GIS Representations Raster Data Models

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GIS RepresentationsRaster Data Models
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A raster data model uses a grid.

• One grid cell is one unit or holds one attribute
• Every cell has a value, even if it is missing.
• A cell can hold a number or an index value
  standing for an attribute.
• A cell has a resolution, given as the cell size
  in ground units.
• Rasters are easy to understand, easy to read and
  write, and easy to draw on the screen

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Rasters

• 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.
• Grid compression techniques used in GIS are
  run-length encoding and quad trees.

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Generic structure for a raster (a grid)
Grid extent
s
w
Grid cell (pixel)
o
R
Resolution
Columns
Generic structure for a grid.
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Rasters are useful in presenting biophysical data
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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.

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Rasters are faster but...

• Points and lines in raster format have to move to
  a cell center.
• Lines can become fat. Areas may need separately
  coded edges.
• Each cell can be owned by only one feature.
• As data, all cells must be able to hold the
  maximum cell value.

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Raster databases

• Usually a single value is assigned to each cell
  in a raster layer
• A raster database contains many congruent raster
  layers with identical numbers of rows and columns
  and an identical geographic extent.

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Geocoding a raster

• row number, column number, resolution (cell
  size), llx and lly.
• row by row from the bottom left
• because the array is geometrically regular and
  ordered, it is not necessary to record the
  locations of every cell
• coordinates appear in a raster data set only in
  order to register the raster to a coordinate
  system.

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Values of a raster layer

• Output from one band of a remote sensing
  satellite (level of radiation) 0-255
• Classified scenes from RS images
• e.g. 1 urban, 2 agriculture, 3 wetlands
• A sandy soils, B loamy soils, C clay
• Digital elevation models elevations (real
  numbers)
• Presence of geographic features
• e.g. 1 road present 0 no roads

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Sampling for rasters
(RS data)
(DEM)
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The mixed pixel problem cells contain more than
one class (value)
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Grids and missing data
GIS data layer as a grid with a large section of
missing data, in this
case, the zeros in the ocean off of New York and
New Jersey.
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Characteristics of a raster

• Resolution
• Orientation
• Values
• Zones
• Classes
• Location

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Topology of rasters

• 4 edge neighbors von Neuman neighborhood of a
  cell, the Rooks case neighbors.
• 8 neighbors if diagonal neighbors are included,
  the total is eight, the Queen's case neighbors
• There are (n-2)(m-2) of these cells with a full
  set of 4 or 8 neighbors
• There are 2(n-2) 2(m-2) of these cells on the
  border that have only three edge-neighbors
• There are four cells at the raster corners with
  only 2 edge-neighbors each
• The total (n-2)(m-2) 2(n-2) 2(m-2) 4 nm
  cells
• cause edge effects on raster modeling.

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Encoding Raster Data

• Every cell must have a value even though there
  may be nothing there
• redundant
• repetitive
• Compression methods to reduce data volume
• (spatial dependency of geographic data)
• Run length encoding
• Quadtree encoding

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Run length encoding
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Use of Rasters I

• Things in raster forms
• a computer display
• photos by digital camera
• images
• geoTIFF is an adaptation of the general-purpose
  TIFF image standard that includes the necessary
  hooks for registering the raster to the Earth,
  plus other geographic features
• certain kinds of data always come in raster form
• digital elevation models
• remote sensing images

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Use of Rasters II

• good for representing fields, but
• never use a raster to represent a network (such
  as a sewer network), if the application requires
  accurate connectivity.
• Never use a raster to represent precise
  boundaries (such as land ownership parcels).
• Rasters have definite yet limited spatial
  resolution cell size is a direct indicator of
  level of geographic details.
• Cell size can cause different results of spatial
  analysis.

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An example of raster applications

• Objective identify areas suitable for logging
• Criteria if an area
• is dominated by commercial hardwood (e.g.,
  average cell value, dominate cell value)
• is well drained
• is not within 500 m of a lake or watercourse

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Raster Map overlay
SOLUTION
SOILS
PARKS
URBAN
FOREST