gv.Polygons DataError When Using OSGB Projection

Question

I have 2 shapefiles for the UK:

In [3]: # SHAPEFILE 1: 
   ...: # WESTMINISTER PARLIAMENTARY CONSTITUENCY UK SHAPEFILE 
           


        

Answer 1

My issue turned out to be caused by my reprojection step from OSGB to WGS 84.

# THE ORIGINAL PROJECTION ON THE SHAPEFILE
In [16]: lad19.crs                                                                                                                                                                                                                       
Out[16]: {'init': 'epsg:27700'}

While the result of the following command would suggest that the reprojection step worked

In [17]: lad19.crs = {'init': 'epsg:4326'} 
    ...: lad19.crs                                                                                                                                                                                                                       
Out[17]: {'init': 'epsg:4326'}

if you look at the geometry attribute you can see that it is still made up of eastings and northings and not longitudes and latitudes as you would expect after reprojecting:

In [8]: lad19["geometry"].head()                                                                                                                                                                                              
Out[8]: 
0    POLYGON ((448986.025 536729.674, 453194.600 53...
1    POLYGON ((451752.698 520561.900, 452424.399 52...
2    POLYGON ((451965.636 521061.756, 454348.400 52...
3    POLYGON ((451965.636 521061.756, 451752.698 52...
4    POLYGON ((419709.299 515678.298, 419162.998 51...
Name: geometry, dtype: geometry

The solution was to instead reproject from the original to the desired projection using this method, with the key part being to include inplace=True:

In [11]: lad19.to_crs({'init': 'epsg:4326'},inplace=True) 
    ...: lad19.crs                                                                                                                                                                                                            
Out[11]: {'init': 'epsg:4326'}

The eastings and northings contained in the geometry column have now been converted to longitudes and latitudes

In [12]: lad19["geometry"].head()                                                                                                                                                                                             
Out[12]: 
0    POLYGON ((-1.24098 54.72318, -1.17615 54.69768...
1    POLYGON ((-1.20088 54.57763, -1.19055 54.57496...
2    POLYGON ((-1.19750 54.58210, -1.16017 54.60449...
3    POLYGON ((-1.19750 54.58210, -1.20088 54.57763...
4    POLYGON ((-1.69692 54.53600, -1.70526 54.54916...
Name: geometry, dtype: geometry

and now gv.Polygons can use this shapefile to successfully produce a choropleth map:

In [13]: gv.Polygons(lad19, vdims='lad19nm', 
    ...:            ).opts(tools=['hover','tap'],  
    ...:                   width=450, height=600 
    ...:                  )                                                                                                                                                                                                   
Out[13]: :Polygons   [Longitude,Latitude]   (lad19nm)