Feature Overlay

Feature Overlay (editing and combination)

Feature layers can participate in the overlay process, but the types of analyses are different.

Here are some of the other types of feature overlay (from the Analysis/Overlay toolbox)

and others are possible in other programs (as seen here http://wiki.gis.com/wiki/index.php/Overlay).

If two or more spatial units need to be “combined” we can use different types of spatial math.  Combinations of spatially intersecting features are available in ArcGIS under the edit menu (for digitizing or for editing a single theme with multiple intersecting polygons) but are generally used across multiple layers from the Geoprocessing Toolbox .  This image shows the effects of the union, intersect, combine and subtract commands on the “big box” and “little box” polygons.  Little box was drawn first.

These are the types of spatial and attribute combinations that are available in the Geoprocessing Toolboxes–they are common to most vector-able GIS.

  • Dissolving will result in feature categories and lines that disappear between spatial units (example: detail land use to general land use categories)
  • Merging brings adjacent maps with the same attribute information (be careful, unstable results if the attributes aren’t the same) \
  • Clipping (also known as stamping) is like a cookie cutter slicing off a piece of an original theme (example:  taking a subset of a geologic map for a study area)
  • Intersect and Union both create a fused theme that incorporates the spatial and attribute nature of both.  This will result in a huge number of polygons for some maps.  Imagine intersecting a landuse and geology theme….. If you intersect a line theme with a polygon theme (input is line, overlay is polygon), the information about the polygon is added to the line segments, and the lines are split into new segments where the polygons cross lines…….see below….note that any length attributes for the original line theme or “connection” information (what’s upstream or downstream) will now be incorrect and must be updated.

Selection by location (although this is really part of topic 6 “location”)

  • in a GIS, selection across layers using database attribute queries can be a “non-spatial” operation.
  • However, because we know the spatial context of each feature class and each feature, we can use their location relative to another as a selection criteria. This is known generally as an “intersect” operation, but has lots of variety of selection types and names in different platforms. Here are some of the possibilities in ArcGIS for features in one layer that. . .

    . . . the features of another layer
  • In my work I find “within a distance of” especially useful.

    here the red rings show a distance around the points of interest, and the highlighted cyan features show the “selected” features of each type.