When measurements are made from a lattice gas simulation it must be in
terms of standard units which are fundamental to the lattice gas model.
The basic unit of time is one time-step, the basic unit of length is a
lattice unit (lu) which is the distance between two neighbouring sites
on the grid. The basic unit of mass is the mass of a particle which is
taken to be unity. Other quantities, such as velocity, are measured in
units derived from these basic units, velocity being measured in
lattice units per time-step. The Reynolds number, which is a dimensionless quantity, can be calculated from equation (3.41)
and the simulation compared to a flow in the
real world with the same Reynolds number.
As stated above the basic length scale is the lattice
unit (lu), the distance between two neighbouring sites. It is sometimes
convenient when describing the grid to refer to the number of
sites in each direction. Thus a 512 by 512 grid is a grid with 512
sites in each direction, the grid is 512 by 443 lu since the
separation between rows of sites is 
lu.