Reference

Base functionality
add_level()	Add a level to the grtsdb
connect_db()	connect to or create an SQLite database
extract_sample()	extract the grts sample
Incorporating legacy sites
add_legacy_sites()	Add legacy sites
drop_legacy_sites()	Drop the table with legacy sites for a given level
extract_legacy_sample()	Extract the GRTS sample with legacy sites
Auxiliary functions
compact_db()	Compact the database by removing the lower levels and all indices
create_index()	Add the index to the table
has_index()	has a table the required index
is_grtsdb()	Check is an object is a grtsdb
n_level()	Calculate the required level based on a bounding box and a cellsize
which_level()	Return a vector of level number which are available in the database