How to fix issues with polygons, geometry validity
I am drawing polygons over a satellite image to classify forest vs non-forest. I have a raster file of polygons from the same image from 2014, and I am updating the borders of those polygons for the 2026 image. I'm completely new to QGIS, and I encountered a ton of issues last night after a couple hours of polygon editing/drawing.
It wouldn't let me save the layer because I had created a polygon with "unknown geometry" / a geometry type that couldn't be saved to the file. I was eventually able to find which polygon that was but I couldn't figure out how to fix it without having to redraw it entirely. Is there another way to do it without having to redraw them?
Often, I need to split polygons and create a new one in between. Either a) a hole inside the polygon, or b) completely dividing an existing "forest" polygon into 2 with a non-forest "corridor" between them. For a, I was using add ring, and then drawing a new polygon in that ring. I have snapping and topological editing on for reference. But that then resulted in all of those polygons being invalid and I wasn't sure why or how to fix them. For b) i haven't figured out a good way to do that yet without making everything invalid.
Often, when I am drawing new polygons it will then say "Add feature: The feature has been added, but at least one geometry intersected is invalid. These geometries must be manually repaired." I really don't know how to fix this or what it even means.
I tried to use Claude to figure these out + going on past threads but I couldn't figure anything out really. If anyone could help that would be much appreciated!