QGIS: Fixed Buffers and Interior Buffers for Polygons with Holes Not Working, What to Do?

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I'm following the online qgis manual available on qgis website. I'm doing the step 21.11.3. (http://docs.qgis.org/2.14/en/docs/training_manual/answers/answers.html#complete-analysis-3).

To make it simple : I have a raster converted to vector, so now it is a polygon (several polygons). I'm doing an interior buffer exactly how it is described in the manual. I didn't obtain the same result. I can see the interior buffer just in some polygons, not in all of them ->

The interior buffers appear just for those polygons, that don't contain a hole. I use qgis 2.18.3.

This is how i created the interior buffer ->

In green you see the input layer : suitable_terrain, in blue the output layer, the interior buffer.

It is not a problem of a projection. I use WGS84 UTM Zone 34S where the units are in meters. I did also the Check validity and i obtain the same result.

Best Answer

I tested the Processing toolbox -> QGIS geoalgorithms -> Vector geometry tools -> -> Fixed distance buffer on a polygon shapefile that has holes, as in your case, and I have got the same result that polygons with holes are not considered for interior buffers of -100 m distance.

Here is the output:

However, when I used the buffer vectors located under Processing toolbox -> GDAL/OGR -> [OGR] Geoprocessing -> Buffer vectors, I got the correct result that you want of distace of -100 m, as you can see below:

Here is the output:

I noticed that the buffer shape is different between the two results, but it is more accurate in [OGR] Buffer vectorstool than in QGIS Fixed distance buffer tool.

You need to use Processing toolbox -> GDAL/OGR -> [OGR] Geoprocessing -> Buffer vectors in your analysis.

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Let me suggest an approach by a Virtual Layer.

SELECT st_difference(st_buffer(Your_layer.geometry, 100), Your_layer.geometry)
FROM Your_layer

Please save this Virtual Layer as another vector file, such as ESRI Shapefile, before you proceed to next steps.

[GIS] Cannot find polygons that are inside a big polygon using GeoPandas

I think what you want is a "spatial join", where we add information of the second dataframe (the land use dataframe, information on the type of land use) to the first dataframe (the buildings) based on their spatial relationship:

gpd.sjoin(b, la[['fclass', 'geometry']], how='left', op='intersects')

This adds a new column to the b dataframe with the land use class in which each building is located.

Some additional notes about your attempts:

Try 1: a overlay can be suited here if you want to create a new geometry (e.g. in the case where a building is only partly inside a big land use polygon, then overlay will actually clip the original builing's geometry). But if you don't want to edit the geometries (take the intersection), the the spatial join (sjoin) shown above is more appropriate.
It is true that overlay is currently very show, as you noticed. There is work underway to improve that in the next version of geopandas: https://github.com/geopandas/geopandas/pull/429.

Try 2: this should also work, and actually for me it does not give a full series of False values. For the exact example you gave (inter = b.within(la.loc[0, 'geometry'])), I get 18 True values (from inter.sum()). However, this is actually equivalent to the spatial join, apart from that the spatial join does this automatically for all elements of the land use dataset.

Best Answer

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