I needed to do some conditionals in the QGIS raster calculator, but it doesn’t support that—or at least doesn’t seem to. But it does support logical operators, with a result of either 0 or 1. For instance, here’s the script I wrote:

# Subtract them
((DavisQuad2012-02-25T16_00_00Z@1  -  DavisQuad2012-02-29T16_00_00Z@1)*
# Multiply by 1 if neither is 255 (NoData), 0 otherwise
(DavisQuad2012-02-25T16_00_00Z@1 != 255 AND DavisQuad2012-02-29T16_00_00Z@1 != 255))
# Subtract 32768 if either one was NoData, giving us -32768 for NoData.
-
(32768*(DavisQuad2012-02-25T16_00_00Z@1 = 255 OR DavisQuad2012-02-29T16_00_00Z@1 = 255))

Of course, you can’t actually put the comments in. But what it does is this: First, I subtract one raster from the other and multiply that by the logical operation that neither one contains NoData. That gives me the difference of the rasters, or 0 if either one contains NoData. Then I subtract 32768 multiplied by the inverse of the aforementioned logical operation, so any pixel with a NoData value in either of the original rasters is -32768 in the new one.

One of the more popular posts on this blog has been my piece on adding basemaps to QGIS. While the OpenLayer plugin is great, one of the things that I find dissatisfying is that it requires reprojecting your data to match the EPSG:3857 basemap. I often work in State Plane, and I’d just as soon have my data stay in that projection, which will also minimize local distortion. Well, as it turns out, one can add tiled map services as GDAL raster layers, with all the benefits that entails (e.g. reprojection). What you need to do is create an XML file like the following (which is lifted almost verbatim from the GDAL website, specifically this file):