In addition, If u can post please following:

select * from all_tables where table_name = 'GRID_MASTER' and owner = 'SPARTIAL';

select * from all_indexes where table_name = 'GRID_MASTER' and owner = 'SPARTIAL';

May be (but not absolutly needed)
analize index III validate structure;
select * from index_stat;
for index on the column GRID_LML.

Then, in sqlplus do following:

set autotrace TRACEONLY EXPLAIN STATISTICS;
select grid_name, grid_lvl2_code From spatial.grid_master Where grid_lvl = 2 and
grid_priority = 1;

and post result of trace of this query.
If we'll have this information, I hope, we can explain "why oracle optimazer use FULL SCAN TABLE?"