Map Analysis with Raster Datasets

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Map Analysis with Raster Datasets

• Francisco Olivera, Ph.D., P.E.
• Department of Civil Engineering
• Texas AM University

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Map Analysis

• Map analysis consists of inferring information --
  necessary for a given engineering/scientific task
  -- from general information contained in digital
  spatial datasets. In map analysis with raster
  datasets, the digital spatial datasets consist of
  grids.

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Spatial Analyst

• To enable raster data analysis capabilities, the
  Spatial Analyst ArcView extension has to be
  loaded.
• The Spatial Analyst adds the Analysis and Surface
  menus, as well as the Histogram button and
  Contour tool, to the ArcView GUI.
• A significant number of additional capabilities
  are also available through Avenue scripts.

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Grid Datasets
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Grid Data Structure

• Cellular-based data structure composed of square
  cells of equal size arranged in rows and columns
  that store the value of a terrain parameter.
• Cells that do not store any value are assigned a
  NODATA code and are called NODATA cells.

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Grid Properties

• Type Integer or Floating Point. Depending on the
  type of numbers the grid cells store, a grid is
  considered Integer (if it stores integer values)
  or Floating Point (if it stores real values).
  Floating Point grids, storing the same
  information as an Integer grid, take
  significantly much more storage space.
• Status Permanent or Temporary. Many grids
  created in ArcView are stored as temporary files
  and are subject to be erased automatically. A
  temporary grid can be saved as a permanent grid
  by clicking on Theme/Save Dataset.

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Value Attribute Table

• Value Attribute Tables (VATs) are tables
  associated with grids which have two fields
  Value and Count. The Value field lists all values
  found in the grid, and the Count field the number
  of cells that have that value.
• Only Integer grids have VATs.
• VATs are created automatically for Integer grids
  with less than 500 unique values or with a range
  of values that does not exceed 100,000.
  Otherwise, the VAT has to be created with Avenue
  requests.

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Analysis Properties

• Analysis Properties are defined for a View
  document.
• Analysis Extent defines the rectangle in which
  the grid is contained. The Analysis Extent can be
  defined same as a specific grid or with
  coordinates.
• Cell Size defines the length of the cell side.
  The Cell Size can be defined same as a specific
  grid or with a number.

It is fundamental to have all grids of a view
aligned and with their cells coinciding exactly.
Therefore, the Analysis Extent and Cell Size must
be set before any grid is created in the View.
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Analysis Properties

• Analysis Properties are defined for a View
  document.
• The Number of Rows and Number of Columns are self
  explanatory. These properties would be redundant
  if the Analysis Extent and Cell Size have been
  defined already.
• The Analysis Mask flags the cells within the
  Analysis Extent where grid values are not
  calculated. Grid values are assigned NODATA
  wherever the mask has NODATA cells. The Analysis
  Mask is defined same as a specific grid.

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Vector-to-Raster Conversion

• Point-to-grid
• Each point is converted into the grid cell where
  it is located. The cell value is a user-selected
  attribute of the point dataset.
• If two points coincide in the same cell, one is
  chosen randomly for the cell value. Coincidence
  of two or more points within a cell might reflect
  inconsistency between the resolution of the grid
  and point dataset.
• To convert a point dataset into a grid, click on
  the Theme/Convert to Grid menu.

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Vector-to-Raster Conversion

• Line-to-grid
• Each line is converted into the grid cells with
  which it intersects. The cell value is a
  user-selected attribute of the line dataset.
• If two lines coincide in the same cell, one is
  chosen randomly for the cell value.
• To convert a line dataset into a grid, click on
  the Theme/Convert to Grid menu.

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Vector-to-Raster Conversion

• Polygon-to-grid
• Each polygon is converted into the grid cells
  whose center it contains (centroid method used by
  ArcView). The cell value is a user-selected
  attribute of the polygon dataset.
• If two (overlapping) polygons coincide in the
  same cell, one is chosen randomly for the cell
  value.
• To convert a polygon dataset into a grid, click
  on the Theme/Convert to Grid menu.

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Raster-to-Vector Conversion

• Grid-to-point and grid-to-line conversions are
  available in ArcView, but not in the standard
  interface.

• Grid-to-polygon
• All adjacent cells (i.e., share a side) with the
  same value are aggregated into a single polygon.
• The cell value is stored in an attribute Grid
  Code of the polygon dataset.
• Polygon outlines are smoothed to avoid
  "jagginess".
• Grids have to be Integer grids.
• To convert a grid into a polygon dataset, click
  on the Theme/Convert to Shapefile menu.

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Raster Functions

• Raster functions create output grids using input
  grids as arguments.
• Raster functions are classified into
• Local functions
• Focal functions
• Zonal functions
• Global functions

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Local Functions

• The value of an output grid cell depends on the
  value of the cells of the input grids that have
  the same location. Neighbor cells have no
  influence on the output values.
• Local functions can have one or many input grids
  as arguments.

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Focal Functions

• The value of an output grid cell depends on the
  value of the cells of the input grids in the
  neighborhood.
• The neighborhood can be defined in different
  ways.
• Focal functions usually have one input grid as
  argument, but could have more than one.

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Zonal Functions

• The value of an output grid cell depends on the
  value of the cells of the input grids of the same
  zone.
• A zones grid has to be one of the function
  arguments.
• Besides the zones grid, zonal functions usually
  have only one more input grid as argument, but
  could have more than one.

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Global Functions

• The value of an output grid cell depends on the
  value of all the cells of the input grids.
• Global functions usually have one input grid as
  argument, but could have more than one.