Fwd: Tricky join and update with same table

Hi all,

Some help if you can please. I have GPS data for a number of people. Each
person turned the GPS on, at home, at a different time of the day. Before
that time, I presume that the person was in their house. I have made a
table for each person for a whole 24 hours (one record per minute), and I
now want to 'fill in' their location for the rows before they turned the
GPS on. So for each person I want to take the first row where the
point_geom is not null, and update all of the rows above it with that
value. It's driving me nuts.

spid | point_time | point_geom
-----------------------------------------------------------
1 | 2012-01-01 00:01:00 |
1 | 2012-01-01 00:02:00 |
1 | 2012-01-01 00:03:00 | POINT(X, Y)
1 | 2012-01-01 00:04:00 | POINT(X, Y)
1 | 2012-01-01 00:05:00 | POINT(X, Y)
2 | 2012-01-01 00:01:00 |
2 | 2012-01-01 00:02:00 |
2 | 2012-01-01 00:03:00 |
2 | 2012-01-01 00:04:00 |
2 | 2012-01-01 00:05:00 | POINT(X, Y)
3 | 2012-01-01 00:01:00 |
3 | 2012-01-01 00:02:00 | POINT(X, Y)
3 | 2012-01-01 00:03:00 | POINT(X, Y)
3 | 2012-01-01 00:04:00 | POINT(X, Y)
3 | 2012-01-01 00:05:00 | POINT(X, Y)

I've managed to select the correct row using this:

SELECT spid, min(point_time) as point_time
FROM hlhs_day
WHERE point_geom IS NOT NULL
GROUP BY spid;

However when I try to add in the column 'point_geom' to the query, it won't
work.

Thanks for your help and suggestions.

James