[Talk-GB] building shapes from OS Street View

Robert Scott lists at humanleg.org.uk
Fri Apr 9 23:49:19 BST 2010


That's great! I can stop reading about Harris operators. I totally agree about orthogonal snapping.

How well would this scale up to the whole country? (!! Not automatically importing the results of course !!) I'm thinking about tile/batch sizes, tile boundary issues, any necessity for porting parts to c++ for speed etc.


On Friday 09 April 2010, TimSC wrote:
> Hi again,
> I have been working on auto tracing buildings and I'm making progress. I 
> was slightly encouraged by Ed Avis's comments. I think one underlying 
> difference is peoples attitude to omissions in map data. Many people 
> think they are a good thing, particularly since they encourage the 
> community to do high quality surveying. But I think omissions are bad in 
> terms of actually using the map. I don't think we should be using the 
> main map to gauge our progress. I suspect what we need is good meta data 
> - how and when data is sourced. Anyway enough rambling...
> Tracing buildings. I have been using the original images, since image 
> transformations tend to introduce degradation of quality. I use colour 
> to select building pixels, then form edge fragments, then form polygons, 
> then simplify the polygons using the Douglas-Peucker algorithm, then 
> group them so we get inner and outer edges, then tranform image 
> coordinates to GBOS then to WGS84 via OSTN02 (I ported the perl code to 
> python), then save as OSM format and load back into JOSM. Screenshot: 
> http://timsc.dev.openstreetmap.org/dev/josm-building-outlines.png
> The next steps are to improve the quality of the polygon shapes, 
> possibly by checking if the edges are nearly orthogonal, and if so 
> making them completely orthogonal. Also I need to write a filter to 
> check for buildings in the area, to avoid importing duplicate buildings. 
> I need to look at the simplification, as sometimes an extra node is 
> added to a polygon (the initial node used as the start of the 
> algorithm). I am also considering detecting roads that overlap buildings 
> in the source images, since this is probably the biggest loss of 
> quality. The result I am getting is already more spatially detailed than 
> my own survey of the University of Surrey campus (although not as rich 
> in information).
> In the medium term, I will import some buildings once I have the quality 
> I want. I want to minimise manual work in JOSM but I don't rule it out. 
> I will only be working in the Guildford area - it's my data to gamble 
> around there :)
> TimSC
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