editingmergepolygonqgisqgis-2.6
I want to merge some polygons into one using the merge tool. But when doing so, one of the polygons just disappears (see screenshot 2). Why does this happen?
When I use the simplify tool before, it does not happen.
My system: QGIS 2.6.0; Linux Mint 17
Check the corresponding geoalgorithm in the Processing Toolbox (Ctrl+Shift+T).
If I understand your comments correctly, you have already created the the dissolved shape and merged it in with your other dataset, but are still seeing the other polygons that were merged together to create the large, dissolved polygon?
If so, (and assuming you still have the separate layer of the dissolved polygon) all you will need to do is select the polygons that are overlapped by the merged polygon and delete them. See Selecting features of vector layers based on another vector layer using QGIS? on performing a Select By Location in QGIS. I am also not very familiar with QGIS, but assume that there is an option to set about using the centroid of your broken out polygons as a selection parameter or selection option. Make sure that only those polygons that are actually covered by your new polygon are the only ones selected--there are selection options that will grab any polygon that touches the newly merged one.
since you are new to this and I am not sure that I 100% understand your problem, please, please, please make a backup copy of your shapefile or data table before deleting data
Best Answer
When specific shapes exhibit strange behavior as a result of a geoprocessing tool or editing operation, one of the first things to check is whether they have valid geometries. Geometry errors can result in null results or other errors. QGIS has a Check Geometry Validity tool, and another option would be to load it into GRASS and run v.clean. There are several tools available in other software as well.