I have a raster layer with travel times from a specified point in my city's downtown. What I'd like to do is select all of the census blocks that lie at least partially within 45 minutes of downtown (i.e. I'd like to select all of the vector features that contain at least one raster pixel with a value of less than 45). I'd like to do this in QGIS if at all possible; suggestions for other FOSS software are also welcome.
- Use the raster calculator to filter your raster based on < 45. Result should be a new raster with 0 and 1, 1 is True.
- Convert the new raster mask to vector with GDALTools (Raster menu) Conversion -> Polygonize
- Intersect or Spatial Select based on intersection, or clip or extract the census layer based on your raster converted to polygon.
This is but one possible way to do such an analysis using QGIS. Spatial databases such as rasterlite or postgis is another option.