# So now I convert my OSGB36 data to LLA ... oh ... that looks ... hard

**URL:** <https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400>\
**Category:** Geo\
**Tags:** question, geo, geodesy\
**Created:** [January 11, 2022, 3:18pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400 "2022-01-11T15:18:56Z")\
**Posts on this page:** 11\
**Page:** 1

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**Author:** ![lawless-m](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lawless-m/32/30869_2.png) [@lawless-m](https://discourse.julialang.org/u/lawless-m)\
**Post date:** [January 11, 2022, 3:18pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/1 "2022-01-11T15:18:57Z")

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Hi,

The UK’s Office of National Statistics publishes data which uses OSGB36 East / North coordinates.

Of which I have 39 million addresses from the ONSUD database

> **[Open Geography Portal](https://geoportal.statistics.gov.uk/search?tags=onsud)**
>
> The UK's official Office for National Statistics Open Data Geography Site

e.g. East/North “426642,380231”

Using an online converter

> **[Co-ordinate converter - OSGB36 to WGS84 - WGS-84 to OSGB - NAC - nearby.org.uk](http://www.nearby.org.uk/coord.cgi?p=SK2664280231&f=more)**
>
> Free Online Coordinate Convertor - converts most UK/Irish Grid References and Worldwide Lat/Longs Painlessly - also includes links to many other sites

this is

`Lat:53.3183427137348 Long:-1.60153684101643`

The other data I have is all in LL

I don’t care too much about the direction of conversion so long as I can use a common format

I tried using Geodesy as this seems to be the tool to use but I don’t understand the results

```julia

julia> ecef = ECEFfromLLA(osgb36)(LLA(53.3183427137348, -1.60153684101643))
ECEF(3.8164622703267145e6, -106705.89246363874, 5.09141104371438e6)

julia> @printf("(%d, %d, %d)", ecef...)
(3816462, -106706, 5091411)

```

But that makes no sense, clearly I have much to learn.

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [January 11, 2022, 4:18pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/2 "2022-01-11T16:18:06Z")

</div>

You can use `Proj4.jl` to convert your OSGB36 X, Y data to WGS84 Lat, Lon:

```julia
using Proj4
src = Projection(Proj4.epsg[27700]) # [m] OSGB 1936 / British National Grid
tgt = Projection(Proj4.epsg[4326]) # [deg] WGS84 = ETRS89 = EPSG: 4326
Xs, Ys = 426642.0, 380231.0
Proj4.transform(src, tgt, [Xs, Ys]) # -1.6015368, 53.3183427

```

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<div class="post-metadata">

**Author:** ![Raf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raf/32/3383_2.png) [@Raf](https://discourse.julialang.org/u/Raf)\
**Post date:** [January 11, 2022, 10:43pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/3 "2022-01-11T22:43:28Z")

</div>

You can also do this with `reproject` in ArchGDAL.jl

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<div class="post-metadata">

**Author:** ![lawless-m](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lawless-m/32/30869_2.png) [@lawless-m](https://discourse.julialang.org/u/lawless-m)\
**Post date:** [January 12, 2022, 1:38pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/4 "2022-01-12T13:38:22Z")

</div>

thank you both

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**Author:** ![visr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/visr/32/17204_2.png) [@visr](https://discourse.julialang.org/u/visr)\
**Post date:** [January 12, 2022, 10:04pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/5 "2022-01-12T22:04:48Z")

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> [@lawless-m](#):
>
> I tried using Geodesy as this seems to be the tool to use but I don’t understand the results

If I understand correctly, with `ECEFfromLLA(osgb36)(LLA(x, y))`, what you are doing is to convert latitude longitude coordinate with no elevation, that lie on the OSGB36 datum, to a [Earth-centered, Earth-fixed](https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_coordinate_system) (ECEF) coordinate system, i.e. measured from the center of the Earth. What you want to do instead is to convert coordinates that you have that are in the OSGB 1936 / British National Grid (EPSG:27700) coordinate system, to latitudes and longitudes.

As mentioned,

> [@rafael.guerra](#):
>
> You can use `Proj4.jl` to convert your OSGB36 X, Y data to WGS84 Lat, Lon:

However I’d strongly recommend to use the new API, both for increased accuracy, and because the old API is about to be removed:

```julia
julia> using Proj4

julia> trans = Proj4.Transformation("EPSG:27700", "EPSG:4326", always_xy=true)
Transformation
    source: OSGB 1936 / British National Grid
    target: WGS 84 (with axis order normalized for visualization)

julia> trans((426642.0, 380231.0))
2-element SVector{2, Float64} with indices SOneTo(2):
 -1.6015358313331525
 53.3183599763518

```

This removal will happen as part of the renaming to Proj.jl in [Rename package to Proj.jl by visr · Pull Request #57 · JuliaGeo/Proj.jl · GitHub](https://github.com/JuliaGeo/Proj4.jl/pull/57). Besides renaming the package, this example will continue to work.

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<div class="post-metadata">

**Author:** ![lawless-m](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lawless-m/32/30869_2.png) [@lawless-m](https://discourse.julialang.org/u/lawless-m)\
**Post date:** [January 13, 2022, 9:37am UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/6 "2022-01-13T09:37:04Z")

</div>

Ah yes, the no-elevation explanation makes sense.

Curiously I get a different result to your example

```julia
julia> trans = Proj4.Transformation("EPSG:27700", "EPSG:4326", always_xy=true)
Transformation
    source: OSGB 1936 / British National Grid
    target: WGS 84 (with axis order normalized for visualization)

julia> trans((426642.0, 380231.0))
2-element StaticArrays.SVector{2, Float64} with indices SOneTo(2):
 -1.601536798482117
 53.31834270000983

```

And both are also different to the previous solution

```julia
julia> Proj4.transform(Projection(Proj4.epsg[27700]), Projection(Proj4.epsg[4326]), [426642.0, 380231.0])
2-element Vector{Float64}:
 -1.601536844755606
 53.31834269853471

```

but for my usage that is far enough down the decimal to not make too much difference, as it is in the 1m realm.

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<div class="post-metadata">

**Author:** ![lawless-m](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lawless-m/32/30869_2.png) [@lawless-m](https://discourse.julialang.org/u/lawless-m)\
**Post date:** [January 13, 2022, 10:57am UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/7 "2022-01-13T10:57:39Z")

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Hi,

an FYI for anyone reading later

this new API is not threadsafe

I replaced this

`en2lalo(e, n) = Proj4.transform(Projection(Proj4.epsg[27700]), Projection(Proj4.epsg[4326]), [e, n])`  
with this  
`en2lalo(e, n) = Proj4.Transformation("EPSG:27700", "EPSG:4326", always_xy=true)((e,n))`

And using mutliple threads (eleven) I get a `received signal: 11` error

Commenting out the `Threads.@threads` made it work again

```julia
##Threads.@threads
    for i in 1:length(readers)
        dbs[i] = generate(readers[i])
    end

```

I’ve raised an Issue with the project - [https://github.com/JuliaGeo/Proj4.jl/issues/58](https://github.com/JuliaGeo/Proj4.jl/issues/58)

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [January 13, 2022, 11:08am UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/8 "2022-01-13T11:08:30Z")

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> [@lawless-m](#):
>
> but for my usage that is far enough down the decimal to not make too much difference, as it is in the 1m realm.

Accuracy in the sixth decimal place in the latitude measurement is worth about 10 cm.  
It is almost certain that common input data is far from that kind of accuracy.

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<div class="post-metadata">

**Author:** ![Raf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raf/32/3383_2.png) [@Raf](https://discourse.julialang.org/u/Raf)\
**Post date:** [January 13, 2022, 11:39am UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/9 "2022-01-13T11:39:52Z")

</div>

> **[Coordinate Precision](https://xkcd.com/2170/)**
>
> 40 digits: You are optimistic about our understanding of the nature of distance itself.

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<div class="post-metadata">

**Author:** ![lawless-m](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lawless-m/32/30869_2.png) [@lawless-m](https://discourse.julialang.org/u/lawless-m)\
**Post date:** [January 13, 2022, 12:02pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/10 "2022-01-13T12:02:29Z")

</div>

creating one en2lalo function per thread solved this issue

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<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [January 13, 2022, 2:15pm UTC](https://discourse.julialang.org/t/so-now-i-convert-my-osgb36-data-to-lla-oh-that-looks-hard/74400/11 "2022-01-13T14:15:23Z")

</div>

I was scratching my head why I had slightly different results with GMT (that uses GDAL for this calculation). It turns out that is not indifferent if the EPSG code is converted into aPROJ4 string or a WKT. (I’m using the `println` to force writing with higher number of decimals)

```julia
println(xy2lonlat([426642.0 380231.0], s_srs=epsg2wkt(27700)))
[-1.601536798482117 53.31834270000983]

println(xy2lonlat([426642.0 380231.0], s_srs=epsg2proj(27700)))
[-1.6000274390369076 53.31806612704705]

```

former result is the same as we get when using the CLI `cs2cs`

```julia
echo 426642.0 380231.0 | cs2cs EPSG:27700 EPSG:4326 -f %.12f
53.318342700010 -1.601536798482 0.000000000000

```

so it must be the most accurate one.  
It’s the first time I see such a sensible difference between the use of `PROJ4` and `WKT` strings.