Units and coordinate systems

SWASH expects all quantities that are given by the user to be expressed in S.I. units:
m, kg, s and composites of these with accepted compounds, such as Newton
(N) and Pascal (Pa). Consequently, the water level and water depth are in m,
flow velocity in m/s, etc. For __wind, (incident) wave and flow direction__ both the
Cartesian and *a* nautical convention can be used (see below). Directions and spherical
coordinates are in degrees (^{0}) and not in radians.

SWASH operates either in a Cartesian coordinate system or in a spherical coordinate system, i.e.
in a flat plane or on a spherical Earth. In the Cartesian system, all geographic locations and
orientations in SWASH, e.g. for the bottom grid or for output points, are defined in one common
Cartesian coordinate system with origin (0,0) by definition. This __geographic origin
may be chosen totally arbitrarily by the user__. In the spherical system, all geographic locations
and orientations in SWASH, e.g. for the bottom grid or for output points, are defined in geographic
longitude and latitude. Both coordinate systems are designated in this manual as the
__problem coordinate system__.

In the input and output of SWASH the __direction of wind, (incident) wave and current__ are defined according to either

- the
__Cartesian convention__, i.e. the direction to where the vector points, measured counterclockwise from the positive*x*-axis of this system (in degrees) or - a
__nautical convention__(there are more such conventions), i.e. the direction where the wind or the waves come__from__, measured clockwise from geographic North.

For regular grids, i.e. uniform and rectangular, Figure 4.1 (in Section 4.5) shows how the locations of the various grids are determined with respect to the problem coordinates. All grid points of curvilinear grids are relative to the problem coordinate system.