Geographic Information Systems CIVE 1188 Hanadi Rifai, PhD, PE Civil and Environmental Engineering University of Houston

Where did GIS come from? GIS is built upon knowledge from geography, cartography, computer science and mathematics Geographic Information Science is a new interdisciplinary field built out of the use and theory of GIS

Defining GIS Different definitions of a GIS have evolved in different areas and disciplines All GIS definitions recognize that spatial data are unique because they are linked to maps (Space matters!) A GIS at least consists of a database, map information, and a computer-based link between them

Spatial and non-spatial data

Map Overlay

The Feature Model

A Brief History of GIS GIS’s origins lie in thematic cartography (manual map overlay) Computer cartography advances in 1950s and 1960s Early influential data sets were the World Data Bank and the GBF/DIME files. Early systems were CGIS, MLMIS, GRID and LUNR The Harvard University ODYSSEY system was influential due to its topological arc-node (vector) data structure GIS was significantly altered by (1) the PC and (2) the workstation During the 1980s, new GIS software could better exploit more advanced hardware User Interface developments led to GIS's vastly improved ease of use during the 1990s During the 1980s, new GIS software could better exploit more advanced hardware

Sources of Information on GIS The amount of information available about GIS can be overwhelming Sources of GIS information include journals and magazines, books, professional societies, the World Wide Web, and conferences GIS has Web Home pages, network conference groups, professional organizations, and user groups Most colleges and universities now offer GIS classes in geography departments

Organizing Data and Information Information can be organized as lists, numbers, tables, text, pictures, maps, or indexes. Clusters of information called data can be stored together as a database. A database is stored in a computer as files.

Flat File Database RecordValue Attribute RecordValue RecordValue

Geographic Coordinates as Data

Building complex features Simple geographic features can be used to build more complex ones. Areas are made up of lines which are made up of points represented by their coordinates. Areas = {Lines} = {Points}

Areas are lines are points are coordinates

Basic properties of geographic features

GIS Analysis Much of GIS analysis and description consists of investigating the properties of geographic features and determining the relationships between them. Two types of systems: –Raster or pixel based –Vector or arc based

RASTER A grid or raster maps directly onto a programming computer memory structure called an array. Grids are poor at representing points, lines and areas, but good at surfaces. Grids are good only at very localized topology, and weak otherwise. Grids are a natural for scanned or remotely sensed data. Grids suffer from the mixed pixel problem. Grids must often include redundant or missing data. Grid compression techniques used in GIS are run-length encoding and quad trees.

Raster Data Formats Most raster formats are digital image formats. Most GISs accept TIF, GIF, JPEG or encapsulated PostScript, which are not georeferenced. DEMs are true raster data formats.

The Vector Model A vector data model uses points stored by their real (earth) coordinates. Lines and areas are built from sequences of points in order. Lines have a direction to the ordering of the points. Polygons can be built from points or lines. Vectors can store information about topology.

VECTOR At first, GISs used vector data and cartographic spaghetti structures. Vector data evolved the arc/node model in the 1960s. In the arc/node model, an area consist of lines and a line consists of points. Points, lines, and areas can each be stored in their own files, with links between them. The topological vector model uses the line (arc) as a basic unit. Areas (polygons) are built up from arcs. The endpoint of a line (arc) is called a node. Arc junctions are only at nodes. Stored with the arc is the topology (i.e. the connecting arcs and left and right polygons).

TOPOLOGY Topological data structures dominate GIS software. Topology allows automated error detection and elimination. Rarely are maps topologically clean when digitized or imported. A GIS has to be able to build topology from unconnected arcs. Nodes that are close together are snapped. Slivers due to double digitizing and overlay are eliminated.

Basic arc topology n1 n A B ArcFromToPLPRn1xn1yn2xn2y 1n1n2ABxyxy Topological Arcs File Figure 3.5 A topological structure for the arcs.

Finding Existing Map Data Map libraries Reference books State and local agencies Federal agencies Commercial data suppliers e.g. GDT, Thompson, ETAK World Wide Web GIS vendors package data with products.

USGS: National Mapping

Terrain data DEM DLG Contours DCW Contours

U.S. Bureau of the Census

NOAA Weather and other data

Distributed active archive center Sioux Falls, SD Operated by USGS Eros Data Center

Digitizing Captures map data by tracing lines from a map by hand Uses a cursor and an electronically- sensitive tablet Result is a string of points with (x, y) values

Scanning Places a map on a glass plate, and passes a light beam over it Measures the reflected light intensity Result is a grid of pixels Image size and resolution are important Features can “drop out”

Field data collection

Attribute data Logically can be thought of as in a flat file Table with rows and columns Attributes by records Entries called values

Database Management Systems Data definition module sets constraints on the attribute values Data entry module to enter and correct values Data management system for storage and retrieval Legal data definitions can be listed as a data dictionary Database manager checks values with this dictionary, enforcing data validation.

Retrieval Operations Searches by attribute: find and browse. Data reorganization: select, renumber, and sort. Compute allows the creation of new attributes based on calculated values.

The Big Eight Form the bulk of operational GIS in professional and educational environments There are some significant differences between these “big eight” systems.

Arc/Info ESRI Redlands, CA Arc/Info Market leader workstation (mostly) remarkable functionality many formats supported ArcEdit ArcGRID ArcPlot INFO

ArcView Versions 1-3, 3.1 PC Windows Avenue Web links Map Objects Extensions

AutoCAD MAP Windows all versions SQL DBF Access Extension to AutoCAD Menu-based Massive installed base Added grid, projection & topology support DB links good. 3D links good

GRASS First UNIX GIS Developed by Army Corps of Engineers UNIX functionality Many unique functions Free until recently Many data sets Baylor University now supports

IDRISI Developed at Clark University, Worcester MA Original in PASCAL, with open code Development uses a specialty Windows/DOS Spatial analysis/stats extensions

Maptitude Caliper Corporation Consultancy TRANSCAD and GIS+ Many network solutions Windows Import/Export Address matching

Microstation MGE CAD software with GIS extensions Intergraph Corp, Huntsville AL Uses Windows NT Many parcel applications Web extensions, server tools etc.

MapInfo Based in Troy, NY Mapping functions Limited GIS functionality Uses Visual Basic Many applications Favored for 911, field