Using Grass to slightly smooth a polygon

Question

I have a geojson file with a lot of features. Many of them look like that:

What I'd like to get is a shape that very much resembles this one but is smoothed a little bit. Ideally, it would look similar to that one:

Do you have any ideas how to do this? I tried using GRASS in QGIS (generalize function) but I wasn't able to find parameters that would yield satisfactory results.

I used all of the algorithims available in GRASS v.generalize method using different parameters.

This is the GEOJson:

{
"type": "FeatureCollection",
"name": "shape",
"crs": { "type": "name", "properties": { "name": 
"urn:ogc:def:crs:OGC:1.3:CRS84" } },
"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": 
"Polygon", "coordinates": [ [ [ 70.297311693548977, 15.511449157884835 
], [ 70.297311693548977, 15.511457528420548 ], [ 70.297088479263266, 
15.511496590920549 ], [ 70.297094059620406, 15.511552394491977 ], [ 
70.297253099798979, 15.51153565342055 ], [ 70.297264260513259, 
15.511597037349121 ], [ 70.297311693548977, 15.511591456991978 ], [ 
70.297311693548977, 15.511588666813408 ], [ 70.297353546227555, 
15.511588666813408 ], [ 70.297353546227555, 15.511449157884835 ], [ 
70.297311693548977, 15.511449157884835 ] ] ] } }
]
}

The most sensible soultion out of those included in v.generalize was to use chaiken method with small "Maximal tolerance value" parameter, but it was also not very good:

The solutions listed there dint't yield a satisfactory result. — mihal277, Nov 15 '17 at 14:00
It would probably be helpful to show how you configured the tool, and explain why it wasn't satisfactory. — RoperMaps, Nov 15 '17 at 14:06
I fear what you want to do will be hard to set as it is. Try to think of a quantified criteria to erase a point and not another one. Smoothing generally gives you a rounded polygon. — gisnside, Nov 15 '17 at 16:08
map shaper.org is a site than import a GIS file, which then can be interactively generalized, then exported to a GIS file format of your choosing. — Tele-Pet, Nov 15 '17 at 16:20
Did you try a simplifying algorithm? Chaiken is a smoothing algorithm which is not the same. @lynxlynxlynx highlights the potential pitfall of simplifying which is thin polygons. — RoperMaps, Nov 15 '17 at 16:53
I may finally have found a solution I didnt think of initially with square buffers (and grass v.buffer). See my answer below. — gisnside, Nov 15 '17 at 17:00

score 1 · Answer 1 · answered Nov 15 '17 at 16:42

I think you'd have to code this yourself, but I fear there is no way to always do it accurately. The chosen parameter(s) will just be off for some cases.

Two approaches come to mind:

traversing the edges and deciding what to do at vertices depending on the running length and angle changes. Modify the geometry if they are small enough;
computing the distance of consecutive vertices and if it is small enough, merging one into the other. For your example, maximising area would work as a way to decide which point to remove. This one is vulnerable to polygons thinner than the threshold though!

gisnside · Answer 2 · 2017-11-15T17:20:12.980

You could try "square" buffering your layer by X distance (you'll have to try depending on your data) then by -X distance. It might be sufficient to remove little parts.

For that, you may be interested by the GRASS algorithm v.buffer with snap distance options activated. See Creating square buffer in qgis

I'll do some more tries to seek the best method to avoid round borders (the problem with my example below), but the main trick is to use a square buffer method.

Using Grass to slightly smooth a polygon

2 Answers2