GIS analysis: spatial joins

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GIS analysis spatial joins

• J426 Class 23
• April 7, 2009

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Overview

• Issues in spatial joins
• Point-in-polygon analysis
• Spatial join relationships

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Spatial joins

• Joins the data in one attribute table for one
  feature layer to the attribute table of another
  feature layer, like any join
• Spatial join joins records in the source table to
  one record in the destination table based upon
  the spatial relationship of the features

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Spatial join relationships

• Can use all of the various spatial relationships
  used in selecting features, for example
• Is contained by
• Is nearest
• Intersect
• Is within distance of
• For each combination of layer type, ArcGIS
  provides the spatial relationships that make sense

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Issue in spatial join relationships

• Some relationships will give unambiguous joins,
  such as
• Join of non-overlapping polygon features to
  points
• Any point can be within only one polygon
• That point gets the records from the containing
  polygon
• Join of points to nearest points

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Issues in spatial join relationships

• Other spatial join relationships can result in
  ambiguous joins, such as
• Layer of overlapping polygons to points
• ArcGIS joins first record and provides count of
  features satisfying the spatial join relationship

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Joining polygon layers to point layers

• Often referred to as point-in-polygon analysis
• The records of the polygon layer are joined to
  the records of the point layer for the points
  inside each polygon
• Example joining block group median family income
  to crime point locations

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Example of data in block group polygon theme
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Example of data in crime point theme
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Block group and point features
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Block group family income data joined to crime
points
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Spatial join relationships

• The following slides indicate the types of
  spatial relationships provided by ArcGIS for the
  various combinations of layer types

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Joining point layers to point layers

• Joins points to nearest points
• Choice of attribute information for first nearest
  point and distance to nearest point, or
• Summary statistics for nearest points and count
  of nearest points

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Joining line layers to point layers

• Joins lines to nearest points
• Choice of attribute information for first nearest
  line and distance to nearest line, or
• Summary statistics for nearest lines and count of
  nearest lines

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Joining polygon layers to point layers

• Joins polygons to points inside the polygon, or
• Joins polygons to nearest point (with being
  inside equivalent to distance of zero)
• Provides count of the number of polygons joined
  to each point

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Joining point layers to line layers

• Joins points to nearest lines
• Choice of attribute information for first nearest
  point and distance to nearest point, or
• Summary statistics for nearest points and count
  of nearest points

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Joining Line Layers to Line Layers

• Joins lines to nearest lines
• Choice of attribute information for first nearest
  line and distance to nearest line, or
• Summary statistics for nearest lines and count of
  nearest lines

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Joining polygon layers to line layers

• Joins polygons to lines completely inside, or
• Joins polygons to nearest lines, or
• Joins polygons to lines intersects and summary
  statistics for intersected polygons and count of
  intersected polygons

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Joining point layers to polygon layers

• Joins points to nearest polygons with distance to
  nearest polygon (inside is distance of zero), or
• Joins points to containing polygons, with summary
  statistics for points and count of number of
  points

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Joining Line Layers to Polygon Layers

• Joins lines to nearest polygons with distance to
  polygon (intersection is distance of zero), or
• Joins lines intersecting polygons, with summary
  statistics for lines and count of number of lines

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Joining polygon layers to polygon layers

• Joins polygons to polygons in which joining
  polygon is completely inside, or
• Joins polygons intersecting polygons, with
  summary statistics for polygons and count of
  number of polygons