Promote Polygon to Multi-Polygon

[normanb]

I will be more active in this forum shortly I hope, but there has been questions about mixing polygons and multi-polygons with GDAL. Some drivers within GDAL support multiple geometry types, and ogr2ogr even has a PROMOTE_TO_MULTI flag.

If however you are using Python and not the GDAL tools then this is the fastest way I know to promote a polygon to a multi-polygon and requires a little bit of knowledge of the WKB spec.

    # promote to multi-polygon using WKB
    byteOrder = ">"
    if g[0] == 1:
        byteOrder = "<"
    
    format = f"{byteOrder}BII"
    multi_geom = struct.pack(format, g[0], 6, 1) + g

Where g is the original WKB geometry, if g[1] == 3 then you know you have a polygon that needs to be promoted, 6 is the WKB code for multi-polygon.

[vcschapp]

Thanks Norman - both tips are appreciated.

I'm curious if you have a question or issue to raise for discussion, or were you more just trying to share this knowledge?

[normanb]

Mainly I was looking to share this knowledge as it has been asked in a few blog posts I have seen. I have been working a lot with the buildings dataset and I have feedback for discussion but I will share that in a separate topic with a different header.

A single geometry type in the distribution might be easier but it was easy enough to code around.