Using Python pyproj

So I have a CSV with a bunch of coordinates on the British National Grid and I want to display these shapes on a map on the web. Almost all web mapping tools expect coordinates in WGS84.

So I needed to convert the coordinates. I cheekily asked data scientist Anna Powell-Smith who suggested GeoPandas.

So, in a fresh conda environment:

conda install geopandas

Wait

I would love to say that in the time it took to download the 2.3 Gigabytes that GeoPandas needs, I found pyproj and wrote my conversion tool using it.

Life isn't quite so poetic, but it is the case that a conda env containing GeoPandas is 2.3 Gigabytes, and a conda env containing pyproj is 156 Megabytes.

(Incidentally, I came across pyproj because I saw it whizz by as all the dependencies for GeoPandas were scrolling by, and it sounded like the thing that actually converted coordinates)

The pyproj documentation is not exactly introductory, but the docstrings in the source were pretty useful in working out how to use it.

Project and transform

I create an object for the source projection, which is British National Grid:

os_proj = pyproj.Proj(init='epsg:27700')

and an object for the target projection, which is WGS 84:

wgs_proj = pyproj.Proj(init='epsg:4326')

The numbers after the epsg are seemingly random, but you can discover them by Googling epsg British National Grid or similar.

So with a pair of x, y coordinates in hand (which are Eastings and Northings in metres for British National Grid), you can transform them from one projection to another with:

lon, lat = pyproj.transform(os_proj, wgs_proj, x, y)

which (because our target project is WGS84) gives a longitude and a latitude (in degrees).

You can mass convert by passing in arrays of coordinates apparently, but I convert a CSV file row-by-row calling pyproj.transform once per row. That converts a 15000 row file essentially instantaneously (< 1 second), so that's fine.

The files and the code are in the sheffield-lsoa repo.

So, despite not using GeoPandas in the end, I do thank Anna Powell-Smith for the recommendation.