GRID Theme Basics

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GRID Theme Basics
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GRIDS

• Can store both
• Discrete Data
• Continuous Data

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Discrete GRIDS

• Discrete grids represent geographic features that
  have definable boundaries

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Discrete Grids
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Continuous Surfaces

• Continuous surfaces represent geographic
  phenomena that can vary spatially. Each cell
  value on the surface is a measure of the
  concentration or level at that location.
• Examples
• contamination levels,
• heat from a fire,
• elevation,
• concentration diminishing from a source

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Continuous Surfaces
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Grid theme location

• Each grid theme has a Cartesian coordinate
  system.
• 2 things are stored
• (1) the coordinates of the grids Cartesian
  coordinate origin (bottom left of grid)
• (2) the cell size

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Cartesian Coordinate System
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Grid Theme Attributes

• Only integer grid themes
• have a theme table.
• One record is stored for
• each unique value in the
• grid theme.

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Grid data sources

• ArcView Spatial Analyst can use data in many
  different formats.
• Some data sets may be converted or imported into
  grids.
• Spatial Analyst also has two export formats to
  output grids.

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Converting themes to grids

• ArcView Spatial Analyst can convert both feature
  and image themes into grids. This process is
  called rasterization. The left part of the
  diagram represents a polygon feature theme. The
  right part of the diagram is its representation
  as a grid theme.

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Convert Image to Grid

• Image conversion formats include
• ERDAS
• IMAGINE (if the Images extension is loaded)
• RLC (run-length compressed files)
• BIL, BIP, BSQ
• Sun Rasterfile
• TIFF
• JPEG (if the Images extension is loaded)
• BMP (Windows bitmap images)

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Converting grids to polygon shapes

• Only integer grids can be converted into polygon
  themes.
• The process is referred to as vectorization