clipdemqgisraster
I have performed a DEM extraction using the clipper tool in QGIS 1.9.0 Master, using a mask layer, the original resolution of the DEM is 20×20 but in the resultant DEM the new resolution is 19.98×19.98. Both in meters.
Is it normal? How can I prevent QGIS clipper tool to change the pixel size?
Note that clipper tool (from raster menu) distort your output, or may distort it by changing cell size. My suggestion is to use the clipper under gdal tools from the toolbox.
There define -tr resx, resy in the additional parameters window; where both resx & resy > 0. Those stand for the input raster resolution, and forces the output to keep it. You can get the resolution from the properties>metadata of your input raster. Note that it will be written like this: 30.0, -30.0; thus modify it to be 30.0, 30.0.
The image below sum it up with an example:
I have used the GDAL tools to rasterize a vector and providing the extension of my desired raster worked but if the data was very specific(i.e. lots of decimal points) I still didn't obtain the extension I needed. So I tried different things that worked in different ways:
a) I modified my rasters to the desired pixel size. If I created a raster from vector I followed this tutorial: http://www.mikemeredith.net/blog/1212_GIS_layer_for_Distance_from_in_QGIS.htm This indicates in the editable box where to provide the pixel size. It also indicates how to provide extension but because my extension was very "long" that didn't work for extension only for pixel size.
b) If you are using QGIS and are preparing your environmental layers for Maxent I suggest using the QSDM plugin and unify all your layers. All layers need to be in the same format (.tiff),same CRS and it helps if they have the same resolution. They will be given an equal extension as new layers that have been "unified". Even if they don't seem like they are the same, checking the metadata will confirm that they have been modified to have the same extension and resolution. Ready to be used in Maxent.
c) Additionally there is a nice tutorial here for Maxent AND QGIS, check out the: "Creating new rasters with GDAL tools" section. Where they cover re-sampling and this is great if you want to change pixel size for your environmental layers. http://clp-foss4g-workshop.readthedocs.org/en/latest/qgis_raster_resample.html
Hope it works and if there are other ways that worked for you I would be happy to know as sometimes for us beginners it is a lot of trial and error.
Best Answer
In newer QGIS versions (I tested it only in QGIS 2.6) you can set the resolution when you save a raster via "save as" (quite similiar with ArcMap)