GIS Geographic Information System Cluster 1 CPAS

What is GIS? Geographic Information System (GIS) is a system of computer hardware, software, and procedures designed to support the compiling, storing, retrieving, analyzing, and display of spatially referenced data for addressing planning and management problems. In addition to these technical components, a complete GIS must also include a focus on people, organizations, and standards.

Three Views of a GIS The Database View: A GIS is a unique kind of database of the world—a geographic database (geodatabase). It is an "Information System for Geography." Fundamentally, a GIS is based on a structured database that describes the world in geographic terms. The Map View: A GIS is a set of intelligent maps and other views that show features and feature relationships on the earth's surface. Maps of the underlying geographic information can be constructed and used as "windows into the database" to support queries, analysis, and editing of the information. The Model View: A GIS is a set of information transformation tools that derive new geographic datasets from existing datasets. These geoprocessing functions take information from existing datasets, apply analytic functions, and write results into new derived datasets.

The Database View The Map View The Model View

GIS data GIS data: All themes within a GIS are based on geographically referenced data, or spatial data. GIS software generally consist of two types of spatial data (vector and raster data) but can also accommodate other components of spatial data, such as attribute data and metadata. Spatial data vector data (feature themes) - attribute data raster data (image themes, grid themes) - attribute data Metadata (data about the spatial data)

Types of Spatial Data Feature themes use vector data, which are represented by points, lines, or polygons (e.g., earthquakes, elevation contours, etc.). Some feature themes have several polygons associated with one or more features and/or features that "overlap" one another sometimes referred to as a "region". Image themes use raster data, which are represented by an array of square pixels or cells. Each cell has a value associated with it that represents a condition found at the corresponding location or area of the "real-world". Aerial photography and satellite imagery are common raster data. Grid themes also use raster data but require special GIS software to utilize them for spatial analyses. The land cover data layer used in GIS is a grid that has a spatial resolution of 28.5 meters, which means that each pixel represents a 28.5 meter x 28.5 meter area of the "real world." In this case, each cell has one of 19 possible values that correspond to the 19 different classifications (e.g., clear water, alder, open conifer forest, and background).

Raster data In its simplest form, a raster consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information, such as temperature. Rasters are digital aerial photographs, imagery from satellites, digital pictures, or even scanned maps.

Vector data The vector data model represents each feature as a row in a table, and feature shapes are defined by x,y locations in space (the GIS connects the dots to draw lines and outlines). Features can be discrete locations or events, lines, or polygons. Most vector geographic information systems support three fundamental geometric objects: Point: A single pair of coordinates. Line: Two or more points in a specific sequence. Polygon: An area enclosed by a line.

Land Cover Layer

Attribute data Attribute (tabular) data is the descriptive data that GIS links to map features. Attribute data is collected and compiled for specific areas like states, census tracts, cities, and so on and often comes packaged with map data.

Metadata Metadata are essentially "data about data." They are an integral part of the spatial data in that they describe the content, quality, and other characteristics of the spatial and attribute data.

Map Legend A map legend defines how colors, symbols, contours, and annotation are used in a map. ArcView allows users to edit map legends to control presentation, and thus interpretation, of the data that are displayed.

Map Features Map Features Locational information is usually represented by points for features such as wells and telephone pole locations, lines for features such as streams, pipelines and contour lines and areas for features such as lakes, counties and census tracts. Point feature A point feature represents as single location. It defines a map object too small to show as a line or area feature. A special symbol of label usually depicts a point location. Line feature A line feature is a set of connected, ordered coordinates representing the linear shape of a map object that may be too narrow to display as an area such as a road or feature with no width such as a contour line. Area feature An area feature is a closed figure whose boundary encloses a homogeneous area, such as a state country soil type or lake.

Map Characteristics In addition to feature locations and their attributes, the other technical characteristics that define maps and their use includes: Map Scale Map Accuracy Map Extent and Data Base Extent

Map Scale Map scale indicates how much the given area has been reduced. For the same size map, features on a small-scale map (1:1,000,0000) will be smaller than those on a large-scale map (1:1,200). Cartographers often divide scales into three different categories. Small-scale maps have scales smaller than 1 : 1,000,000 and are used for maps of wide areas where not much detail is required. Medium-scale maps have scales between 1 : 75,000 and 1 : 1,000,000. Large-scale maps have scales larger than 1 : 75,000. They are used in applications where detailed map features are required.

Map Resolution Map resolution refers to how accurately the location and shape of map features can be depicted for a given map scale. Scale affects resolution. In a larger-scale map, the resolution of features more closely matches real-world features because the extent of reduction from ground to map is less. As map scale decrease, the map resolution diminishes because features must be smoothed and simplified, or not shown at all.

Map Accuracy Many factors besides resolution, influence how accurately features can be depicted, including the quality of source data, the map scale, your drafting skill and the width of lines drawn on the ground. A fine drafting pen will draw line's 1/100 of an inch wide. Such a line represents a corridor on the ground, which is almost 53 feet wide.

Types of Information in a Digital Map Three general types of information can be included in digital maps: Geographic information, which provides the position and shapes of specific geographic features. Attribute information, which provides additional non-graphic information about each feature. Display information, which describes how the features will appear on the screen.

Feature Colors and Linetypes – should be chosen to make the map's meaning clear. Naming Roads – are important for proper map interpretation. This information should be legible, positioned in the center of the road or offset from the center, and drawn at intervals suited to the scale of the final map or its purpose. Landmark Symbols – should be used to indicate landmarks, such as hospitals, schools, churches, and cemeteries. The symbols should be sized appropriately in relation to map scale. Polygon Fill – Polygon features, such as lakes or parks, should be filled with an appropriate color or hatch pattern. Zoom Layer Control – should be set up so that detailed, high-density information only appears when the user zooms in for a close-up of part of the map. For example, when a large area is displayed, only the major roads should appear; for a smaller area, both major and minor roads should appear.

Layering Most GIS software has a system of layers, which can be used to divide a large map into manageable pieces. For example, all roads could be on one layer and all hydrographic features on another. Major layers can be further classified into sub-layers, such as different types of roads - highways, city streets, and so on. Layer names are particularly important in CAD-based mapping and GIS programs, which have excellent tools for handling them.

Types of Digital Maps Topographical Maps It is a reference map showing the outline of selected man-made and natural features of the earth. It often acts as a frame for other features Topography refers to the shape of surface represented by contours or shading. It also shows lands, railway and other prominent features. Thematic maps Thematic maps are an important source of GIS information. These are tools to communicate geographical concepts such as Density of population, Climate, movement of goods and people, land use etc. It has many classifications.

GPS Global Positioning System is a system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver.