Clipping Vector by Custom Raster Extent in QGIS?

Question

I have a raster image (elevation map from DEM) where I have limited the rendered elevation to only 3,500' and higher. I would like to convert the boundary/extent of this rendered image into a vector file, so that I can upload as a shapefile into other programs. I have not been able to do so yet after much Googling and experimentation.

Among other things, I have attempted to clip a large vector by the raster (I get an error saying "Could not load source layer for OVERLAY."). I have attempted to polygonize a two color raster (black and white) and keep only one of those. I believe they are all on the same CRS. I can convert it into a vector, but that converts all the contours, which is not my goal. I only want the boundaries.

Picture below of the raster extent (in white) that I want in vector form.

How do I clip out the black portion?

Answer 1

You're on the right track by polygonizing the raster. Currently you're trying to polygonize the elevation raster, and the tool is trying to create a separate polygon for every elevation value (ie a polygon for 3500', another polygon for 3501', etc.) You could select and merge all the polygons with values > 3500, but there's a much faster method. First create a raster with only values of 0 and 1 (0 where elevation < 3500, 1 where elevation > 3500). Then you can polygonize that raster, and it will have create a polygon of the area above 3500 feet.

1. Use the raster calculator with this expression (substitute the name of your raster band):

   "rastername@1" * ("rastername@1" > 3500)
   

2. Polygonize the output from step 1.

3. The output should be a polygon layer with two polygons. Select and delete the polygon with 0 value.