Generalization in GIS

Caitlin Dempsey

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Sometimes GIS data contains an excess of detail or spatial information than what is needed for the scale of the map being prepared.  Generalization is the method used in GIS to reduce detail in data.  For example, a small scale map of the United States does not need detailed coastlines or a map of California does not need to show every road in the state.

Generalization can be achieved by removing detail, such as only showing major roads, showing only the boundary of a state instead of all the counties.  In GIS generalization is also used to smooth out lines, removing small detail such as the nooks and crannies of a coastline or the meanderings of a stream.

Since detail about a geographic feature is simplified during generalization, generalized data is less spatially accurate.  Those using generalized data to calculate length, perimeter, or area will incur errors in the calculations.

Generalization in ArcGIS

Depending on whether you are generalizing vector or raster data, there are different tools for generalizing GIS data using ArcGIS.  There is a toolset in the Spatial Analyst toolbox in ArcGIS that allows for several different methods of generalization on raster data.  The generalization tools in the toolset are grouped into three categories:  Aggregating zones of data (Nibble, Shrink, Expand, Region Group, and Thin), smoothing data edges (Boundary Clean and Majority Filter), and reducing the resolution of a raster (Aggregate).  For vector data, ArcGIS has a Generalize tool in the Editing toolset which uses the Douglas-Peucker simplification algorithm to simplify lines.  For additional generalization methods, the Generalization toolset found in the Cartography toolbox offers a range of tools for simplifying and reducing resolution of vector data for cartographic purposes.


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Smoothing a line using the Generalize tool in ArcGIS.  Image: Esri.
Smoothing a line using the Generalize tool in ArcGIS. Image: Esri.

Generalization in QGIS

Generalization of vector data in QGIS can be achieved via the Simplify Geometries tool which is found by selecting Vector -> Geometry Tools -> Simplify geometries. From the popup window, you can then select the line or polygon layer you want to simplify and set the tolerance.

Generalize function in QGIS.
Generalize function in QGIS.

MapShaper

If you need to generalize a GIS dataset independent of a desktop GIS application, MapShaper is a free online tool that allows you to upload a shapefile, GeoJSON, or TopoJSON file.  You can then select the simplification method and whether to repair intersections, auto-snap, or prevent shape removal. Once the process has been run, a download link is provided with the newly generalized GIS data.

MapShaper generalization tool.
MapShaper generalization tool.
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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.