[Qgis-user] Raster calculator output layer is smaller than expected

[Nicolas Cadieux]

Hi, 
You have lots of steps I do not fully understand (I am on vacation after all...).  
This is what I would do and I figure these steps will help you. 
-make sure rasters have the same CRS, resolution and the same "nodata" values. If you multiply rasters, the no data value on one layer will cancel out value on other layers.  If needed, you can remove the nodata value with gdal_warp. 
-If you use the calculator, I would carefully select the output file size. 
-You can also use the merge function "raster\misc\merge".  If the .tiff is over 4GB, use the -bigtiff switch if you have problems.  
-You can also merge by creating a virtual raster and then by loading the virtual rasters and then saving it as a tiff.  Both options will  give you better control on the end result.  Always specify the bigtiff option if the file stops growing at around 4GB. 
-If you clip a raster in the middle of a pixel, you will alter it's size and definition. I usually make a vector grid to make sure I don't split pixels in half. 
Hope this helps! 
Cheers 
Nicolas Cadieux M.Sc. 
Les Entreprises Archéotec inc.  
8548, rue Saint-Denis Montréal H2P 2H2 
Téléphone: 514.381.5112  Fax: 514.381.4995 
www.archeotec.ca 
On Jul 5, 2015 5:15 AM, "GOODFELLOW A. [via OSGeo.org]" <ml-node+s1560n5214332h53 at n6.nabble.com> wrote: 

	








Hi all. 
I'm using QGIS 2.6 on a Mac 10.7.5 system. I'm attempting to create a cost surface from several raster layers. The raster layers themselves have been created from a transect across a series of vector shapefiles of a larger geographical extent. 


I used Raster Calculator to add the different layers. The output appeared to successfully combine all the layers, but covered only about 1/20 of the transect area I need. 


I tried the process again, manually checking the Min/Max values for X and Y, and got the same result. 

Because not all the features in the shapefiles are distributed evenly within the transect I've clipped and rasterised, the resulting rasters are approximately, but not exactly the same size. The pixel resolutions of the raster layers are also different - most
 are 3000x30000, one is 5000x50000, and one is 8000x80000. I'm not sure if either of these would cause the issue, as the output isn't distorted, it's just smaller than I require. 

Many thanks for any assistance. 
Adam 





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