OpenStreetMap and Ordnance Survey Master Map – Beyond good enough
15 July, 2009
In June, Aamer Ather, an M.Eng. student at the department, completed his research comparing OpenStreetMap (OSM) to Ordnance Survey Master Map Integrated Transport Layer (ITN). This was based on the previous piece of research in which another M.Eng. student, Naureen Zulfiqar, compared OSM to Meridian 2.
There are really surprising results. The analysis shows that when A-roads, B-roads and a motorway from ITN are compared to OSM data, the overlap can reach values that are over 95%. When the comparison with Master Map was completed, it became clear that OSM is of better quality than Meridian 2. It is also interesting to note that the results of higher overlap with ITN were achieved under stricter criteria for the buffering procedure that is used for comparison.
As noted, in the original analysis, Meridian 2 was used as the reference dataset, the ground truth. However, comparing Meridian 2 and OSM is not like with like, because OSM is not generalised and Meridian 2 is. The justification for treating Meridian 2 as the reference dataset was that the nodes are derived from high-accuracy datasets and it was expected that the 20 metres filter would not change positions significantly. It turns out that the generalisation impacts the quality of Meridian more than I anticipated. Yet, the advantage of Meridian 2 is that it allows comparisons for the whole of England, since the file size is still manageable, while the complexity of ITN would make an extensive comparison difficult, time-consuming and lengthy.
The results show that for the 4 Ordnance Survey London tiles that we’ve compared, the results put OSM only 10-30% from the ITN centre line. Rather impressive when you consider the knowledge, skills and backgrounds of the participants. My presentation from the State of the Map conference, below, provides more details of this analysis – and the excellent dissertation by Aamer Ather, which is the basis for this analysis, is available to download here.
The one caveat that will need to be explored in future projects is that the comparison in London means that OSM mappers had access to very high-resolution imagery from Yahoo! which have been georeferenced and rectified. Therefore, the high precision might be a result of tracing these images, and the question is what happens in places where high resolution images are not available. Thus, we need to test more tiles and in other places to validate the results in other areas of the UK.
Another student is currently comparing OSM to 1:10,000 map of Athens, so by the end of the summer I hope that it will be possible to estimate quality in other countries. The comparison to ITN in other areas of the UK will wait for a future student who will be interested in this topic!