Calculating total area overlapping specific raster DEM color interval whose boundaries are confined by another vector?

Question

I know this sounds quite difficult, but maybe it isn't for you or at least it shouldn't if I explain it correctly.

(I'm using QGIS 2.14) I'd like to know to calculate total area overlapping a specific raster DEM color (user-specified altitude interval) whose boundaries are confined by another vector.

So basically there's a raster DEM as background map, I assigned a specific color to an altitude interval. I applied a vector I created to show where boundaries of a certain area area. Now within this area, I applied another raster which partly overlaps the altitude interval I applied in the background DEM.

I'd like to know what is the area, within the vector boundaries, that overlaps my altitude interval in the DEM map.

EDIT: I tried running raster calculator on intersections then zonal statistics checking "count" and "sum" but I get a huge table of 8000+ rows and two columns, however each cell usually have a value around 0.4 or 1.x, so I don't know how to interpret this or if I should somehow sum all those cells together to get the number of pixels falling in my area of interest (overlapping altitude range, shaded and within vector bounday).

Answer 1

To put things clear, I assume that you want to compute the area which responds to the following conditions:

• located inside your DEM altitude interval
• located inside your vector boundaries
• located where your second raster (let's call it raster2) has values other than "nodata" (this is the unclear part of your post, feel free to correct me if I misunderstood).

There are several ways to perform this; here's what I think is the most simple approach (let's call z1-z2 your altitude interval):

1. first, your data need to have an equal-area projection if you want to compute meaningful areas. So, if it's not already the case, choose a projection that fits the region you're mapping, then reproject your raster and vector layers to it (using "Save As" for vector layers and "Raster > Projections > Warp" for rasters).

2. use the raster calculator to create a mask representing the parts of raster2 located inside your DEM interval. The formula will be something like ("raster2@1" != -9999) * ("dem@1" >= z1) * ("dem@1" <= z2). You should get a raster with 1s in your altitude interval and 0s or "nodata" everywhere else. (Note: the"raster2@1" != -9999 part should be fine, but depending on raster2's values you might have to adjust it).

3. install the Zonal Statistics plugin. This plugin computes statistics on a raster, within polygons. Once installed, it will be located in the "raster" menu. Choose the previously created mask as "raster layer", and the vector layer which contains your boundaries as "Polygon layer". If this vector layer contains a lot of polygons, you might prefer to export the polygon representing your boundaries as a separate vector layer (it will be less time consuming). If everything goes well, you'll get additional fields in your vector layer; among them will be a "count" column which represents the number of mask pixels inside every polygon from the vector layer. You just have to multiply the number in this column by the area of a pixel (cellsize) - which depends on the projection - to get your final area.