[Talk-GB] OSMUK response to Accuracy of cadastral parcel alignment

[Rob Nickerson]

Hi all,

I worked on getting the Cadastral Parcel layer online so can shed some
light on what we mean by "perfectly aligned".

Firstly what it is not. It is not a statement about the accuracy of land
boundaries. As others have pointed out, determining the precise point of
land ownership is complex and Land Registry don't claim that this is what
their data shows. As Tom notes the Land Registry describe it as a "general
position of the boundaries".

So what do we mean by "perfectly aligned"? The answer is in two parts, but
the starting assumption is that the Ordnance Survey use the right
alignment. If you don't agree with that assumption then you can stop
reading!

Part 1: Show that the INSPIRE data aligns to OS data
For this we tested the INSPIRE data against other OS data including OS Open
Data (e.g. the OS Open Greenspace data). The data aligns. I've had someone
subsequently confirm it also aligns to OS MM.

Part 2: Show that we are transforming the data accurately
The INSPIRE data (and other OS Open Data) uses the OS National Grid map
projection. To use the layer in the OSM editors we therefore need to
transform the data to a different projection. This is where the OSM
community (and especially Adrian) helped us. Once we thought we had it
correctly set up we tested several points in an OS online tool to confirm
that the transform was in deed correct and as per the OS recommended
method. These tests passed with exact matches.

Given these two things, plus the fact that the data is GB wide, we feel it
is a good source to align all of our mapping and imagery layers to.

P.S. As other have noted, if you try to open the source data in JOSM you
will get a different result. This is because JOSM fails on part 2 above.
JOSM has the wrong in built transformation.

I hope this helps.

Best regards and happy mapping,
*Rob*
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