How Many Volunteers Does It Take To Map An Area Well?

1. How Many Volunteers Does It Take To Map An Area Well? Dr Muki Haklay Department of Civil, Environmental and Geomatic Engineering, UCL m.haklay@ucl.ac.uk AamerAther (M.Eng 2009), Sofia Basiouka (MSc GIS 2009) and NaureenZulfiqar (M.Eng 2008) Ordnance Survey data was kindly provided by the Ordnance Survey research unit. OSM data was provided by GeoFabrik & CloudMade

2. Outline • A bit about quality of geographical information • Evaluation of OSM with Meridian data set • Evaluation of OSM with MasterMap • Linus’ low –more users: higher quality?

3. The quality issue • How good it the data? • First question: good for what? Subjective quality – fitness for purpose/use • Second question: how to measure? Objective quality – but need to evaluate it in light of the first question

4. The quality issue • How good it the data? • Positional accuracy – the position of features or geographic objects in either two or three dimensions • Temporal accuracy – how up to date is the data? Does it presents the existing situation and when will it be updated? • Thematic/attribute accuracy – for quantitative attributes (width) and qualitative attributes (geographic names) • Completeness – The presence and absence of objects in a dataset at a particular point in time • Logical consistency –adherence to the logical rules of the data structure, attribution and relationships

5. The ‘problem’ “We know little about the people that collect it, their skills, knowledge or patterns of data collection” “Loose coordination and no top-down quality assurance processes – can’t produce good data” “It is not complete and comprehensive – there are white areas”

6. Coverage and completeness

7. Coverage and completeness

8. Completeness – difference by user?

9. Patterns of collaboration

11. Users • 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) • Translate to VGI it might mean:“The more users there are per area, the better is the positional and attribute quality” • But does Linus’ law apply to OSM (and to VGI)?!?

12. Accuracy and Completeness- Study I • Comparing OSM to OS Meridian 2 roads layer • 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... • Nodes are derived from 1:1,250-1:2,500 mapping, with 20m filter around centre line generalisation

13. Positional Accuracy • Meridian 2 and OSM – Motorway comparison

14. Goodchild and Hunter (1997), Hunter (1999) method • Assuming that one dataset is of higher quality • Create buffer around the dataset with known width • Calculate the percentage of the evaluated dataset that falls within the buffer

15. Motorway comparison • Buffer of 20m • Average of 80% - ranging from 59.81% to 88.80%

16. Comparison II – Ordnance Survey Master Map • Data used for comparison: OS MasterMap Integrated Transport Network (ITN) layer • ITN consists of road network information • The most accurate and up-to-date geographic reference for Great Britain’s road structure • Any major real world changes are updated within 6 months • Used for numerous applications • e.g. Transport management systems, road routing, emergency planning...

17. Four test locations chosen: TQ28se TQ38se TQ17ne TQ37sw

18. Comparison methodology • Buffer analysis – again based on Goodchild and Hunter (1997) buffer comparison technique: Buffer width (X): X OSM ITN

19. Results of Master Map comparison • Buffer overlap results: • 109 roads examined covering over 328 km

20. TQ38se (East London) TQ28se (North/Central London)

21. TQ37sw (South London) TQ17ne (West London)

23. Quality not linked to length

24. Completeness – bulk method • Assumption: as Meridian 2 is generalised, so for each sq km: If Total length(OSM roads)>Total length(Meridian 2 roads) Than OSM is more complete than Meridian 2 • The comparison can also includes attributes, by testing for the number of objects with complete set of values

25. Methodology 1 3 2 4 5

26. Change in completeness Mar 2008 – Mar 2010

27. England – March 2008

28. England – March 2009

29. England – October 2009

30. England – March 2010

31. Completeness with attributes • The test for completeness with attributes checks that roads and streets names have been completed • Until the release of Ordnance Survey data in 1st April 2010, this was a good indication for ground survey of an area

32. England – March 2008

33. England – March 2009

34. England – October 2009

35. England – March 2010

37. Linus’ law and OSM

38. Conclusions • OSM quality is high – and it is assumed that the quality is coming from aerial imagery • Linus’ Law does not seem to apply in a straight forward manner – at least not from 5 and above • More research is required for lower numbers or participants and different quality of imagery

39. Further reading • 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. • Haklay, M. And Weber, P., 2008, OpenStreetMap – User Generated Street Map, IEEE Pervasive Computing. • Haklay, M., Singleton, A., and Parker, C., 2008, Web mapping 2.0: the Neogeography of the Geoweb, Geography Compass • Haklay, M., 2008, Open Knowledge – learning from environmental information, presented at the Open Knowledge Conference (OKCon) 2008, London, 15 March. • 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. • To get a copy, write to m.haklay@ucl.ac.uk , or get them on povesham.wordpress.com