The visibility is something that is calculated for publication after a DA has been derived.
You can't calculate a visibility requirement if you don't know the MDA/DA. Once the visibility required is calculated there is then an adjustment for HIAL (if there are any). So no, the DA is not adjusted to meet a visibility requirement.
Again the DA is calculated solely based on the obstacle environment. Once the DA has been sorted, then a visibility required is calculated.
As for the Sydney ILS's there is an allowance for 200ft shipping passing to pass through the final approach area. It appears this is why the minima is as it is.
Last edited by alphacentauri; 6th Feb 2013 at 04:29.