polygonqgisraster
I have OS street view mapping as raster images. Is it possible (without the obvious of tracing each dwelling) to identify all polygons on each tile.
To clarify,
My ultimate aim is to produce a shapefile layer of polygons for each component….particularly dwellings in order to create a 260m buffer around each which is easy done.
Using QGIS
You could convert the raster to points (Conversion Tools\From Raster) and join that to the polygon layer using a spatial join (layer's context menu\Joins and Relates\Join...\Join data from another layer based on spatial location). That gives the option to collect some statistics, as well as provides a field with a count.
The sticky point is that assumes the centroid for the cell equates to the majority of the cell, which might not always be true. In other words, a point could fall within a polygon, but the majority of the actual cell resides in a different polygon.
Depending on your needs, that may not be significant.
You can create square and rectangular grids using the Vector Grid tool under Vector > Research Tools > Vector Grid.
To get the required coordinates, I suggest using the Coordinate Capture tool and then input the Xmin, Xmax, Ymin, Ymax from these captured points. Note that the units will be in the coordinate system currently used, so you might want to reproject your raster into a coordinate system with Meters. To create a rectangular grid, simply unlock the 1:1 ratio on the tool menu. You can create the grids as both polygons and polylines
Here's an example of the Vector Grid parameters:
and the output:
Best Answer
It is possible but the polygons are a little ragged, see the attached picture. Street View images are (single-band) indexed TIFFs. Using the raster value tool told me that the pixel value for buildings was 204 and I used this value in the Raster Calculator to make a new TIFF in which all of the pixels in the image having a value of 204 were converted to ones and all of the pixels in the image having a value not equal to 204 were converted to zeros. The expression to do this is 'filename@1 = 204' (without the quotes).
It was then simply a matter of polygonising the new TIF: Raster -> Conversion -> Polygonize (Raster to vector).
In the picture I've overlaid the original image with just the 'buildings' polygons.
Low-down dirty stuff really.
Nick.