I am using QGIS 2.4 (Chugiak) via the kyngchaos build for OS-X. My question is–when using the Rasterize tool to convert a point vector layer to a raster and using the "Raster resolution in map units per pixel" option, how are "map units" defined?
So far as I can tell thus far, "map units" are decimal degrees regardless of other settings. The "Canvas units" setting under Project Properties –> General does not change this. Changing the coordinate system under Project Properties –> CRS does not change this. "Map units" are not dependent on the CRS of the input layer, nor on the CRS of the first layer added to the project. Is decimal degrees the only option and not modifiable in any way, or is there some other setting I have yet to find that defines "map units" in this context?
The reason I am curious is that if you have a point shapefile you wish to rasterize, and that shapefile uses a regular grid that defined in a projected coordinate system (e.g., a grid with points at 1000 m in NAD83 Z13) rather than a geographic coordinate system, there is no "Raster resolution in map units per pixel" that will be appropriate except over rather small areas if decimal degrees are the only map units that can be used. What you want to tell QGIS in this context is "rasterize this vector layer with a pixel size of 1000 m by 1000 m in NAD83 Z13" not "rasterize this vector layer with some pixel size of X by Y degrees". It seems there must be a way to get QGIS to do this, but I have yet to find it.
Best Answer
Map units are defined by the source layer's projection. You should check your original source file, confirm that it is actually in UTM (and showing projected coordinates in metres, as in the example below) and not simply set as UTM (which can be done falsely).
Add a point layer:
Confirm it's in UTM (also note the displayed coordinates in the above screenshot, '703656,9798666', which are clearly not degrees):
Rasterize it - in this case, looking for a 2500x2500m pixel:
Output grid is 2500m pixels: