Raster GIS

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Raster GIS
full-resolution image
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Discrete data values only at points, lines or
within polygons Continuous data implied values
everywhere - surface analysis
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Precipitation
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TINs versus raster DEMs
TIN elevations at triangle vertices of uniform
facets DEM elevation values, one for each grid
square
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Raster versus vector
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Images and Continuous data raster
required Elevation DEM or TIN Discrete
data vector more common (depending on GIS
activity)
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RASTER FILES Simple 'grid' of rows and columns
-cells or pixels cell value is based on a
selected attribute Resolution is based on size
of  cell -gtsmaller higher resolution pixel size
usually in round values, e.g. 10, 25, 250, 1000
metres X and Y coordinates of upper left
and lower right corners. georeferencing is
'implicit' (based on header information)
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• Advantages
• A simple data structure
• Overlay operations are straight forward
• High spatial variability is efficiently
  represented (e.g. relief).
• Only raster can easily store image data (e.g.
  photos)
• Disadvantages
• Data structure is not compact
• Limited in attribute management each pixel has
  one data value
• output can appear 'blocky'
• Raster formats (file header files)
• tif, gif, jpg, bmp (Graphics formats)
• img (Imagine image)

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• Vector Data model
• Features are coded as points, lines (arcs) and
  areas (polygons)
• Defined by single points, connected nodes, and
  arcs
• Vector files contain information attached to
  features
• georeferencing is explicit - coordinates on each
  point or vertex
• Advantages
• Compact data structure for generally homogenous
  areas
• Efficient encoding of topology ( containment,
  contiguity, connectivity)
• Strong in database management
• Better suited for map output
• Disadvantages
• More complex data structure
• Some types of analysis are more complex
• Cannot store image data

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Converting points to raster
Raster form at is not compact
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Lines to raster
Lines are contiguous pixels
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Polygons to raster
Similar areas have adjacent pixels Attribute
table shows number of pixels in each value,
graphed in a histogram
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GIS analysis

• Location what is here
• Condition where can I find these features
• Patterns how is one layer related to another
• Trends what has changed
• Modelling what if e.g. a condition changed

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Location and Condition
Queries Vector data are based on features and
have x and y coordinates What are the
characteristics of this feature ? Query returns
information for that feature Condition returns
similar features in an attribute table Raster
data are based on pixels, with a grid system of
rows /columns. What is at this (pixel) location
? Query returns information for that
pixel Condition returns similar pixels in a
histogram (value versus frequency)
(assuming integer values)
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GIS overlay analysis
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GIS overlay analysis
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DEMs analysis per pixel TINs analysis by
triangular facet
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Raster analysis SINMAP (Stability INdex MAPping)
Inputs slope, moisture, surface geology etc..
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Viewshed analysis line of sight
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Summary
Raster analysis, modelling Vector
inventory, output GIS Software Arcmap mostly
vector spatial analyst (formerly
GRID) Raster GIS software GRASS
IDRISI Examples Raster Modelling
http//www.pinebeetle.ca/infestation_impact.html
Vector http//www.esri.com/software/arcgis/exten
sions/networkanalyst/index.html