ST_ClosestPointOfApproach
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## Name

ST_ClosestPointOfApproach — Returns a measure at the closest point of approach of two trajectories.

## Synopsis

`float8 ST_ClosestPointOfApproach(`geometry track1, geometry track2`)`;

## Description

Returns the smallest measure at which points interpolated along the given trajectories are at the smallest distance.

Inputs must be valid trajectories as checked by ST_IsValidTrajectory. Null is returned if the trajectories do not overlap in their M ranges.

See ST_LocateAlong for getting the actual points at the given measure.

Availability: 2.2.0

This function supports 3d and will not drop the z-index.

## Examples

    -- Return the time in which two objects moving between 10:00 and 11:00
    -- are closest to each other and their distance at that point
    WITH inp AS ( SELECT
      ST_AddMeasure('LINESTRING Z (0 0 0, 10 0 5)'::geometry,
        extract(epoch from '2015-05-26 10:00'::timestamptz),
        extract(epoch from '2015-05-26 11:00'::timestamptz)
      ) a,
      ST_AddMeasure('LINESTRING Z (0 2 10, 12 1 2)'::geometry,
        extract(epoch from '2015-05-26 10:00'::timestamptz),
        extract(epoch from '2015-05-26 11:00'::timestamptz)
      ) b
    ), cpa AS (
      SELECT ST_ClosestPointOfApproach(a,b) m FROM inp
    ), points AS (
      SELECT ST_Force3DZ(ST_GeometryN(ST_LocateAlong(a,m),1)) pa,
             ST_Force3DZ(ST_GeometryN(ST_LocateAlong(b,m),1)) pb
      FROM inp, cpa
    )
    SELECT to_timestamp(m) t,
           ST_Distance(pa,pb) distance
    FROM points, cpa;

                   t               |     distance
    -------------------------------+------------------
     2015-05-26 10:45:31.034483+02 | 1.96036833151395

## See Also

ST_IsValidTrajectory, ST_DistanceCPA, ST_LocateAlong, ST_AddMeasure

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