This was always the case in New Guinea
I wouldn't go so far as to say "always", but it was one of the first things I implemented in PNG. That would've been back around 1986 or so. It was my response to the Pans Ops requirement to cater for "mountainous areas" in a country that is basically all mountains.
I started off by looking at what other countries were doing to meet the requirement. I checked the AIP for New Zealand, Oz, the USA, Singapore and the UK. My conclusion after all that reading was that everybody was copping out on the problem (ie taking the easiest option). I couldn't do that because it would've been much too pessimistic in PNG.
I undertook a review of TAF QNHs over a 5 year period all over the country and found that our QNHs are pretty stable in a fairly narrow range for most of the time. Yes, we get some massive differences in QNH between coastal and highlands regions, but I found that they were pretty well graduated from the low QNH in coastal regions, typically around 1010 mbs, to around 1017 mbs in the Highlands.
This work resulted in the progressive increase in MOC, as explained in my previous post, to account for progressively higher obstacles.
don't forget the factor of Altimeter accuracy as vertical displacement from the QNH source increases
I account for this as well. I adopted the Australian practice of adding a value of 100 feet for QNH accuracy. This value results from the requirement for an Area QNH to be within plus or minus 3 millibars of the local QNH for all aerodromes covered by the Area QNH.
3mbs = 90 feet, which is simply rounded up to 100 feet to make the calcuation simpler.
I apply this 100 feet to all minimum altitudes, regardless of whether it is an Area Minimum Altitude, LSALT, MSA, or a limiting altitude on an approach. It is also applied to the MDA and DA for all procedures and pilots are permitted to subtract 100 feet from the MDA or DA upon receipt of the local QNH.
Of course, in common with other countries, we also require pilots to make an appropriate adjustment for PEC.
check the criteria used to create your data
The topo charts that are available in PNG are not updated very often. In fact, all of them continue to show "Edition 1", even after an update, so I am constantly worried about that.
Thus, whenever I'm in doubt as to the accuracy of the topo data, I go out and take a look. I always do that for approach and departure procedures, right up to MSA. I will only do it for LSALT or AMA if I'm uncertain - that doesn't happen very often because, after 15 years of procedure design in PNG - and a lot of flying around the country myself - I already know most of the areas that are shown appropriately on the charts.