QGIS – Converting Raster to Vector (Polygons) with Generalisation

lidarpolygonizeqgisraster

I have created rasters showing different types of vegetation (bushes, trees, open land) with help from lidar data. Now I'm looking for a method to turn these images into polygons.

I'm using a Mac and preferably QGIS, GDAL, GRASS etc. I do have the possibility to use Windows but I'd prefer staying in Mac OS X.

Here is the source image:

Blue (with some red) means trees and red is bushes. White is open land.

And this image (drawn by hand) is kind of what I'd like to achieve:
target image

Any ideas?

I've tried r.neighbors some but I'm not satisfied.

Update: I have now tried Majority and it looks better. But the narrow part in the south east part is not being included. I guess those dots are not in "majority". How could I make them being included? It's like if I need minority but not extreme minority…

See image below (note that I have only included bushes, the red color from the original image):

majority

Best Answer

You can use a Majority Filter followed by Polygonize for this type of operation. QGIS has a majority filter in the SAGA toolbox. Polygonize is found in the Raster tab > Conversion > Polygonize (Raster to Vector...). For more cartographically pleasant vector features, use Simplify Geometry to smooth the rough vector edges: Vector > Geometry Tools > Simplify Geometries...

enter image description here

Another option is to use a Guassian Filter in the SAGA toolbox and then threshold that output using the Raster Calculator. In this second example, I used a double pass circular Guassian Filter with a stdev of 3 and a search radius of 30. I then thresholded the resulting image using the following syntax in the raster calculator:

"Filtered Grid@1" > 0.7

The resulting output is overlaid in transparent blue on the original:

enter image description here

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For GRASS GIS Zonal statistics, see http://grasswiki.osgeo.org/wiki/Zonal_statistics

It looks as if you want to use "v.vect.stats": count points in areas, calculate statistics from point attributes (needs GRASS 7).

QGIS – Remove Islands and Completely Surrounded Polygons After Polygonization

You can use for example this python script to highlight or delete the features that are completely within another feature.

layer = iface.activeLayer()  #or the layer you want to use
features_a = layer.getFeatures()
features_b = [feat for feat in layer.getFeatures()]
within_list = list()
for feat_a in features_a:
    for feat_b in features_b:
        if feat_b.id() != feat_a.id():
            if feat_b.geometry().within(feat_a.geometry()):
                within_list.append(feat_b.id())

To highlight the features you can use:

layer.setSelectedFeatures(within_list)

And to delete the highlighted features use:

caps = layer.dataProvider().capabilities()
if caps & QgsVectorDataProvider.DeleteFeatures:
    res = layer.dataProvider().deleteFeatures(within_list)

As a test result I get for example:

But before you execute this script please save your qgis file because if you want to process a large file this script probably crashes your qgis.

Best Answer

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