arcgis-desktoparcmapfields-attributesmergeshapefile
I'm trying to merge all the features in a shapefile that have the same value under a certain field in the attribute table. Right now, I open the attribute table and use "Select by Attributes" to select all the features of a certain value, merge those, then move on to the next value.
Is there a way to do this automatically?
I had the same issue dossolving polygons, the problem was that there were Geometry Errors. Bare in mind my process was a Standalone application, however the concept should help. Before performing the merge I ran
validate= processing.runalg("grass7:v.clean",CleanedOutput,0,0.1,extents,-1,0.0001,InFeature,errorsOutput)
This tool is simply a GRASS tool called v.clean, you can search for it from your processing toolbox. This tool cleans the geometry of the input vector layer, read about this more HERE
If you don't trust the v.clean to solv the problems, You can also use the Check Geometry Validity from fTools plugin, to inspect the file manually, this tool will highlight all geometry errors, enter edit mode to solve the errors by deleting duplicate Nodes, Polygon overlaps etc.
Once the Geometry errors are dealt with the files should merge, dissolve, intersect, without losing features.
Another method I used was the GDAL tools ogr2ogr
To create a file to merge into use the following command. This will take a shapefile and copy it to a file called merge.shp (this filename is up to you).
ogr2ogr -f ‘ESRI Shapefile’ merge.shp filename1.shp
Then merge the following files by using:
ogr2ogr -f ‘ESRI Shapefile’ -update -append merge.shp filename2.shp -nln merge
ogr2ogr -f ‘ESRI Shapefile’ -update -append merge.shp filename3.shp -nln merge
*Note you need the OSGeo4W shell provided with the For Advanced Users Download * This should help defeat the geometry errors Hopefully this solves your issue, I remember how frustrating it was with me.
If I understand your question right you can use the 'Dissolve Tool' in 'Geoporcessing Tools' in QGIS.
"Bayelsa Wards" would be your 'Input vector layer' and "Ward code" the 'Dissolve field'.
Best Answer
You can use Dissolve tool selecting the attribute that you want to merge.