[Talk-GB] building shapes from OS Street View

[Seventy 7]

 <blatant lie>Yeah, me too. I was just about to do that! 
</blatant lie>
Fantastic!

  ----- Original Message -----
  From: "Graham Jones"
  To: TimSC
  Cc: talk-gb at openstreetmap.org
  Subject: Re: [Talk-GB] building shapes from OS Street View
  Date: Fri, 9 Apr 2010 22:32:40 +0100

  Hi Tim,That is exactly the sort of thing I had envisaged writing -
  you have nearly finished before I got started - well done!
  Graham.

  On 9 April 2010 22:25, TimSC <mapping at sheerman-chase.org.uk> 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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  --
  Dr. Graham Jones
  Hartlepool, UK
  email: grahamjones139 at gmail.com

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