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4B_1_How many volunteers does it take to map an area well


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4B_1_How many volunteers does it take to map an area well

  1. 1. How Many Volunteers Does It Take To Map An Area Well? <br />Dr Muki Haklay <br />Department of Civil, Environmental and Geomatic Engineering, UCL <br /><br />AamerAther (M.Eng 2009), Sofia Basiouka (MSc GIS 2009) and NaureenZulfiqar (M.Eng 2008)<br />Ordnance Survey data was kindly provided by the Ordnance Survey research unit. <br />OSM data was provided by GeoFabrik & CloudMade<br />
  2. 2. Outline<br />A bit about quality of geographical information<br />Evaluation of OSM with Meridian data set<br />Evaluation of OSM with MasterMap<br />Linus’ low –more users: higher quality? <br />
  3. 3. The quality issue<br />How good it the data? <br />First question: good for what? Subjective quality – fitness for purpose/use<br />Second question: how to measure? Objective quality – but need to evaluate it in light of the first question<br />
  4. 4. The quality issue<br />How good it the data? <br />Positional accuracy – the position of features or geographic objects in either two or three dimensions<br />Temporal accuracy – how up to date is the data? Does it presents the existing situation and when will it be updated? <br />Thematic/attribute accuracy – for quantitative attributes (width) and qualitative attributes (geographic names)<br />Completeness – The presence and absence of objects in a dataset at a particular point in time<br />Logical consistency –adherence to the logical rules of the data structure, attribution and relationships <br />
  5. 5. The ‘problem’<br />“We know little about the people that collect it, their skills, knowledge or patterns of data collection”<br />“Loose coordination and no top-down quality assurance processes – can’t produce good data”<br />“It is not complete and comprehensive – there are white areas”<br />
  6. 6. Coverage and completeness <br />
  7. 7. Coverage and completeness <br />
  8. 8. Completeness – difference by user?<br />
  9. 9. Patterns of collaboration<br />
  10. 10.
  11. 11. Users<br />Limited ‘on the ground’ collaboration. Important as this can be the main source of quality assurance - ‘Given enough eyeballs, all bugs are shallow’ (Raymond, 2001) <br />Translate to VGI it might mean:“The more users there are per area, the better is the positional and attribute quality”<br />But does Linus’ law apply to OSM (and to VGI)?!?<br />
  12. 12. Accuracy and Completeness- Study I <br />Comparing OSM to OS Meridian 2 roads layer<br />Maridian 2 -Motorways, major and minor roads are... Complex junctions are collapsed to single nodes and multi-carriageways to single links... some minor roads and cul-de-sacs less than 200m are not represented... Private roads and tracks are not included...<br />Nodes are derived from 1:1,250-1:2,500 mapping, with 20m filter around centre line generalisation<br />
  13. 13. Positional Accuracy<br />Meridian 2 and OSM – Motorway comparison<br />
  14. 14. Goodchild and Hunter (1997), Hunter (1999) method<br />Assuming that one dataset is of higher quality<br />Create buffer around the dataset with known width <br />Calculate the percentage of the evaluated dataset that falls within the buffer<br />
  15. 15. Motorway comparison<br />Buffer of 20m<br />Average of 80% - ranging from 59.81% to 88.80%<br />
  16. 16. Comparison II – Ordnance Survey Master Map<br />Data used for comparison: OS MasterMap Integrated Transport Network (ITN) layer<br />ITN consists of road network information<br />The most accurate and up-to-date geographic reference for Great Britain’s road structure <br />Any major real world changes are updated within 6 months <br />Used for numerous applications<br />e.g. Transport management systems, road routing, emergency planning...<br />
  17. 17. Four test locations chosen:<br />TQ28se<br />TQ38se<br />TQ17ne<br />TQ37sw<br />
  18. 18. Buffer analysis – again based on Goodchild and Hunter (1997) buffer comparison technique:<br />Buffer width (X):<br />X<br />ITN<br />OSM<br />Comparison methodology <br />
  19. 19. Buffer overlap results:<br />109 roads examined covering over 328 km<br />Results of Master Map comparison <br />
  20. 20. TQ38se (East London)<br />TQ28se (North/Central London)<br />
  21. 21. TQ37sw (South London)<br />TQ17ne (West London)<br />
  22. 22.
  23. 23. Quality not linked to length <br />
  24. 24. Completeness – bulk method<br />Assumption: as Meridian 2 is generalised, so for each sq km:<br />If Total length(OSM roads)>Total length(Meridian 2 roads)<br />Than OSM is more complete than Meridian 2<br />The comparison can also includes attributes, by testing for the number of objects with complete set of values<br />
  25. 25. Methodology<br />1<br />3<br />2<br />4<br />5<br />
  26. 26. Change in completeness Mar 2008 – Mar 2010<br />
  27. 27. England – March 2008<br />
  28. 28. England – March 2009<br />
  29. 29. England – October 2009<br />
  30. 30. England – March 2010<br />
  31. 31. Completeness with attributes<br />The test for completeness with attributes checks that roads and streets names have been completed<br />Until the release of Ordnance Survey data in 1st April 2010, this was a good indication for ground survey of an area<br />
  32. 32. England – March 2008<br />
  33. 33. England – March 2009<br />
  34. 34. England – October 2009<br />
  35. 35. England – March 2010<br />
  36. 36.
  37. 37. Linus’ law and OSM <br />
  38. 38. Conclusions<br />OSM quality is high – and it is assumed that the quality is coming from aerial imagery <br />Linus’ Law does not seem to apply in a straight forward manner – at least not from 5 and above<br />More research is required for lower numbers or participants and different quality of imagery <br />
  39. 39. Further reading <br />Haklay, M., 2008, How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England, submitted to Environment and Planning B.<br />Haklay, M. And Weber, P., 2008, OpenStreetMap – User Generated Street Map, IEEE Pervasive Computing.<br />Haklay, M., Singleton, A., and Parker, C., 2008, Web mapping 2.0: the Neogeography of the Geoweb, Geography Compass<br />Haklay, M., 2008, Open Knowledge – learning from environmental information, presented at the Open Knowledge Conference (OKCon) 2008, London, 15 March. <br />Haklay, M., 2007, OSM and the public - what barriers need to be crossed?presented at State of the Map conference, Manchester, UK, 14-15 July.<br />To get a copy, write to , or get them on <br />