Unir dos líneas y obtener su diferencia

Question

En la foto hay dos líneas que son paralelas entre sí. Quiero llevarlas a las mismas coordenadas y realizar la diferencia entre ellas, que será la pequeña parte de la línea roja del fondo. Para acercarlas estoy utilizando ST_Snap y ST_SnapTogrid, pero no llegan al mismo sitio. Siempre que veo sus valores WKT, hay una diferencia entre ellos. ¿Hay alguna otra forma de hacerlo? Esto es sólo un ejemplo, y en mi caso tengo un gran número de líneas. El método que estoy utilizando es:

    Create Table Mytable As select Difference(
    ST_snaptogrid(st_snap(area2.geometry,area1.geometry,10),1)
    ,ST_snaptogrid(st_snap(area1.geometry,area2.geometry,10),1))
As Geometry from Area1 area1,Area2 area2 

Mi problema es que siempre hay una diferencia entre la línea por mucho que aumente el valor de ST_Snap

El área 1 WKT es:

LINESTRING(328073.546864 6807587.379884, 328083.270982 6807590.696921, 328091.225316 6807593.259645)
LINESTRING(327797.341809 6809087.203224, 327817.646694 6809124.933343, 327834.793422 6809156.142693)
LINESTRING(327858.73345 6809194.273138, 327846.069419 6809176.233451, 327834.793422 6809156.142693)
LINESTRING(327797.341809 6809087.203224, 327817.646694 6809124.933343, 327834.793422 6809156.142693)
LINESTRING(327858.73345 6809194.273138, 327846.069419 6809176.233451, 327834.793422 6809156.142693)
LINESTRING(327981.300935 6808365.031003, 327985.016518 6808372.66537, 327989.310438 6808385.47058, 327993.133348 6808398.916238, 327996.174084 6808410.698549, 327997.657363 6808418.929391, 327998.059613 6808425.623853, 327997.609842 6808433.239406, 327994.421005 6808440.092431, 327991.176344 6808443.450785, 327987.15875 6808445.332367, 327981.975209 6808445.775286, 327977.130078 6808444.892307, 327971.232208 6808442.864326, 327962.314799 6808440.573971, 327955.585748 6808437.708019, 327949.390128 6808437.284516, 327943.311077 6808440.85768, 327940.628234 6808445.778085, 327941.422165 6808452.041745, 327941.927263 6808456.636409, 327941.133349 6808463.27772, 327935.344827 6808472.914984)
LINESTRING(327933.042454 6808901.492707, 327949.590932 6808892.427465, 327982.597201 6808872.637156, 328004.02018 6808856.149622, 328024.80521 6808828.733857, 328044.07552 6808799.16736, 328059.914356 6808775.343777, 328069.481633 6808762.452965, 328076.685954 6808755.869418)
LINESTRING(328076.685954 6808755.869418, 328087.981025 6808752.712386, 328107.793822 6808752.857009, 328133.149461 6808753.121076)
LINESTRING(327933.042454 6808901.492707, 327949.590932 6808892.427465, 327982.597201 6808872.637156, 328004.02018 6808856.149622, 328024.80521 6808828.733857, 328044.07552 6808799.16736, 328059.914356 6808775.343777, 328069.481633 6808762.452965, 328076.685954 6808755.869418)
LINESTRING(328076.685954 6808755.869418, 328087.981025 6808752.712386, 328107.793822 6808752.857009, 328133.149461 6808753.121076)
LINESTRING(328132.250771 6808620.469034, 328137.46438 6808632.649227, 328141.963598 6808644.902069, 328149.764701 6808663.874684, 328153.257259 6808673.914148)
LINESTRING(327840.846786 6808834.303038, 327842.301786 6808804.3626, 327843.600265 6808785.782563, 327847.983413 6808771.875459, 327858.662089 6808755.338149, 327867.331653 6808742.453688, 327871.202073 6808730.517056, 327871.980273 6808717.386586)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327840.846786 6808834.303038, 327811.866351 6808832.593119, 327779.316555 6808832.200754, 327755.113508 6808832.555611, 327747.690049 6808833.240576)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327840.846786 6808834.303038, 327842.301786 6808804.3626, 327843.600265 6808785.782563, 327847.983413 6808771.875459, 327858.662089 6808755.338149, 327867.331653 6808742.453688, 327871.202073 6808730.517056, 327871.980273 6808717.386586)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327840.846786 6808834.303038, 327811.866351 6808832.593119, 327779.316555 6808832.200754, 327755.113508 6808832.555611, 327747.690049 6808833.240576)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327840.846786 6808834.303038, 327842.301786 6808804.3626, 327843.600265 6808785.782563, 327847.983413 6808771.875459, 327858.662089 6808755.338149, 327867.331653 6808742.453688, 327871.202073 6808730.517056, 327871.980273 6808717.386586)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327840.846786 6808834.303038, 327811.866351 6808832.593119, 327779.316555 6808832.200754, 327755.113508 6808832.555611, 327747.690049 6808833.240576)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327906.53266 6808913.088692, 327907.246699 6808912.828987, 327923.50676 6808906.284046, 327933.042454 6808901.492707)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327741.101971 6808968.246202, 327782.687231 6808955.211629, 327836.305251 6808936.888488)
LINESTRING(327906.53266 6808913.088692, 327907.246699 6808912.828987, 327923.50676 6808906.284046, 327933.042454 6808901.492707)
LINESTRING(327836.305251 6808936.888488, 327836.651557 6808919.926536, 327837.637871 6808889.766109, 327840.134355 6808848.2158, 327840.846786 6808834.303038)
LINESTRING(327836.305251 6808936.888488, 327838.246057 6808936.212788, 327898.756704 6808915.717157, 327906.53266 6808913.088692)
LINESTRING(327933.042454 6808901.492707, 327949.590932 6808892.427465, 327982.597201 6808872.637156, 328004.02018 6808856.149622, 328024.80521 6808828.733857, 328044.07552 6808799.16736, 328059.914356 6808775.343777, 328069.481633 6808762.452965, 328076.685954 6808755.869418)

y el WKT del Área2 es

LINESTRING(328098.741578 6807595.992692, 328091.228 6807593.261)
LINESTRING(327834.793 6809156.148, 327838.634078 6809163.160107)
LINESTRING(327834.793 6809156.148, 327838.634078 6809163.160107)
LINESTRING(327834.793 6809156.148, 327843.409 6809171.877)
LINESTRING(327834.793 6809156.148, 327843.409 6809171.877)
LINESTRING(327933.086892 6808443.109881, 327940.284 6808445.234, 327949.041176 6808448.53097)
LINESTRING(328077.154479 6808747.885264, 328076.684 6808755.871)
LINESTRING(328077.154479 6808747.885264, 328076.684 6808755.871)
LINESTRING(328077.18164 6808747.424254, 328076.684 6808755.871)
LINESTRING(328077.18164 6808747.424254, 328076.684 6808755.871)
LINESTRING(328153.261 6808673.914, 328155.571 6808678.871, 328156.421398 6808681.259019)
LINESTRING(327840.843 6808834.31, 327840.436553 6808842.291967)
LINESTRING(327840.843 6808834.31, 327840.436553 6808842.291967)
LINESTRING(327840.843 6808834.31, 327840.436553 6808842.291967)
LINESTRING(327836.468448 6808928.892544, 327836.303 6808936.884)
LINESTRING(327836.468448 6808928.892544, 327836.303 6808936.884)
LINESTRING(327836.468448 6808928.892544, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.135 6808848.214, 327837.64 6808889.774, 327836.654 6808919.93, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.135 6808848.214, 327837.64 6808889.774, 327836.654 6808919.93, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.135 6808848.214, 327837.64 6808889.774, 327836.654 6808919.93, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.135 6808848.214, 327837.64 6808889.774, 327836.654 6808919.93, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.135 6808848.214, 327837.64 6808889.774, 327836.654 6808919.93, 327836.303 6808936.884)
LINESTRING(327836.479375 6808928.364739, 327836.303 6808936.884)
LINESTRING(327836.479375 6808928.364739, 327836.303 6808936.884)
LINESTRING(327836.479375 6808928.364739, 327836.303 6808936.884)
LINESTRING(327840.843 6808834.31, 327840.436595 6808842.291149)
LINESTRING(327840.843 6808834.31, 327840.436595 6808842.291149)
LINESTRING(327840.843 6808834.31, 327840.436595 6808842.291149)
LINESTRING(327843.878678 6808934.311913, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327843.878678 6808934.311913, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327843.878678 6808934.311913, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327906.529 6808913.09, 327898.958813 6808915.656595)
LINESTRING(327906.529 6808913.09, 327898.958813 6808915.656595)
LINESTRING(327844.362871 6808934.147921, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327844.362871 6808934.147921, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327844.362871 6808934.147921, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327906.529 6808913.09, 327898.76 6808915.724, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327906.529 6808913.09, 327898.76 6808915.724, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327906.529 6808913.09, 327898.76 6808915.724, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327906.529 6808913.09, 327898.76 6808915.724, 327838.245 6808936.22, 327836.303 6808936.884)
LINESTRING(327933.194508 6808892.287715, 327933.047 6808901.5)

El área2 tiene líneas adicionales que quiero en la diferencia. Estas no son geometrías completas porque aquí no puedo incluirlas todas debido a la restricción de palabras

Answer 1

Si tienes dos líneas (no necesariamente paralelas), cada una definida con dos vértices: punto inicial y punto final. Entonces puedes encontrar dos puntos en la línea 1: p1 el más cercano/cercano al punto inicial de la línea 2 y p2 el más cercano desde el punto final de la línea 2.

Entonces la parte entre p1 y p2 es la parte de la línea 1 donde la línea 2 "cubre" la línea 1. La "diferencia" restante puede tener hasta dos partes: La parte entre el punto inicial de la línea 1 y el primero de (p1, p2), y la parte entre el segundo de (p1, p2) y el punto final de la línea 1.

Para reducir la complejidad asumo que tiene un con pares de líneas en las columnas redline y blackline. Entonces esta consulta debería devolver la "diferencia" (redline - blackline), que son las partes de redline que no están "cubiertas" por blackline:

-- create sample data: one pair of lines corresponding to your graphic
WITH mytable(id, redline, blackline) AS (VALUES
    (1,
     geometry 'LINESTRING(73 8, 79 201)',
     geometry 'LINESTRING(67 9, 72 166)')),
-- first expression: find points on redline
-- ST_Line_Locate_Point returns a value between 0 (start point)
-- and 1 (end point)
s1 AS (SELECT
    id, redline,
    ST_Line_Locate_Point(redline, ST_StartPoint(blackline)) p1,
    ST_Line_Locate_Point(redline, ST_EndPoint(blackline)) p2
    FROM mytable),
-- second expression: calculate and return both possible parts of
-- the "difference"
s2 AS (SELECT
    id,
    ST_Line_Substring(redline, 0.0, least(p1, p2)) geom
    FROM s1
    WHERE least(p1, p2) > 0.0
UNION SELECT
    id,
    ST_Line_Substring(redline, greatest(p1, p2), 1.0) geom
    FROM s1
    WHERE greatest(p1, p2) < 1.0)
-- ST_Union: if possible, return a simple linestring, else a multilinestring
--   no result, if blackline "covers" the full length of redline
SELECT
    id,
    ST_AsEWKT(ST_Union(geom)) diff
    FROM s2
    GROUP BY id;

resultado:

id  diff
1   MULTILINESTRING((73 8,73.0252648518171 8.81268606678289),(77.9062089312056 165.816387287113,79 201))