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I know that the EPSG 4326 is the WGS84 (standard) projection, and is the projection that I am using in my code, but by a mistake, I have wrote 4236 and this worked in the code.

The question is what espg=4236 work? And which is the difference between 4326 and 4236?

The Python code:

geodf = gpd.read_file('./zone.shp')
geodf = geodf.to_crs(epsg='4236')
geodf.to_file('./zone.geojson', driver='GeoJSON')
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EPSG 4326 ("WGS84") and EPSG 4236 (Hu Tzu Shan 1950) are both valid EPSG identifiers, so of course they worked in your codes. However, reverse projecting a set of planar coordinates to EPSG 4326 geodetic coordinates, and to EPSG 4236 geodetic coordinates won't give the same result. (See screen capture below.)

enter image description here

A reason is EPSG 4326 uses the WGS84 ellipsoid, while EPSG 4236 uses the International Ellipsoid of 1924 ("Hayford-Ellipsoid" of 1909). See screen captures below.

enter image description here

enter image description here

Another item to take note is that EPSG 4236 includes transformation parameters - "+towgs84=-637,-549,-203,0,0,0,0". And depending on the datum of your source data, reverse projecting will also include datum transformation, thus causing further error.

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