google maps - Quality of free geodata compared to commercial sources

What are the sources of excellent geo-location data as possessed by Google Maps ?

I am wondering that, even with large amount of volunteer work involved, why is OSM data not comparable to theirs in terms of quality ?

Answer

See this paper (draft version):

Haklay, M. (2010), How good is volunteered geographical information? A comparative study of OpenStreetMap and Ordnance Survey datasets. Environment and Planning B: Planning and Design 37(4) 682 – 703.

for more rigorous assessment (in UK context).

This one for assessment (in comparison wth Google Maps & Bing Maps) in Ireland:

Ciepłuch , B., Jacob, R.,Mooney, P., Winstanley, A. (2010) Comparison of the accuracy of OpenStreetMap for Ireland with Google Maps and Bing Maps. Proceedings of the Ninth International Symposium on Spatial Accuracy Assessment in Natural Resuorces and Enviromental Sciences 20-23rd July 2010 . p. 337.

And this for assessment in France (paywall):

Girres, J.-F. & Touya, G. (2010) Quality Assessment of the French OpenStreetMap Dataset. Transactions in GIS, 14, 435-459.

Germany is covered in this paper:

Zielstra, D., Zipf, A. (2010) Quantitative Studies on the Data Quality of OpenStreetMap in Germany. Paper presented at GIScience 2010.

Very thorough comparison with TomTom data (in Germany as well) is covered here:

Helbich, M., Amelunxen, C., Neis, P. (2012): Comparative Spatial Analysis of Positional Accuracy of OpenStreetMap and Proprietary Geodata. Int. GI_Forum 2012. Salzburg. Austria.

German street network is also discussed in detail here:

Neis, P.; Zielstra, D.; Zipf, A. (2012) The Street Network Evolution of Crowdsourced Maps: OpenStreetMap in Germany 2007–2011. Future Internet 4, 1-21.

There also two Master's dissertations dealing with this topic: one from UCL:

Kounadi, O. (2009) Assessing the quality of OpenStreetMap data.

and one presented at GI_Forum:

Stark, H-J. (2010) Quality assessment of crowdsourced geocoded address-data within OpenAddresses

Also article from Cartographica (paywall) might be of interest:

Mondzech, J. & Sester, M., (2011) Quality Analysis of OpenStreetMap Data Based on Application Needs. Cartographica: The International Journal for Geographic Information and Geovisualization, 46, 115-125.