Friday 14 January 2011

How good is MasterMap?

I've been reflecting on the buildings which have recently been demolished on the Edwards Lane Estate. Both Rosecroft Court and Hayward School still appear on the Ordnance Survey MasterMap available at Nottingham City Council's Nomad site. Now I don't know how frequently this data is refreshed, so it may be that the council only update occasionally.

I thought I'd look a bit closer to home, and selected this little bounding box shown above in aerial photos Bing from 2008 (slightly earlier aerial photos from Google here). I'd like to show the actual MasterMap data, but I suspect that posting on a blog is well outside "for private use".

Anyway within this bounding box I can find the following inaccuracies compared to what is on the ground, with a planning application reference when its been easy to find it quickly:
  1. Pavilion on playing field, Jubilee Campus. Originally built as changing rooms for Cottesmore School, for the past 10 years this was used as a storage facility by the Estates department of the university. Demolished in 2009.
  2. Cycleway on playing field, Jubilee Campus. Installed during 2009 to reflect the brutal reality that this was the route the vast majority of cyclists and pedestrians used, rather tahn the longer path to the west.
  3. Large garage, Lenton Lodge. Built around 1995-6 (95/00584/PFUL3).
  4. Former Police Station, Charnock Walk. Demolished sometime during the 1970s.
  5. Grass area in front of Lenton Lodge. Fenced in around 1977, when the Lodge ceased to be used as tied accomodation for workers in the council Parks department.
  6. Former Ladies toilet, including access, in south wing of Lenton Lodge. Closed in the '70s either on, or before, change of use for the lodge.
  7. 17, Wollaton Hall Drive, Extension built to rear (91/10013/PFUL3)
  8. 18, Wollaton Hall Drive. Extension built to the east of the house, perhaps 20 years ago.
  9. 20, Wollaton Hall Drive. Extension to rear.
  10. 22, Wollaton Hall Drive. Extension to rear. (92/01923/PFUL3)
  11. 49, Charnock Avenue, Large extension built to rear.
  12. 51, Charnock Avenue, Two extensions built, last in 2010.
Note that these are just the obvious ones culled from memory, with one or two checked against the planning applications site. If I went and specifically surveyed for them (and used Bing, the council site etc.,) I'd expect to double the size of this list.

There is a high degree of change, which is, no doubt, a reflection of the area.

In the past 15 years many houses in a whole swathe of the city have been bought by private landlords for renting to students. Larger houses are also attractive to extended families with couples of two or more generations living in the same property (both often identifiable by the presence of large numbers of cars, and most of the front garden given over to parking).

The same phenomenon of change is, however, true of the Dales Estate in Sherwood, which I surveyed earlier in the week. My impression was that this area is still owner-occupied by single-family units, but there are many obvious extensions, and quite a bit of building activity happening right now. In contrast, the houses on the Edwards Lane Estate (last week's survey target) mainly have their original appearance. Many are no doubt still social housing (plenty of Nottingham City Homes vans in evidence), but there were a few "For Sale" signs indicating some owner-occupation. It would be interesting to see if similar inaccuracies apply to these areas.

The Ordnance Survey makes much of the speed, frequency and precision of MasterMap updates. However this cursory look suggests that there side of the story is partial at least. I'm sure there is much I have missed, but hese are the implications which I can see initially:
  • By drawing building outlines, from up-to-date aerial imagery, OSM can offer more accurate information than the OS, although we will never achieve their precision. Note, how much the trade-off between information currency and precision is reflected here (maps "good enough" for their purpose).
  • Planning applicants are required to submit MasterMap output with their applications, and the derived fee income must form be a significant part of the OS revenue: I would guess a few million quid. The OS is a monopoly provider and has no incentive to keep this information up to date for its captive market.
  • Councils raise revenue through council tax on domestic properties. Inaccurate information on the size of properties could affect the accuracy of rating valuations.
  • Property size, gardens converted into parking areas, and so forth have other effects. The most obvious is increasing the direct run-off from rainfall. It is known that this has affected the hydrography of many parts of England (although changed farming practice may be much more significant), with consequences for storm drainage and flood defence. Again, failure to track change may result in under-estimation of any ensuing problems.

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