Circular Linestring: Make a new start and endpoint along the line (with normalized distance) via PostGIS

Question

I want swap/change the order of the coordinates inside a circular string which should be easy as it is a circle.

-- circular linestring --
CIRCULARSTRING(5 2,-3 1.999999, -2 1, -4 2, 5 2)

to something like
CIRCULARSTRING(-3 1.999999, -2 1, -4 2, 5 2, -3 1.999999)

The ideal solution would also contain a get me the normalize distance on some point of the line like I want the point at 0.6 to be my new start and end point.

Are there any options to do that in PostGIS in a SQL Statement?

Answer 1

To reverse the order of the vertices within the CIRCULARSTRING I would suggest to use ST_Reverse, but it actually doesn't work with CIRCULARSTRING: it returns no error, but returns the original geometry unaltered.

Therefore, I suggest this workaround:

1. Convert the geometry to text - ST_AsText
2. Replace the CIRCULARSTRING for LINESTRING in the WKT of the geometry
3. Create a LINESTRING from the text - ST_GeomFromText
4. Reverse the LINESTRING - ST_Reverse
5. Convert the resulting geometry to text - ST_AsText
6. Replace the LINESTRING for CIRCULARSTRING in the WKT of the geometry
7. Create a CIRCULARSTRING from the text - ST_GeomFromText

Here's the sample code:

WITH t AS
(
    SELECT ST_GeomFromText('CIRCULARSTRING(5 2,-3 1.999999, -2 1, -4 2, 5 2)') AS geom
)
SELECT
    ST_AsText(
        ST_GeomFromText(
            REPLACE(ST_AsText(
                ST_Reverse(
                    ST_GeomFromText(REPLACE(ST_AsText(geom), 'CIRCULARSTRING', 'LINESTRING'))
                )
            ), 'LINESTRING', 'CIRCULARSTRING')
        )
    )
FROM t

Result from the query above: "CIRCULARSTRING(5 2,-4 2,-2 1,-3 1.999999,5 2)"