raster based gis analysis. what is a raster gis? 4 a graphic representation of features and...
TRANSCRIPT
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Raster Based GISAnalysis
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Raster Based GISAnalysis
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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
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While feature themes use Coordinates and lines (vectors) to represent geographic features
Raster Grid themes use cells to represent geographic
features
Raster GIS represents elements of the real world for analysis:
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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
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Points, Lines and Areas represent entities– Points - City, Tree– Lines - River, Road– Areas - Forest, Lake
Analysis includes:– Buffering– Intersecting– Network Analysis (shortest path)
In A Vector GIS...
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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
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Raster Themes Include:
Images Scanned maps, Air photos, Satellite images
GridsDEM, Surfaces
In A Raster Based GIS...
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Grids represent entities– Grids made of cells– Value applied to cell
Analysis Includes:– Buffering/Proximity– Reclassification– Hill-shade/Slope– Interpolation– Surface Calculation
In A Raster GIS...
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•Map Algebra•Hill Shades
•Slope•Aspect
•Raster Modeling•Raster and Vector Integration
•Raster to polygon conversion•Contour Generation•Surface Interpolation from point data
Tools for Raster Data Analysis...
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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
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A Vector to Raster Comparison...
Vector Model
Raster Model
Vector GIS Manages Discrete Features
•Distinct boundaries•Stored as integer values•Land use, zoning,
vegetation, lakes, roads, rivers
A Raster GIS Manages Continuous Data
•Stores data as floating point values•Examples: Elevation, noise pollution, rainfall, slope and temperature
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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.
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Row00 11 22 33
332211
00
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-Wetlands2-Water3-Forest4-Recreation
•Cells with the same value make up a zone
Raster Grid Structure
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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.
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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
Raster GIS Analysis
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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
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Data AnalysisWhat can we map using GIS (Spatial Analysis Tools)?
Map where things are- Location AnalysisMap the most and least- Quantitative AnalysisMap concentrations- DensityMap what is inside- MonitoringMap what is nearby- RelationshipsMap Change- Prediction
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What are the
effects of
Global
Warming?
Land cover and temperature relationships are made clear when the data are seen at once.
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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.
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How can I
reduce my
fertilizer Input?
GIS is used in making crop management decisions to maximize yields and minimize fertilizer input.
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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.
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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.
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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.
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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.
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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.).
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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.