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

[Tom Crocker]

Thanks very much Rob.

On Wed, 3 Mar 2021 at 18:14, Rob Nickerson <rob.j.nickerson at gmail.com>
wrote:

> 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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