“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 lines in a roads vector layer that intersect should be joined by the same node. Topology is necessary for carrying out some types of spatial analysis, such as network analysis.” 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.
The vector topology model stipulates that:
Goal: To learn the basics of maintaining vector topology and how to clean data so that it adheres to the vector topology model.
Use the topology checker to validate against any topology errors from the layer that has no duplicates (clean).
What kind of errors do you see for the road? How can we clean up the layer ?**
Reproject the road layer (clean) without duplicates to the specified CRS.
NB: Ideally start by using a very big number like 100 and note the difference. Then start again this time starting with small numbers with small increments.
Name | Expectation |
---|---|
Background Layer |
Google Satellite or topology_cleaning/ 2821AD_21_2014_747_RGB_RECT.tif |
data |
roads_unclean |
Validity tools (Plugins) |
Topology checker, Geometry validity |
CRS |
EPSG:3857 |
Cleaning tools |
V.clean |
Roads Style |
Simple line, Colour - #e31a1c, Stroke width - 0.86 |
The vector topology model stipulates that:
Data that does not adhere to the topological model inhibits spatial analysis to be done and will always give erroneous results. 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.