I have a strange error with a large complex polygon shapefile (representing a hypothetical historic flood level of a lake) that I cannot resolve. Using QGIS 2.18.14, the polygon was derived as follows...
- Reclassify a digital elevation model (DEM) into two classes- one class above the desired flood level, one below the desired flood level.
- Polygonize the class below the desired flood level (Raster--> Conversion--> Polygonize).
- Subsequently, I edited the geometry of the resulting polygon to merge small islands with the larger lake level polygon. I also adjusted some boundaries manually using the edit nodes tool.
Now, when I open the shapefile, toggle editing, and use the nodes tool, validation starts as usual, except that the entire polygon consists of duplicate nodes. Because the polygon is large and complex (~35,000 nodes), it takes some time to validate and is really inefficient to manually inspect. I've tried the following remedies:
- Use the GRASS v.clean tool
- Use the MMQGIS Modify--> Remove Duplicate Geometries Tool
- Import the shapefile into SAGA and convert the polygon to edges + nodes, then subsequently remove duplicate points with the Shapes - Points--> Remove Duplicate Points tool
- Export only the edges from the previous step, saving them as a polygon
None of these remedies seem to work. I've not been able to find this same problem posted elsewhere online. I would ordinarily give up at this point and try to re-create the flood level polygon to better document what may have caused it to go wrong along the way (I suspect something happened during my manual editing, but I can't imagine what). I put several hours into step 3 above (manual editing), and so it would be great if there was a way to solve this issue.
I included a link to download the shapefile below if anyone wishes to play with it.
p.s. - you may be tempted to criticize why I am even dealing with this type of analysis by converting an artificially flooded DEM into a polygon... it is a client's request to have the result as a shapefile.