ArcGIS applications are built with the assumption that the feature's geometry follows certain specifications. When the software encounters data which does not follow the specifications, the result can be unexpected software behaviors (including crashes) and incorrect analysis results.
Origins of bad geometry
The Shapefile format is an open format to which many software packages write to natively. Unfortunately some of these software packages (sometimes due to bugs) do not follow the documented specification for the Shapefile format.
Also of note is that when Shapefiles are loaded into personal or file geodatabase, the features are copied as they are (with the geometry problem) to these other formats, so the same caution and care which needs to be used when using Shapefile data needs to be taken when using personal and file geodatabase feature classes.
The exception to this when the data is loaded to an SDE Geodatabase. Before it is stored in the feature class, each record's geometry is checked for geometry problems, and repaired if necessary.
The onus is on the data's user to verify that it contains features with valid geometries before using it. Once check geometry has been run on a dataset, it is not necessary to check it again if the data has not been modified.
Finding and fixing geometry problems
The Check Geometry
tool will generate a report all features with geometry problem within the feature classes provided. Then to "fix" the problems use the Repair Geometry
Below is the list of geometry problems:
- Short segment: Some segments are shorter than allowed by the system units of the spatial reference associated with the geometry. Short segment errors will be deleted by the Repair Geometry tool.
- Null geometry: The feature has no geometry or nothing in the SHAPE field. Null geometry errors will be deleted.
- Incorrect ring ordering: A polygon is topologically simple, but its rings may not be oriented correctly (outer rings—clockwise, inner rings—counterclockwise). Incorrect ring ordering will be updated to be valid.
- Incorrect segment orientation: Individual segments are not consistently oriented. Incorrect segment orientation will be updated to be valid.
- Self-intersections: The interior of each part must not intersect with itself or other parts. Self-intersections will be updated to be valid.
- Unclosed rings: The last segment in a ring must have its "to" point incident on the "from" point of the first segment. Unclosed rings will be updated to be valid.
- Empty parts: The geometry contains an empty part. Empty parts will be updated to be valid.