--- a/schema/isrs_functions.sql	Thu Mar 07 12:15:14 2019 +0100
+++ b/schema/isrs_functions.sql	Thu Mar 07 12:20:00 2019 +0100
@@ -21,6 +21,28 @@
     LANGUAGE sql
     STABLE PARALLEL SAFE;
 
+-- Return a multipoint with coordinates of stretch end points
+CREATE OR REPLACE FUNCTION ISRSrange_points(
+    stretch isrsrange
+) RETURNS geometry
+AS $$
+DECLARE result_geom geometry;
+BEGIN
+    SELECT geom
+        INTO STRICT result_geom
+        FROM (
+            SELECT ST_Collect(CAST(geom AS geometry)) AS geom
+                FROM waterway.distance_marks_virtual
+                WHERE location_code IN(lower(stretch), upper(stretch))) AS pts
+        -- Ensure both have been found
+        WHERE ST_NumGeometries(geom) = 2;
+
+    RETURN result_geom;
+END;
+    $$
+    LANGUAGE plpgsql
+    STABLE PARALLEL SAFE;
+
 -- Clip waterway axis to a stretch given by a pair of ISRS location codes.
 -- Uses the table waterway.distance_marks_virtual to map ISRS location codes
 -- to their geo-location.
@@ -31,12 +53,9 @@
     -- in m, up to which linestrings will be connected at their boundary
 ) RETURNS geometry
 AS $$
+DECLARE result_geom geometry;
+BEGIN
     WITH RECURSIVE
-        -- Get coordinates of location codes
-        points_geog AS (
-            SELECT geom FROM waterway.distance_marks_virtual
-                WHERE location_code = lower(stretch)
-                    OR location_code = upper(stretch)),
         utm_zone AS (
             -- Find best matchting UTM zone
             SELECT best_utm(stretch) AS z),
@@ -48,8 +67,8 @@
         points AS (
             SELECT ST_ClosestPoint(
                     wtwaxs,
-                    ST_Transform(points_geog.geom::geometry, z)) AS geom
-                FROM points_geog, utm_zone, (
+                    ST_Transform(geom, z)) AS geom
+                FROM ST_Dump(ISRSrange_points(stretch)), utm_zone, (
                     SELECT ST_Collect(wtwaxs) AS wtwaxs
                         FROM axis) AS ax),
         axis_snapped AS (
@@ -88,9 +107,11 @@
         axis_segment AS (
             -- Fetch end result from snapping
             SELECT wtwaxs AS line
-                FROM axis_snapped
-                WHERE array_length(ids, 1) = (
-                    SELECT max(array_length(ids, 1)) FROM axis_snapped))
+                FROM axis_snapped,
+                    (SELECT ST_Collect(points.geom) AS pts
+                        FROM points) AS points
+                -- Return end result only if both distance marks were connected
+                WHERE ST_Covers(ST_Buffer(wtwaxs, 0.0001), points.pts))
         -- Use linear referencing to clip axis between distance marks.
         -- Simplification is used to work-around the problem, that
         -- ST_LineSubstring might generate very small line segments at an
@@ -102,12 +123,16 @@
         SELECT ST_SimplifyPreserveTopology(ST_LineSubstring(
                     axis_segment.line, min(fractions.f), max(fractions.f)),
                 0.0001) AS line
+        INTO STRICT result_geom
         FROM axis_segment, LATERAL (
             SELECT ST_LineLocatePoint(axis_segment.line, points.geom) AS f
                 FROM points) AS fractions
-        GROUP BY axis_segment.line
+        GROUP BY axis_segment.line;
+
+    RETURN result_geom;
+END;
     $$
-    LANGUAGE sql
+    LANGUAGE plpgsql
     STABLE PARALLEL SAFE;
 
 -- Clip an area to a stretch given by a pair of ISRS location codes.

--- a/schema/isrs_tests.sql	Thu Mar 07 12:15:14 2019 +0100
+++ b/schema/isrs_tests.sql	Thu Mar 07 12:20:00 2019 +0100
@@ -34,6 +34,23 @@
     ,
     'isrs_asText() is the inverse of isrs_fromText()');
 
+SELECT throws_ok($$
+    SELECT ISRSrange_points(isrsrange(
+            ('AT', 'XXX', '00001', '00000', 0)::isrs,
+            ('AT', 'XXX', '00001', '00000', 99999)::isrs))
+    $$,
+    'P0002', NULL,
+    'ISRSrange_points fails if not both distance marks can be found');
+
+SELECT throws_ok($$
+    SELECT ISRSrange_axis(isrsrange(
+            ('AT', 'XXX', '00001', '00000', 0)::isrs,
+            ('AT', 'XXX', '00001', '00000', 2)::isrs),
+        0)
+    $$,
+    'P0002', NULL,
+    'ISRSrange_axis fails if no contiguous axis can be constructed');
+
 SELECT ok(
     ISRSrange_area(isrsrange(
             ('AT', 'XXX', '00001', '00000', 0)::isrs,

--- a/schema/run_tests.sh	Thu Mar 07 12:15:14 2019 +0100
+++ b/schema/run_tests.sh	Thu Mar 07 12:20:00 2019 +0100
@@ -28,7 +28,7 @@
     -c 'SET client_min_messages TO WARNING' \
     -c "DROP ROLE IF EXISTS $TEST_ROLES" \
     -f tap_tests_data.sql \
-    -c 'SELECT plan(58)' \
+    -c 'SELECT plan(60)' \
     -f isrs_tests.sql \
     -f auth_tests.sql \
     -f manage_users_tests.sql \