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In H2GIS, geometry can be converted to geography with function "ST_Transform" For example:

SELECT ST_Transform(ST_GeomFromText(
'POINT(584173 2594514)', 27572), 4326);
-- Answer: POINT(2.1145411092971056 50.345602339855326)

How can I convert in the opposite way? How can I convert lat/lng(2.1145411092971056 50.345602339855326) to a coordinate in Cartesian coordinate system?

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At first you have to know a SRID (EPSG) code of the coordinate reference system (CRS) to which you want to convert lat/lng coordinates - each of existing CRS has any kind of distortion (see this). For example, for Europe, the EPSG:3035 is used for statistical mapping at all scales and other purposes where true area representation is required (equal area projection). Then you can define transformation like this:

SELECT ST_Transform(ST_GeomFromText(
'POINT(2.1145411092971056 50.345602339855326)', 4326), 3035);

EPSG:4326 is used for geographic coordinates (WGS 84 system - lat/lon), EPSG:3035 is used as a target CRS...

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