Hi,
I need to calculate the vertical offset between 3D lines that really
does not have a common intersection point.
Example: the following perpendicular lines, 100m wide, crossing at the
center, have a vertical offset of 25 m at the intersection point. One is
flat, the other goes from 0 to 50m.
ST_MakeLine(ST_PointZ(-43000, -276500, 0, 3763),
ST_PointZ(-42900, -276500, 0, 3763)) As line1,
ST_MakeLine(ST_PointZ(-42950, -276450, 0, 3763),
ST_PointZ(-42950, -276550, 50, 3763)) As line2
My approach is top calculate the 2D intersection point, and then
calculate the z difference between the lines at that point.
To do so, I and up with this LISP style query, that returns 25 as expected.
SELECT
st_z( ST_3DLineInterpolatePoint( line2, ST_LineLocatePoint(line2,
ST_Intersection( line1, line2)))) -
st_z( ST_3DLineInterpolatePoint( line1, ST_LineLocatePoint(line1,
ST_Intersection( line1, line2))))
FROM (
select
ST_MakeLine(ST_PointZ(-43000, -276500, 0, 3763),
ST_PointZ(-42900, -276500, 0, 3763)) As line1,
ST_MakeLine(ST_PointZ(-42950, -276450, 0, 3763),
ST_PointZ(-42950, -276550, 50, 3763)) As line2
) As foo;
Using, for example, ST_3DClosestPoint will return wrong results, because
the closest point are calculated in any direction. The following query
returns 22.5 m, for example.
SELECT
st_z(ST_3DClosestPoint(line2, ST_Intersection( line1, line2))) -
st_z(ST_3DClosestPoint(line1, ST_Intersection( line1, line2)))
FROM (
select
ST_MakeLine(ST_PointZ(-43000, -276500, 0, 3763),
ST_PointZ(-42900, -276500, 0, 3763)) As line1,
ST_MakeLine(ST_PointZ(-42950, -276450, 0, 3763),
ST_PointZ(-42950, -276550, 50, 3763)) As line2
) As foo;
Does anyone has a more elegant way to compute this vertical distance?
A variant of ST_InterpolatePoint that works on the Z value would be nice
to have; or a ST_3DLineInterpolatePoint function that receives a POINT,
instead of a fraction.
Regards,
Jorge
