How to merge adjacent polygons with no common attributes in GRASS?

Question

Many topics are talking about merging polygons based on their common attributes, but few (or none) give a real answer to location-based merging problem.

Explanation.

Here is a fraction of the base map:

Some polygons are adjacents but belong to different entities. I want them to merge when they touch and stay as they are if they don't. Then, I want to extract the area of the new entities out of it.

Due to the large size of the maps, I'm working on Grass.

To solve the problem, I first made a tiny buffer:

grass_util.g('v.buffer', input='map', output='map_buffer', type=area distance=0.001)

My output is this one:

Everything looks pretty good, except that making a buffer brings the entities to merge into the one same entity (as displayed by the attr table that I can display here due to my small reputation). More precisely, the number of entities is almost the same, a bit less, the ones that didn't merge plus the new merged ones, but they are all affected "1" in the only category: "cat".

So we still have our different polygons, different entities, but they are merged.

Following this, when I try to add a new column and fill it with geometrics (my area in ha):

grass_util.g('v.db.addcolumn' map='map_buffer' col="area int") grass_util.g('v.to.db', map='map_buffer', option=area, column=area, unit=h)

It lamely fails ! The area isn't calculated for each entity and attributed to it. It is calculated for the whole package, and every entity gets the area of the whole.

I have to say that I don't really see how to get this thing sorted. I tried dissolve, v.clean, v.build, multiple to one and one to multiple, I tried to deconstruct the polygones to lines and to reconstruct them again.

An option which need the processing module that I don't have yet is to use this command:

processing.runalg('qgis:exportaddgeometrycolumns', input, calc_method, output)

which works when I try on a small area in Qgis Gui.

The question is twofold:

1. Find an other way to merge the adjacent polygons based on their location (to avoid the destructive buffer)
2. Find a way to get back my entities and retrieve their personal areas.

Answer 1

What you need to do is add a single category value to all polygons in a second layer, then run v.dissolve using that layer 2. This will remove all common boundaries. Here are the steps (say your original is named "polys"):

v.category polys opt=add layer=2 out=polys2 cat=1 step=0
v.dissolve input=polys2 layer=2 out=polys3
v.db.addtable polys3 column="area_sqm double precision"
v.to.db polys3 option=area units=meters column=area_sqm

The parameters "cat=1 step=0" insure that all categories in layer 2 are=1. Then v.dissolve works on all common boundaries.