Geography datatype

[JakobMiksch]

PostGIS has a Geography datatype. This is useful to store worldwide POI data and perform proximity analysis using ST_DWithin.
However, ( to my knowledge), osm2pgsql currently does not support importing the OSM geometry to a Geography column. Only these types seem to be allowed: https://osm2pgsql.org/doc/manual.html#the-flex-output
Would it be of interest for osm2pgsql to be able to create Geography columns? What do you think about this?

[joto]

It shouldn't be too difficult to add this. We'll just need a way to tell osm2pgsql in the config file that we want geography instead of geometry columns. We probably also need to check the SRID, because geography types only allow lat/lon SRIDs and we want to give a nice error message instead of failing with an SQL error. I don't have much experiences with the geography type, but I think that's probably it.

[pnorman]

Operating in EPSG:4326 and changing a couple bits of the WKB should be all that's necessary, but I haven't looked into WKBs in enough detail.

[joto]

PostGIS happily converts geometry WKBs from osm2pgsql into whatever geography uses internally. So we don't need to change anything there.

[JakobMiksch]

I guess the next step is to create an issue for that. My initial thoughts about necessary tasks in this context are:

• the implementation itself
• useful error message if user provides wrong CRS
• Tests
• Docs
• Example Lua Config

Do you have any further requirements?

Then I can create an issue on that ...

[joto]

Is there any need for supporting other projections except 4326 for the geography data type? If not then there is an easy backwards-compatible way to implement this. The projection parameter on geometry columns already support either a number (ie SRID) or a few strings like mercator and latlon (https://github.com/osm2pgsql-dev/osm2pgsql/blob/master/src/flex-table-column.cpp#L101-L125). This could be extended to allow geography as name, use -1 or something like that as m_srid and then use that in https://github.com/osm2pgsql-dev/osm2pgsql/blob/master/src/flex-table-column.cpp#L127-L178 to switch to geography type. Feel free to create a PR for that. And yes, we'll need some tests.

[JakobMiksch]

Only EPSG:4326 is relevant here. Thanks for the links to the code. I will have a look 👀

[mboeringa]

Only EPSG:4326 is relevant here. Thanks for the links to the code. I will have a look 👀

Not to sure about that, if we do want Geography support, it seems more natural to me to implement a generic solution also supporting all other geodetic SRSs PostGIS supports, to have maximum compatibility with PostGIS.

Support for other geodetic SRSs than EPSG:4326 was already added back in PostGIS 2.2 according to https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography, so should be well established and tested by now.