A join will work out for you.

<pre>INSERT INTO AnotherTable
SELECT a.GridCode, a.State, a.Area
FROM Table
JOIN (SELECT GridCode, max(Area) as Area
FROM Table
GROUP BY GridCode) b ON a.GridCode = b.GridCode AND a.Area = b.Area</pre>

If each GridCode contains only one state (I assumed that this was not the case).

<pre>INSERT INTO AnotherTable
SELECT GridCode, State, max(Area)
FROM Table
GROUP BY GridCode, State</pre>

The first example I gave is the more precise example.