The recent flooding in Britain has led to a degree of questioning about how we should hold such data, but it has also reawakened an interest of mine in seeing if OpenStreetMap data can be used for simple modelling of hydrological systems.
First I discuss transient data and look at a different aspect of flooding, the availability of suitable data, in the second part of this post.
|Flooding in the Somerset Levels mapped on OSM |
using key natural=water
copyright OSM contributors, CC-BY-SA
|Typical works on NET Phase 2 creating road closures|
© Copyright David Lally and licensed for reuse under this Creative Commons Licence.
The fundamental gotcha is not that we cant add this type of data quickly, but that it is really difficult to remove it once it is no longer current. For a typical closed footbridge, or lengthy roadworks, it means checking the status on a regular basis. For collapsed buildings in disaster zones there is no equivalent of the immediate post disaster aerial imagery and usually hardly anyone on the ground. Even for something as close to home as the Somerset Levels, as the flood waters recede someone ought to be mapping the change, but in practice I doubt if anyone is doing this.
Even when some transient activity has a known predicted time interval we have no way of reversing the original edit at some time in the future. (In practice, most of these predicted times have a habit of being estimated much too conservatively).
So how can we use OpenStreetMap intelligently to add transient data in such a way that it does not interfere with the standard persistent data, but can readily be integrated with it?
Typically transient data is added by one of the following means:
- Extra tags, mostly adding an access restriction to highways affected by closures, or using a tag specific to a particular event (for instance damaged buildings after an earthquake).
- Additional nodes and ways (areas of flooding, additional barriers etc).
The scheme I envisage is to use a relation to link together the persistent data with transient data. The relation would contain information about the event and when it should be automatically terminated, and its members would be persistent and transient OSM objects denoted as such with roles. The transient OSM objects should be also tagged as such as well. In many cases this is all that is required as searching for transient relations and their transient members would all that any bot would need to do. For cases such as closure of bridges data consumers might have to merge the tags of the persistent and transient members of the relation to use the data accurately.
- Somerset Levels flooding. A single relation with several temporary ways as transient members each tagged natural=water.
- Burning Man (and other festivals). Again a single relation with many temporary ways as transient members. Some highway access may require transient ways associated with permanent ways, for instance when a private road becomes a oneway service road providing access for parking.
- Closure of a footbridge. The relation has a pair of members, the permanent footbridge and an additional way with the extra tags describing the temporary state of affairs.
These are fairly crude thoughts, but probably more practical than the long term solution which we want from Open Historical Map.
Now onto flooding.
Hydrological Data and OpenStreetMapThe floods in Southern England have received a huge amount of media coverage and consequently government attention. From a plain geo-viewpoint this reached a pitch with a heavily attended and publicised FloodHack event at Google's London City campus the weekend before last. I share a certain scepticism about the value of such events, but it did have one very powerful effect. It brought home at a senior level in the government why things like flood data need to be much more open than they are now. See Owen Boswarva on this, and Charles Arthur's piece in the Guardian today.
|Flooding Attenborough Village 2000|
Environment Agency All Rights Reserved.
River Erewash, which was at least couple of metres higher than the workings at the time. One effect is that in times of flood the Erewash catchment waters enter the Trent further downstream and much faster than they used to. Also the Erewash catchment is full of former coal mine workings and I believe is rather more polluted than similar tributaries of the Trent.
|Sketch of changes in water flows from Erewash into the Trent following extension of Attenborough Gravel Workings beyond the Erewash River. The underlying map shows the state of the workings around 1950 and is from OpenStreetMaps rectified out-of-copyright 1:25k Ordnance Survey Maps. Blue represents original flow from the Erewash catchment, Cyan the new flow. CC-BY-SA|
Attenborough NR, showing re-profiled Erewash outflow
View Larger Map
Aspects of the hydrology were studied as part of a recent PhD thesis at the University of Nottingham. However, it would be of considerably interest if we could build our own simulations, not least in order to test assertions made by people like the Environment Agency with access to sophisticated data sets. In fact, given availability of suitable data and skills I imagine a number of wildlife charities would use modelling of catchments. This would be most useful in the early planning stages when it is often difficult to challenge the assumptions of the various agencies.
|Construction of flood defences on Corbett's Meadow near Attenborough Village|
- River Catchments and flowlines (NHD data set)
- Landcover for modelling runoff and permeability
- A digital elevation model (DEM)
- Precipitation data
- Point data from monitoring stations
- Water quality data
The key data which cannot be readily accessed or approximated is the stuff that the Environment Agency holds.
For this type of small scale modelling it is perfectly feasible to use approximate data to start with and then incrementally improve the data over time. Details of a DEM are not absolutely required because there are several hundred people who know exactly which paths start to flood first and how deep the flooding needs to be before you stop wandering down the path in wellies. I imagine it would be relatively straightforward to approximate the immediate area to an accuracy better than 1 m vertical elevation just using peoples personal knowledge.
Nor are the results of the model likely to be used to make multi-million pound decisions. It is therefore misleading as some commenters on the Guardian article who have belittled the ability of the broader community to be able to make use of this data. The mantra of 'Good Enough' needs to be heard more frequently. It is something we have shown regularly with OpenStreetMap for a range of applications.
Unfortunately I don't see myself having the time to explore whether my suggested approach using BASINS and OSM data is viable. It is something which I think might make an interesting dissertation project.
To finish I want to note how OpenSource data has actually been used by at least one governmental actor during the UK floods. Simon Miles, the GIS officer at Windsor and Maidenhead Council obviously had to do a few long evenings:
Last day at Bronze command. 70+ hardcopy maps printed, 10+ new layers created, over 8 days, all delivered via #qgis #postgis & @astuntechAnd this was one of the maps he produced:
— Simon Miles (@geosmiles) February 19, 2014
|Flooding around Datchet and Wraysbury Feb-2014|
Even in small ways Open Data can be very useful.
The inspiration for the second half of this blog lies in extensive discussions with Norman Lewis (Attenbrough Volunteer Reserve Manager 1964-1974) and Sandy Aitken (Volunteer Reserve Manager 2007-) who between them have over 90 years of familiarity with the reserve.