[GIS] Combine multiple rows in attribute table into one

Question

I'm running into problems while trying to edit some data.

I'm trying to merge a number of land-use categories into less categories. So for example I have 5 types of "forest" which I want to merge into just one attribute.

All of this is in a single layer and thus in a single attribute table.

After I have my desired categories, I want to use another layer with the same categories to compare the difference over time.

Yet I can't figure out how to effectively combine the categories. Selecting by attribute and creating a new layer with that selection works, but it leaves me with a number of layers which still have different categories for the attributes when I only want one. For example, the layer "Forest" still has the separate polygons for different kinds of forests in the attribute table. I just want one big polygon for each category.

The image is an example of one of the layers, which is just arable land. I want to combine all of the polygons into one big one.

I also tried dissolving the separate layers I created by selecting by attribute, but on some layers it gives me the error:

Invalid Topology [incomplete void poly.]

Answer 1

Use the dissolve tool on your "Type" category. This will dissolve all polygons with like attributes into larger polygons