With QGIS 2.18, the "rasterize" command offered the possibility of defining a raster target. I used this option to "update" a existing raster with the value of a polygon layer.
I'm not able anymore to find this possibility in QGIS 3.2. Any idea?
QGIS 3.2 does not allow you to use a raster file as a target. However, once you add the raster file as a layer in your project, it can be used to define the extent of the output (by selecting
Use layer/canvas extent). This applies also to QGIS 2.18 when Rasterize is activated from the processing toolbox.