For standard flat plate collector the incidence angle modifier IAM is assumed to be the same in the transverse and the longitudinal direction.
For vollector with an asymmetric IAM, separate IAMs need to be determined in two or more of the quadrant from east to west (transversal IAM) and from north to south (longitudinal IAM).
Quality Assurance in Solar Heating and Cooling Technology. A Guido to the Standard EN 12975