Looks like std::vector::rend() does not return const_reverse_iterator.
I have got CE on the line
for( std::vector< Spot >::const_reverse_iterator it = spots.rbegin(); it != spots.rend(); ++it )
but
for( std::vector< Spot >::/*const_*/reverse_iterator it = spots.rbegin(); it != spots.rend(); ++it )
was ok.