Agree with MNII, you count the bars - simple, and done in a millisecond, providing you know what you need to see (i.e. the number of bars) and not have to do calculations - e.g. 4 bars X 150 mtrs = etc,etc.
Fatalbert/Velikiye Luki, pretty much it.
compylot/icarus, best better think your replies before posting.