11.12.4 IJK Grid Origin

A frequently-asked question is: How do I determine the location of the origin of an IJK grid in the field of view? Generally, this is a function of the data, not of the schema, but certain inferences can be made from the grid parity and from the specification of the K direction. A number of observations can be made:

• In practice, almost all grids have their origin on the left of the field of view, which specifies the direction of the I axis. Origins on the right, are possible, but are extremely rare in practice.
• The K direction can be used to determine if the origin is on the upper face of a model (K direction = down) or on the lower face of a model (K direction = up).

• The direction of the J axis is determined uniquely by the grid parity, given the directions of the I and K axes.

Calculation of the Jacobian for volume and the transmissibility for flux, are based on the triple-cross product of the cell or grid tangent vectors. To ensure that they are correctly calculated, they also need to know the parity of the 3D coordinate reference system.