If the data file is a standard output file from MIKE 21 HD, i.e. containing the items H Water depth, P-flux and Q-flux and with Data type = 1, you get the additional option of selecting one or more of the following derived items: W Surface Elevation, S Current Speed, and Vx and Vy (current velocities). These items are calculated from h, p and q by the program as described below.
NOTE: The calculations of derived items are based on the assumption of the absolute delete value being very small, e.g. -10-30. The use of other delete values may lead to incorrect derived results in grid cells next to cells holding delete values.
Figure 25.1 shows the definition of the grid items with respect to the position in the grid. The saved h, p and q values in the file are positioned in the grid as shown to the left, the derived items are all positioned in the center point of the grid element as shown to the right.
Figure 25.1 Position in grid for items.
Left: Water depth (h) and flux components (p,q)
Right: Derived current speed (S) and velocity components (Vx,Vy)
The equations used for the calculation of the derived items are as follows:
(25.1) 
(25.2) 
(25.3) 
(25.4) 
where zj,k is the bathymetry value stored in the static item.
