Topology Errors and Cleaning
- 1. Topology Errors & Cleaning Mayuresh Padalkar
- 2. Introduction • Topology expresses the spatial relationships between connecting or adjacent vector features (points, polylines and polygons) in a GIS. • Topological or topology-based data are useful for detecting and correcting digitising errors. • Topology is necessary for carrying out some types of spatial analysis, such as network analysis.
- 3. Why to clean the data? • Certain applications like TUFLOW, InfoWorks ICM don’t require errors • There cannot be two runoff coefficients/roughness applied to the same polygon for specific landuse feature • In case of network analysis, tracing becomes easier
- 4. Type of errors • Polylines 1. Undershoot 2. Overshoot 3. Zero length - rmline 4. Snapping - snap 5. Intersections – break – use rmdupl after breaking 6. Dangles – rmdangle – lengths lesser than threshold are removed • Polygons 1. Overlaps 2. Gaps (Slivers) 3. Duplicates 4. Small areas
- 5. Steps to clean the polygon topology in QGIS • Remove duplicate vertices – Simply geometries with tolerance equal to 0 • Merging the small polygons with largest common boundary – Calculate the area and decide the threshold • Reduce the number of polygons of same type i.e. buildings, roads, grass, etc. – dissolving • We don’t want to select everything in one go - Convert multipart to singlepart • Removing overlaps – bpol in v.clean • Removing duplicates – MMQGIS>Delete Duplicate Geometries
- 6. Tip/Advice • With the steps mentioned in previous slide, not all the polygons get dissolved, but are free from errors which can be used for further processing • Therefore, it’s always better to spot check few locations of area of interest because we have cleaned it using tools and not manually • If working in TUFLOW, errors will still persist due to cell size • If working in InfoWorks ICM, it’s always recommended to remove minimum mesh element areas of less than 0.1m2 in order to increase simulation time • Therefore, it’s always better to select the minimum area which is of least importance, in order to merge with largest common boundary
- 8. Thank You!!!