ST_DistanceCPA
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## Name

ST_DistanceCPA — Returns the distance between the closest point of approach of two trajectories.

## Synopsis

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

## Description

Returns the minimum distance two moving objects have ever been each other.

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

Availability: 2.2.0

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

## Examples

    -- Return the minimum distance of two objects moving between 10:00 and 11:00
    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
    )
    SELECT ST_DistanceCPA(a,b) distance FROM inp;

         distance
    ------------------
     1.96036833151395

## See Also

ST_IsValidTrajectory, ST_ClosestPointOfApproach, ST_AddMeasure, |=|

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