Assigning land cover type (in shapefile) to hydrologic soil shapefile in QGIS?

Question

I have a land cover and a hydrologic soil group shapefiles which cover my watershed. They have different polygons. For example, a forest type land cover (one polygon in land cover shapefile) may correspond to three different soil types (three polygons in soil type shapefile). What make things worse is these three polygons are having other land cover types for the areas outside the boundary of the forest type land polygon, which I need to assign these areas to another land cover type. So I need to:

1. create a new shapefile which is the combination of the two shapefiles I mentioned above. A bigger polygon in one shapefile will need to be split to smaller ones by the polygons in another file.
2. I need to assign the soil type and land cover attributes to the new features in the new shapefile accordingly.

Answer 1

I think I understand what you are looking for. You are needing to assign the Soil Type to a Specific Landcover, within a larger watershed.

This is a pretty straight forward task using the "Intersect" tool in QGIS.

Step 1: Get both your Soils and Landcover maps into the QGIS Map window. Image 1 below is the soils layer (its all one color because I havent figured out how to classify the symbology in QGIS yet). The soil types are A,B and C.

The following image is the Landcover on top of the Soils:

As you can see, the forest area covers all three soil types and the urban is just in Soil Type A.

Step 2: Run the intersection tool. It is located in QGIS under the "Vector" window -> "Geoprocessing Tools" -> "Intersection".

Step 3: Choose the Soils and Landcover as the two input datasets, and if you want choose a location to save this information. Run the tool.

Step 4: The result will a new polygon, showing only those areas where an intersection has occurred. See in Purple below:

The thing to note here is the attribute table. The Polygons now have attributes stating the Name_LC (Landcover) and the associated soil (Name). You can now determine what Landcover Type is underlain by which soil type.

This is a very rough example, one would expect your watershed would have its entirety covered with both soils and landcover. Using this method you can determine the Soils / Landcover combinations.