I see that the draft revised FAA Heliport Design Advisory Circular - AC 150/5390-2C - is out for comment; it now includes use of the TD/PM circle, for accuracy in positioning the helicopter.
Interestingly, there is an objection to use of the touchdown positioning marker (or touchdown positioning circle - why change the name?) by Rex Alexander - president of the National EMS Pilots Association (NEMSPA) - stating that whilst it may work at ground level, it might not be so useful on a rooftop pad. Not sure I understand the logic of the argument as it will provide accurate placing, and access for loading should not be adversely effected.
For information and arguments for use of the TDPM(C), see the paper previously pointed to in an earlier post:
http://www.helidecks.org/download%20...r%20V1%209.pdf
Jim