[Talk-GB] OSM UK's first tile layer

[Rob Nickerson]

Thanks for this.

I'm still confused as to what exactly I need but at least I can run some
tests using these places. If it helps anyone, below is the xml we are using
in Mapnik. The Map srs is from the OpenStreetMap default render and relates
to the web projection. The Layer SRS is the standard for WGS84.

The intent going forward is to not transform from the british national grid
to WGS84 before feeding in to Mapnik. Instead I will give Mapnik the raw
data and the relevant Layer srs. I believe this is "+proj=tmerc +lat_0=49
+lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
+datum=OSGB36 +units=m +no_defs" based on the link at
https://spatialreference.org/ref/epsg/osgb-1936-british-national-grid/ I
will test this but if you or others have feedback at this stage, please let
me know.

<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
+y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over">

  <Style name="My Style">
    <Rule>
      <LineSymbolizer stroke="rgb(100%,0%,0%)" stroke-width="4" />
    </Rule>
  </Style>

  <Layer name="world" srs="+proj=longlat +ellps=WGS84 +datum=WGS84
+no_defs">
    <StyleName>My Style</StyleName>
    <Datasource>
      <Parameter name="type">shape</Parameter>
      <Parameter name="file">york.shp</Parameter>
    </Datasource>
  </Layer>

</Map>

*Rob*


On Mon, 19 Oct 2020 at 20:02, Adrian <ar2988-os2 at yahoo.co.uk> wrote:

> The Ordnance Survey provides a transformation between OSGB36 and ETRS. It
> is described on this page
> https://www.ordnancesurvey.co.uk/gps/transformation/ and on the pages
> linked from there. The transformation is definitive. In other words, OSGB36
> is redefined as being what you get when you apply the transformation to
> sets of ETRS co-ordinates. This must mean that if you compare an OS 1:1250
> National Grid plan, with an older version of the plan from the era of
> OSTN02, features may have shifted slightly.
>
> The transformation involves a mathematical transformation, and an
> adjustment based on a look-up table, to make the result match the errors in
> the old triangulation system. The OS provides applications to do the
> transformation, both ways, for a range of platforms. It also provides
> source code, the look-up table, and details of the mathematical
> transformation.
>
> JOSM handles projections using proj. If you want to know what JOSM does
> with EPSG:27700, you need to know how it is defined in proj. The source
> code of JOSM includes the OSTN02 look-up table (15MB), but it can't be in
> the jar (also 15MB), so I don't know how that works.
>
> Rob asked about position errors from the Helmert transformation without a
> look-up table. Here are some examples.
> Larger errors
> Place             error, m
> St Kilda           4.9
> Scilly             4.7
> Lizard Point       4.1
> Butt of Lewis      3.2
> King's Lynn        2.7
> Mallaig            2.6
> Flamborough Head   2.4
> Colchester         2.4
> Plymouth           2.4
> Nottingham         2.3
> Anglesey           2.1
> Northampton        2.0
> North Foreland     1.9
> Isle of Man S      1.9
> Carmarthen         1.9
> Smaller errors
> St Catherine's Pt  1.4
> Carlisle           0.8
> Edinburgh          0.6
> Aberdeen           1.8
> Thurso             1.6
> Orkney             1.0
> Foula (Shetland)   1.2
> The errors are particularly small near Bristol, Edinburgh and Fair Isle.
> They exceed 2m in South Devon, Cornwall, East Anglia, Nottinghamshire,
> Lincolnshire, the East Riding of Yorkshire, Pembrokeshire, Anglesey and
> Western Scotland.
>
> +1 for referencing GB to ETRS.
>
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