Std, the texture direction, is determined by the APSDF and is a measure of the angular direction of the dominant lay comprising a surface. Std is defined relative to the Y axis. Thus a surface with a lay along the Y axis will return a Std of 0 deg.
(Note that Std is not strictly defined in the ISO 25178-2 but was established earlier in the research which contributed to ISO 25178-2. Future sections of ISO 25178-2 may address parameters such as Std)
Std= Major direction of lay derived from APSDF
Since this surface is spatially isotropic there is no lay and thus Std is indeterminate.
Std is useful in determining the lay direction of a surface relative to a datum by positioning the part in the instrument in a known orientation. In some applications such as sealing, a subtle change in the surface texture direction may lead to adverse conditions. Std may also be used to detect the presence of a preliminary surface modification process (e.g. turning) which is to be removed by a subsequent operation (e.g. grinding followed by tumbling).