GIS (geographic information system)

Another other kind of reclassification is simplification of a discrete or categorical theme based on attributes.  In the below example, a US State data layer that has census information attached to the state spatial file can be subdivided based on population density (this was done in the layer's properties box, using the query builder but could also be "queried" using Select by Attributes option from teh Selection menu.. 

If two or more spatial units need to be "combined" we can use different types of spatial math.  Combinations of spatially interseting features are available in ArcMap 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 identical or the order of attributes is different together) \
• Clipping (also known as stamping) is like a cookies 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 (only extent is different?).  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 segements, and the lines are split into new segments where the polygons cross lines.......see below....note that any length attributes for the orginal line theme or "connection" information (what's upstream or downstream) will now be incorrect and must be updated. 
  

• in a GIS, selection using database attribute queries is a "nonspatial" operation
• 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 seletion t ypes 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.