sql - How to interpolate GPS Positions in PostGIS

I have a PostGIS table of GPS positions for every five seconds:

2011-01-01 00:00:05, POINT(x1,y1)
2011-01-01 00:00:10, POINT(x2,y2)
2011-01-01 00:00:15, POINT(x3,y3)
...

I'm looking for a query that will return values (timestamp and point) for every second. It's ok to assume that points are connected by a straight line.

I'm specifically looking for a way to do this inside the database and not by writing some external script.

Answer

hallo

If your original table is called gps_p, your timestamp field is called ts and the points is called th_geom:

SELECT (geom).geom,  ts1 + (((geom).path[1]-1) ||' seconds')::interval FROM 
    (SELECT ts1, ST_DumpPoints(ST_Segmentize(geom, ST_Length(geom)/5)) as geom FROM 
        (SELECT ts1, ST_LineFromMultipoint(ST_Union(geom1, geom2)) as geom FROM
            (SELECT p1.ts as ts1, p2.ts as ts2, p1.the_geom as geom1, p2.the_geom as geom2 
                FROM gps_p p1 INNER JOIN gps_p p2 on p1.ts + '00:00:05'::interval = p2.ts
            ) a
        )b
    ) c

WHERE (geom).path[1] <= 5;

What it does is that it builds lines between the points and use st_segmentize to divide the line in 5 segments.

If it is not exactly 5 seconds between your original points it will not work. Then you can just add an id field with a sequence and use that to selfjoin the table with id1+1 = id2 instead.

HTH

/Nicklas