Tools to check and edit map data
 
The topological rules used by GIS packages mean that certain functions require vector data resources to contain topologically correct (simple) features for these functions to operate successfully. For example, the tools in the Geoprocessing Wizard of ArcGIS™ (dissolve, merge, union, intersect) may fail if the input shapefiles have features with topological errors (often known as non-simple features). In shapefiles generated from seabed habitat mapping data, common topological problems are: features oriented in an anti-clockwise direction rather than clockwise; ‘bow-ties’ caused by self-intersecting features; and, dangling segments within features. A set of simple diagrams illustrating non-simple features is shown.
 
GMHM4-59_Non-simple_features_according_to_ESRI.png
Illustrations of non-simple features according to ESRI™ topological rules for features
 
Therefore, if you are generating shapefiles that you plan to analyse or edit using spatially analytical tools, it is important to check their topology and simplify any non-simple feature where possible.
 
Time will obviously be saved if shapefiles are checked in batches. For this purpose, MESH has developed a suite of tools for use in ArcGIS™ that can process shapefiles in batches:
 
  • Document shapefiles: identifies shapefiles containing non-simple features;
  • Simplify shapefiles: simplifies shapefiles containing non-simple features, and;
  • Dissolve shapefiles: dissolves a batch of shapefiles based on an attribute specified by the user (useful for making boundary polygons).
 
Sometimes the Simplify shapefiles tool is unable to simplify all the features in a shapefile. In this case there is an additional set of tools that process the shapefiles one at a time to help trace the error:
 
  • Find non-simple features: finds and simplifies non-simple features in a shapefile and deletes features with an empty geometry;
  • Split multipart features: splits multipart features into individual features which retain the attributes of the original multipart polygon;
  • Remove interior rings: removes interior rings from features which is useful for removing artefact slivers resulting from union operations, and;
  • Re-order shapefile: a useful tool for visualising data which helps prevent larger features obscuring smaller ones by drawing the largest polygon first.
 
The ArcGIS MESH Tools are available for anyone to use, but please note that neither the MESH Project nor JNCC offer any software support for these tools; they are used at your own risk. To date they have been used only in ArcGIS™ 8.2 and 8.3. Please keep back-up copies of all data before using the tools.
 

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