Boundaries, be they land ownership or administrative boundaries, form an essential part of geo-data. Was it therefore a success for the open data campaign when Ordnance Survey released the Boundary-Line product, containing all electoral and administrative boundaries, under the OS OpenData Licence? Not Quite.
The problem is that boundary is the “Boundary-Line” data are not boundaries! There are two issues, and these apply to all geospatial data released as independent vector layers; scale and context.
Large scale maps (such as walking maps), have a higher resolution than medium, or small scale maps (such as country of global maps). The larger the scale, the higher the resolution and therefore the more detail shown. As boundaries twist and turn following streams, rivers and hedges, it is important to use a large scale/high resolution. The Ordnance Survey provide boundary data in two products; the free “Boundary-Line” product, and the non-free “OS MasrterMap” product. According to Ordnance Surveys own admission:
Boundary-Line is captured against a lower resolution mapping backdrop and the boundaries are captured to represent the data at a nominal 1:10000 viewing scale. The process of generalising the data may have caused some features to be moved from their true ground position for the purpose of map clarity.
So scale, or to give it it’s proper name, generalisation, is the first problem we should be aware of before using Boundary-Line data in OpenStreetMap.
Lets see what Ordance Survey have to say about their other boundary product – OS MasterMap:
OSMM Topography Layer currently holds the definitive and more accurate boundary information as the boundaries are mered (aligned to) real-world features on the ground. Captured at mapping scales of 1:1250, 1:2500 (for urban areas) and 1:10k (for rural).
Okay, we can see that MasterMap uses a larger scale / higher resolution which helps provide more accurate data, but what is that reference to “mereing” (alignment). Mereing, is the process of establishing a boundary relative to ground features present at the time of a survey (source: ESRI). As such any boundary line extracted from a map of any resolution cannot therefore be used on its own to describe a boundary – once you extract the boundary, you loose the context provided by the rest of the map!
Lets look at an example.
In the example above we see the boundary as a black dashed line. Along the line are some descriptions in pink. The first couple are relatively easy to decipher – “Co Const Met Dist & CP Bdy” is shorthand for County Constituency, Metropolitan District and Civil Parish boundary. Similarly “Boro Const Bdy” tells us it is a Borough Constituency boundary. As we follow the boundary we reach more abstract shorthand:
- 1.22m RH – RH stands for Root of Hedge, and 1.22m equates to exactly 4ft. The boundary is therefore 4ft away from the stem line of the hedge. This unusual convention comes from the presumption that landowners planted hedgerows slightly in from the edge of their land so as to not encroach on their neighbours plots. The distance differs between 3ft and 5ft depending on which parish the land lies within.
- Def – This is a “defaced boundary”, meaning that the original feature that the boundary was aligned to no longer exists. This was probably a hedge that was removed when the houses were built. Had the hedge roots not been removed then the boundary would be marked as “Tk H” implying track of hedge.
- Und – An “undefined boundary” is one where there was no real-world feature to align the boundary to when it was surveyed.
For a list of abbreviations click here.
Any vector extract of boundary data will be problematic due to generalisation (scale) and the loss of information about mereing (loss of context). If you are really keen on determining the exact boundary you may want to read more about boundary presumptions or go and dig out the original surveyors notes (Perambulation Cards) at the National Archives.
So for OpenStreetMap should we be linking boundary ways to streams and hedges on the acknowledgement that Boundary-Line data is not perfect and we are not able to provide any certainty as to which side of the hedge the boundary actually resides on? Please leave your thoughts as comments below.