Checking original geometry when using ST_Split function with MultiPolygon and LineString in PostGIS?

Question

This is an example where a MultiPolygon type geometry is divided with the ST_Split function, the result is two polygon type geometries that join with ST_Union to build the original geometry.

SELECT ST_AsText(ST_Multi(ST_Union(array_agg(t2.splitgeoms)))) as checkgeom FROM (SELECT (ST_Dump(ST_Split(t.origingeom, t.blade))).geom as splitgeoms FROM (SELECT ST_GeomFromText('MULTIPOLYGON(((422724.4094 2114051.9141,422711.9283 2112127.0763,421360.0089 2111149.7395,421156.2348 2110653.5001,421068.9332 2110401.6095,421399.133 2109806.3043,422216.3184 2109103.8077,422461.6364 2108607.605,418963.3179 2109710.2973,419056.2332 2110446.3435,419102.3735 2110751.7852,419112.8234 2110952.7946,418467.9193 2111330.2952,418013.3755 2111671.7966,417903.7447 2112280.1212,417894.2011 2112993.5989,417710.6037 2113424.1606,419402.8895 2114964.4682,419497.7536 2114911.159,421835.4837 2114297.3669,422724.4094 2114051.9141)))',32614) AS origingeom,
                          ST_GeomFromText('SRID=32614;LineString (416126.35021602374035865 2112131.29296381864696741, 419745.03656649653567001 2112394.47015294386073947, 421784.65978221758268774 2113052.41312575712800026, 423042.97571772278752178 2112604.18947552843019366)',32614) AS blade) as t) as t2

Subsequently, the geometry obtained with the original geometry is checked with ST_Equals. In theory the geometries should be equal but they are not and with ST_Difference we see where they are not equal.

SELECT ST_Equals(t.checkgeom, t.origingeom), ST_Difference(t.checkgeom, t.origingeom) FROM (select ST_GeomFromText('MULTIPOLYGON(((422715.77775723 2112720.74038302,422711.9283 2112127.0763,421360.0089 2111149.7395,421156.2348 2110653.5001,421068.9332 2110401.6095,421399.133 2109806.3043,422216.3184 2109103.8077,422461.6364 2108607.605,418963.3179 2109710.2973,419056.2332 2110446.3435,419102.3735 2110751.7852,419112.8234 2110952.7946,418467.9193 2111330.2952,418013.3755 2111671.7966,417907.224718196 2112260.81110943,417903.7447 2112280.1212,417894.2011 2112993.5989,417710.6037 2113424.1606,419402.8895 2114964.4682,419497.7536 2114911.159,421835.4837 2114297.3669,422724.4094 2114051.9141,422715.77775723 2112720.74038302)))',32614) AS checkgeom, ST_GeomFromText('MULTIPOLYGON(((422724.4094 2114051.9141,422711.9283 2112127.0763,421360.0089 2111149.7395,421156.2348 2110653.5001,421068.9332 2110401.6095,421399.133 2109806.3043,422216.3184 2109103.8077,422461.6364 2108607.605,418963.3179 2109710.2973,419056.2332 2110446.3435,419102.3735 2110751.7852,419112.8234 2110952.7946,418467.9193 2111330.2952,418013.3755 2111671.7966,417903.7447 2112280.1212,417894.2011 2112993.5989,417710.6037 2113424.1606,419402.8895 2114964.4682,419497.7536 2114911.159,421835.4837 2114297.3669,422724.4094 2114051.9141)))',32614) AS origingeom) AS t

How do I verify that the geometry division is equal to the original geometry?

Answer 1

In general overlay operations such as ST_Split are liable to change vertices slightly, due to numeric rounding. So generally it is NOT the case that

A = ST_Union(ST_Split(A, B))

You could try comparing the areas of the original and the split geometries - they should be close (but will not be equal, due to the above behaviour).