Raster Based GIS Analysis

Raster Based GIS Analysis

What is a RASTER GIS? A graphic representation of features and attributes Often looks more like an image than a map Made of grid cells which contain information Cells can be any size but are often limited by several criteria

Raster GIS represents elements of the real world for analysis: While feature themes use Coordinates and lines (vectors) to represent geographic features Raster Grid themes use cells to represent geographic features

Vector GIS We have already used a vector based GIS It is superb at drawing maps It is excellent for managing attribute data It is very efficient for performing calculation along routes and for areas with matching geographies

In A Vector GIS... Points, Lines and Areas represent entities Points - City, Tree Lines - River, Road Areas - Forest, Lake Analysis includes: Buffering Intersecting Network Analysis (shortest path)

Raster GIS Very different from the Vector GIS Uses more images and grids Stores data differently Uses square grids to hold data Uses more “codes” to interpret data Uses much more processing power Can generate many more mathematical outputs

In A Raster Based GIS... Raster Themes Include: Images Scanned maps, Air photos, Satellite images Grids DEM, Surfaces

In A Raster GIS... Grids represent entities Analysis Includes: Grids made of cells Value applied to cell Analysis Includes: Buffering/Proximity Reclassification Hill-shade/Slope Interpolation Surface Calculation

Tools for Raster Data Analysis... Map Algebra Hill Shades Slope Aspect Raster Modeling Raster and Vector Integration Raster to polygon conversion Contour Generation Surface Interpolation from point data

How does it work? A imaginary grid is placed over an area Each cell in the grid is given a numeric code for the attribute that is dominant in that cell Each cell can be... numeric qualitative a feature identifier

A Vector to Raster Comparison... Vector GIS Manages Discrete Features Distinct boundaries Stored as integer values Land use, zoning, vegetation, lakes, roads, rivers Vector Model A Raster GIS Manages Continuous Data Stores data as floating point values Examples: Elevation, noise pollution, rainfall, slope and temperature Raster Model

The Raster Grid Raster grids are always square but can be displayed as other shapes Each group of cells can be considered a layer Various layers can be overlain on each other Calculations can be done between layers if the grids line up.

Raster Grid Structure Row 1 2 3 Column Cell (2,3) Grid themes are an organized matrix of cells Cells are organized into rows and columns Rows and columns have an index position number Top left cell is at the 0,0 position Cells Store a numeric value Numeric values are codes that represent geographic features 1-Wetlands 2-Water 3-Forest 4-Recreation Cells with the same value make up a zone Row 1 2 3 Column Cell (2,3)

Geographic Analysis Raster systems are superb at analyzing some geography problems such as... Data that changes continuous over a surface elevation vegetation rainfall temperature, etc. There can be used to create new information such as erosion potential.

Raster GIS Analysis Questions and answers about something geographic Leads to decisions that affect people Involves budgeting money and resources Spatial query examples Show me the pine trees Select the freeway Spatial analysis examples Which trees are ready to harvest? Identify some corridors for the new freeway

What is GIS Analysis From simple to Complex Simple - Sometimes by just making a map you are doing analysis Complex - Sometimes we use many layers of data to simulate real world events

Data Analysis What can we map using GIS (Spatial Analysis Tools)? Map where things are- Location Analysis Map the most and least- Quantitative Analysis Map concentrations- Density Map what is inside- Monitoring Map what is nearby- Relationships Map Change- Prediction

What are the effects of Global Warming? Land cover and temperature relationships are made clear when the data are seen at once.

Will the new building spoil the Park Scenery? Park planners use GIS to determine if a new Visitor Centre can be seen from the peak.

How can I reduce my fertilizer Input? GIS is used in making crop management decisions to maximize yields and minimize fertilizer input.

Managing Raster Data Raster systems are data intensive they must record data at every cell even if nothing is present use large amounts of hard drive space Raster systems have the advantage of being uniformly defined. This means that very powerful analytical tools can be used with a raster system.

Summary Raster systems use grids to store information Images are raster grids Images can be given attributes by pixel Grid can be analyzed for various purposes Raster data can be overlaid with other grids for greater analysis power.

VECTOR! Vector Data Represented by point, line and polygon. Relatively small file size (small data volume) Excellent representation of networks. A large no. of attributes can be attached, hence more information intensive and a number of thematic maps can be prepared from a single layer. Features are more detailed & accurate. Creating, cleaning and updating data is more time and labour consuming.

Vector con’t Topology-based analysis & operations are easier to perform (like network analysis etc.). Can not represent continuous values like land use, elevation etc very well. Assigning projection and transformations are less time taking and consumes less memory of the computer system. Topology makes data structure complex.

RASTER! Points, line & polygons everything in the form of Pixels. Large file size. Networks are not so well represented. Only one pixel value represents each grid cell. Generalization of features (like boundaries) hence accuracy may decrease. Simulations and modeling is easier (spatial analysis, terrain modeling etc.).

Raster con’t Maintaining is easier. Excellent for representing data containing continuous values (like land use, elevation etc.) Coordinate-system transformations take more time and consume a lot of memory. Grid cells or pixel makes simpler data structure.