British National Grid shapefile prj subtle differences change polygon accuracy

Question

I'll preface this question with the statement I know very little about projection systems so this is coming from a position of total ignorance.

I have two shapefiles. One is in England and one is in Scotland, the Scottish one comes from a Scottish goverment website and when converted to geojson with https://github.com/calvinmetcalf/shapefile-js which uses proj4 under the hood, the polygons are several 100 meters offset from the basemap terrain beneath them. The English shapefile is 100% correct.

Upon inspecting the .prj files I can see subtle differences between their definitions:

English shapefile:

PROJCS["British_National_Grid",GEOGCS["GCS_OSGB_1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0.0],UNIT["Decimal_Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",400000.0],PARAMETER["False_Northing",-100000.0],PARAMETER["Central_Meridian",-2.0],PARAMETER["Scale_Factor",0.9996012717],PARAMETER["Latitude_Of_Origin",49.0],UNIT["Meter",1.0]]

Scottish shapefile:

PROJCS["British National Grid",GEOGCS["Ordnance Survey Great Brit",DATUM["Ordnance Survey Great Brit",SPHEROID["Airy 1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0.0],UNIT["Decimal_Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",400000.0],PARAMETER["False_Northing",-100000.0],PARAMETER["Central_Meridian",-2.0],PARAMETER["Scale_Factor",0.9996012717],PARAMETER["Latitude_Of_Origin",49.0],UNIT["Meter",1.0]]

The differences are:

Scotland -> England

SPHEROID["Airy 1830" -> SPHEROID["Airy_1830"

GEOGCS["Ordnance Survey Great Brit" -> GEOGCS["GCS_OSGB_1936"

PROJCS["British National Grid" -> PROJCS["British_National_Grid"

DATUM["Ordnance Survey Great Brit" -> DATUM["D_OSGB_1936"

If I simply replace the entire Scotland .prj file with the England .prj file it lines up perfectly

I then tried to work out exactly what part of the Scotland .prj file was causing the issue and I narrowed it down to:

DATUM["Ordnance Survey Great Brit"

If I just replace that with DATUM["D_OSGB_1936" then the polygons align with the terrain perfectly.

So now when I'm handling shapefile uploads I'm going to unzip and find and replace any mention of DATUM["Ordnance Survey Great Brit with DATUM["D_OSGB_1936".

I'm just wondering is that the correct way to approach it? And I wonder why the Scottish goverment would be exporting an incorrect shapefile config, or if I'm missing something and this is a proj4 error in parsing it?

Answer 1

The fix for now is:

Using jszip package

// Find filename in list of zip files that has .prj extension
let prjFileName = Object.keys(zip.files).find((item) => item.endsWith(".prj"));

const prjFile = zip.file(prjFileName);

if (prjFile) {
  const prjFileContent = await prjFile.async("string");

  if (prjFileContent.includes('DATUM["Ordnance Survey Great Brit"')) {
    zip.file(
      prjFileName,
      prjFileContent.replace(
        'DATUM["Ordnance Survey Great Brit"',
        'DATUM["D_OSGB_1936"'
      )
    );
  }
}