. Vector makes better graphics. Easier to write algorithms and integrate data in raster. Source of data - remote sensing, elevation data come in raster form Vector used for problems where parcels and lines are issues (AM/FM, Property Mgt) Raster is better for continuous data and integrating data used in modeling Important Databases for Hydrology National Hydrology Dataset (NHD) USGS (nhd.usgs.gov) EPA (NHDPlus: www.horizonsystems.com/nhdplus/) National Elevation Dataset (NED) USGS (ned.usgs.gov) National Land Cover Data (NLCD) EPA (www.epa.gov/mrlc/) Soils NRCS (soildatamart.nrcs.usda.gov)
Five Ms of Applied GIS Mapping location of entities Measuring distance, counts Monitoring change over time Modeling integration of data Managing decision-making