x_mapping
xkey max_x
1 1.0
2 3.3

y_mapping
ykey max_y
1 2.5
2 3.5

  my $xkey = $dbh->selectrow_array("SELECT xkey FROM x_mapping WHERE ? < max_x ORDER BY max_x ASC LIMIT 1", {}, $x );
  my $ykey = $dbh->selectrow_array("SELECT ykey FROM y_mapping WHERE ? < max_y ORDER BY max_y ASC LIMIT 1", {}, $y );