[GIS] Getting adjacent polygons to merge in QGIS

mergeqgis

I was given a shapefile made up of a few thousand squares. The data in the squares has 5 different values. I wanted to merge all the common values into the same polygon to simplify things. This is what it looked like.

I used the the Select by Expression tool to select all the polygons of the same values and then used the Merge Selected Features tool to merge the polygons together. I was hoping I would be left with 5 geometries that held each value. However after the merge I had vertical stripes through each of the zones as shown here.

I suspect that I'm looking at polygons that have points that are not exactly at the same coordinates. I also suspect that it is a problem created by the software program that generated the shapefile. I was able to confirm this by using the Snap tool to move the points and snap it back over the other one and then I could merge the polygons together. I don't have the time nor the patience to do that for every point on the map.

I've tried to use the GRASS clean tools to get rid of these points as well as a few other tools I could find and have searched the forum for solutions, but after an afternoon of not getting anywhere, I've decided to see if anyone could help me figure this out. I've provided the (Shapefile Data) here if someone wants to try to do it.

How do I clean this up?

I'm running 2.18.14.

Best Answer

Here is the workflow on how to solve this problem using your data:

Download Multipart Split from the plugin manager
Start editing your polygon layer -> Select all -> Use Multipart Split tool from step 1 to explode the polygons into separate polygons. (# Singlepart to Multiparts tool did not work)
Agree with @Matte solution, you need to use Snap points to grid from QGIS Geoalgorithms -> Vector general tool -> Snap points to grid with Horizontal and vertical spacing of 0.000001 (the minimum you can).
Use Dissolve tool to merge all the polygon together based on field attribute QGIS Geoalgorithms -> Vector general tool -> Dissolve, and uncheck the option (Dissolve all (do not use field))

Here is the result of step 2:

Result of step 4:

However, there are some missing tinny polygons, and I think this is due to the CRS used. You are using geographic (unprojected) data, and working with unprojected data for cleaning polygons may produce undesired outputs.

Related Solutions

[GIS] Merging large number of polygons using QGIS

Using QGIS, apart from the Vector -> Geprocessing Tools - Dissolve Menu entry, you can also use Processing (formerly named Sextante), which offers dissolving by GRASS and SAGA.

[GIS] pyQGIS Merge raster layers

You don't need processing to execute gdal_tools you can use subprocess module to execute your command. Even Processing use the subprocess so why don't you ? here a function to use it :

import sys
from subprocess import Popen, PIPE, STDOUT
# Create a class herit from Exception class
class ProcessException(Exception):
    pass
# function to run the command
def execute(command):

    process = Popen(command.split(" "), shell=True, stdout=PIPE, stderr=STDOUT, stdin=PIPE)
    # Poll process for new output until finished
    while True:
        nextline = process.stdout.readline()
        if nextline == '' and process.poll() is not None:
            break
        # You can add here some progress infos

    output = process.communicate()[0]
    exitCode = process.returncode

    if (exitCode == 0):
        return output
    else:
        raise ProcessException(command, exitCode, output)

To use it :

try:
    execute(u"gdal_merge.py -n 0 -a_nodata 0 -of GTiff -o /path/of/your/output/mozaik.tif /path/of/your/input/terrain_corrected01.tif /path/of/your/input/terrain_corrected02.tif")
except ProcessException as p:
    print(u"something goes wrong : {}".format(p))

--EDIT-- Where is those command

Gdal comes with QGIS as you install QGIS. In Windows they are in the :

C:\path\of\OSGEO4W\bin\

In linux it's in the :

/usr/bin/
# for the manual
/usr/share/man/manl/gdal_merge.l.gz

In my opinion if you don't need the QGIS API it will be faster and easy to use it in a bash script. But it depends on your need.

Best Answer

Related Solutions

[GIS] Merging large number of polygons using QGIS

[GIS] pyQGIS Merge raster layers

Related Question