[Talk-GB] building shapes from OS Street View

TimSC mapping at sheerman-chase.org.uk
Fri Apr 9 22:25:46 BST 2010

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: 

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 :)


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