“Topology expresses the spatial relationships between connecting or adjacent vector features (points, polylines, and polygons) in a GIS. Vector data has to conform to the topological model e.g. two lin”
In this module, we will inspect vector data for topology errors and explore ways in which to clean the dataset so that it conforms to the vector topology model.
Goal: To learn how the basics of maintaining vector topology.
What are duplicates?
What are the units are used in the algorithm?
Can you use v.clean.snap with the polygon ?
NB: To run multiple tools at the same time use v.clean.advanced
Name | Expectation |
---|---|
Background Layer |
Google Satellite or appendix3-local-data/ 2821AD_21_2014_747_RGB_RECT.tif |
data |
roads_unclean,polygon_unclean |
Validity tools (Plugins) |
Topology checker,Geometry validity |
CRS |
EPSG:3857 |
Cleaning tools |
V.clean,v.clean.snap |
QGIS version |
QGIS with grass |
Vector topology is important because it ensures data quality and integrity i.e. topology enables detection of lines that do not meet and lines that cross over each other. Vector data always follows a topological model which states that boundaries should not cross each other (boundaries that cross each other should be split at their intersections). Lines should also be split at the intersection by a single node. Two adjacent polygons should share a single boundary.
When the topological model is not adhered to it introduces error in the datasets and this inhibits spatial analysis to be done. Typical errors are:
GRASS GIS is a fully topological GIS package and using the GRASS algorithms to check and clean data helps in generating clean vector data which can be used in an analysis.