extract-by-maskqgisrastershapefile
My study area is in black, and in grey the shape of Portugal.
I am creating my study area. It corresponds to the coastal area of Portugal. I have a shapefile for Portugal. I want to crop a raster that corresponds to my study area ( a rectangle), with the shape of Portugal, but what I need is the ocean, so the part that is outside of the mask, the complementary part.
How can I do that?
The Raster|Extraction|Clipper tool will help you to do this.
You can open the tool and then click and drag in the raster image to select the area you want to export as a new raster (Clipping mode: Extent), and then refine the exact coordinates in the Extent fields (if necessary).
Probably no way to do contours on only a selection of a raster; clip it this way first!
You say you have a shapefile with appropriate regions, so you can use that for cutting. In the parlance of QGIS, it sounds like you are after Vector->Geoprocessing Tools -> Clip.
If you need to assign the data from the input layer to smaller units in the region layer, you could intersect or union instead, in the same menu.
edit (by Kurt): here is an image for clarification:
yellow: input layer (china)
green: clipmask-layer
violett: clipped output-layer
Best Answer
(1) First, create a rectangle which encompasses your raster.
(1a) Start Extract layer extent tool (in
Layer toolsgroup).(1b) Select your raster as the Input Layer. Run the tool.
(2) You will get a
Extentlayer.(2a) Now start Difference tool (in
Vector overlaygroup).(2b) Select newly created
Extentas the Inpur Layer. Also select your coastal polygon layer as the Overlay Layer. Then run the tool.(3) You would see
Differencelayer (i.e. your ocean area).(3a) Start Clip raster by mask layer tool (in GDAL > Raster extraction).
(3b) Select your raster as the Input Layer, and Difference polygon as the Mask Layer. Click on the checkbox for
Create an output alpha band. Run the tool.(4) Hope your final output is something like below.