[Talk-GB] building shapes from OS Street View

TimSC mapping at sheerman-chase.org.uk
Tue Apr 6 12:46:23 BST 2010

Hi again,

Thanks for the feedback on building traces. The consensus seems to be 
for a JOSM plugin while others saying all surveying should be done on 
the ground. Surveys are only practical where there is permissive or 
public access to buildings, but the majority of buildings are 
inaccessible to the public. Therefore, we need a source besides us doing 
the survey, assuming we want building outlines at all. I think building 
outlines would be useful for navigation, planning, analysis and many 
other uses of the map data.

In response to the comment "no imports ever", I would point out that 
building imports is a completely different situation than that of public 
roads. A glance at OS Street View suggests that about 99% of buildings 
are not publically accessible. If we use only manual surveying, we can 
only achieve coverage of about 1%. I don't think that is satisfactory. 
Imports are therefore very much appropriate for buildings.

The point that buildings may need local knowledge for a high quality 
import is a better point against a mass import. But again, local 
knowledge only really applies to the minority of buildings that are 
accessible. The vast majority of the data can't be improved by local 
knowledge. I suggest the minority of buildings that can be improved can 
be done once they have been imported into the OSM database.

Regarding the technical difficulty, it remains to be seen if automatic 
tracing is possible and of sufficient quality. But it is presumptuous to 
assume it is impossible at this stage.

I observed there are slight artifacts in the rectified tiles with are 
not present in the original set (link below). Notice the "a" in stag is 
distorted and also some of the letters in "Court". An attempt to 
automatically trace those tiles would end up with noticeable glitches. 
Until that stuff gets ironed out, a JOSM plugin, which would presumably 
grab these tiles via WMS, would have poor automatic tracing performance.

Also the rectified tiles seem to be not the highest zoom level 
available? Having the highest resolution makes automatic tracing much 
more accurate. There are various service roads that are on the OS 
opendata site (search for SU986502) that are not in the rectified tiles:

One option is to automatically trace objects from the original images 
and then transform the polygons into WGS84. The main thing I am missing 
is an practical (and open) OSTN02 implementation (in python). I will 
investigate this and perhaps trial it in my local area.

One question, is this data set going to be maintained by OS in the 
future? And don't worry, I won't be doing a mass import without a great 
deal of work and discussion.


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