trigger updating all rows, not just inserted - sql-server-2008

Question

I know I'm missing something obvious here. This trigger is updating all rows in the table (killing performance) when all I want it to do is perform the update on the new inserted row.
CREATE TRIGGER [dbo].[update_location_topo_name]
--fires at each row insert, queries topo map layer (must be present!) and inserts name of topo into new location record
on [dbo].[TBL_LOCATIONS]
after insert
AS
BEGIN
update TBL_LOCATIONS
set TOPO_NAME = dbo.QD24K_GRSM.NAME
FROM dbo.tbl_locations
inner join dbo.QD24K_GRSM
on TBL_LOCATIONS.Location_ID = TBL_LOCATIONS.Location_ID
WHERE (QD24K_GRSM.Shape.STContains(TBL_LOCATIONS.SHAPE) = 1)
END

Answer 1

You need to reference the INSERTED pseudo table to get only the rows that were inserted.
Additionally your join condition of TBL_LOCATIONS.Location_ID = TBL_LOCATIONS.Location_ID makes no sense at all.
Probably better to do this as an INSTEAD OF trigger to modify the rows before insert rather than after they are inserted.
CREATE TRIGGER [dbo].[update_location_topo_name]
ON [dbo].[TBL_LOCATIONS]
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO TBL_LOCATIONS
(foo,
bar,
TOPO_NAME)
SELECT foo,
bar,
dbo.QD24K_GRSM.NAME
FROM INSERTED I
LEFT JOIN dbo.QD24K_GRSM
ON QD24K_GRSM.Shape.STContains(I.SHAPE) = 1/* Will insert additional
rows if more than one match*/
END

Answer 2

I would suggest to create trigger "Instead of Insert" rather than "After Insert".
That way you can modify newly inserted rows as you need before actually inserting them in target table.
And there is absolutely no need to join "inserted" table with target table.
Something like this (syntax might be wrong as not actually run that code):
CREATE TRIGGER [dbo].[update_location_topo_name]
on [dbo].[TBL_LOCATIONS]
Instead Of insert
AS
BEGIN
Insert Into TBL_LOCATIONS (...., TOPO_NAME)
Select ..., CrTable.NAME as TOPO_NAME
From Inserted Cross Apply
(
Select top 1 QD24K_GRSM.NAME from QD24K_GRSM where
QD24K_GRSM.Shape.STContains(Inserted.SHAPE) = 1
) as CrTable
END