postgis distance between two points on a line

Question

I have a route (as a LINESTRING) and two vehicles with a position (as a POINT). I need to calculate the distance between the two points over the route.

As an added difficulty I think I also need to measure the distance from the point to the closest point on the line in the case that the vehicle is not on the route at that moment.

I'm using this to find the closest point on the route:

SELECT ST_AsText(ST_ClosestPoint(pt,line)) AS cp_pt_line, 
    ST_AsText(ST_ClosestPoint(line,pt)) As cp_line_pt
FROM (SELECT 'POINT(100 100)'::geometry As pt, 
    'LINESTRING (20 80, 98 190, 110 180, 50 75 )'::geometry As line
) As foo;

Is that possible in a query?

Answer 1

SELECT     ST_Length(ST_GeographyFromText(ST_AsText(ST_Line_Substring(line,ST_Line_Locate_Point(line,ST_ClosestPoint(line,fpt)),ST_Line_Locate_Point(line,ST_ClosestPoint(line,tpt)))))) As length_m,
                                                                   ST_Distance(ST_ClosestPoint(line,tpt)::geography, tpt::geography) as to_point_to_line_m,
                                                                   ST_Distance(ST_ClosestPoint(line,fpt)::geography, fpt::geography) as from_point_to_line_m,
                                                                   ST_AsText(ST_ClosestPoint(line,tpt)) as to_point_on_line,
                                                                   ST_AsText(ST_ClosestPoint(line,fpt)) as from_point_on_line,
                                                                   ST_AsText(tpt) as to_point,
                                                                   ST_AsText(fpt) as from_point
                                           FROM ( SELECT 'SRID=4326;POINT(1)'::geometry As tpt,
                                                                                          'SRID=4326;POINT(2)'::geometry As fpt,
                                                                                          ST_Segmentize('SRID=4326;LINESTRING(123)'::geometry,  0.00001) As line
                                                       ) As foo;

Distance length_m, distance to_point_on_line and from_point_on_line. :)