We didn't get GPS in PNG until about 91 or 92.
Before that we were route endorsed to each new strip until such time as we had enough experience in various parts of PNG and then gained a CAO28 exemption.
Basically this meant you learned PNG in small pieces as you were based here and there over the first couple of years.
The essential level of knowledge to safely fly around was akin to that you would have of your home suburb when driving your car. In bad, but still flyable, weather a GPS is of very limited value in PNG. I remember taking a chap new to PNG from Moresby to Lae in my C185 years ago. He had one of the then very new a flash Garmin 100s and set it up with the aerial suction capped to the windscreen. The weather was crap so I tracked NW from POM to the saw mountain at the southern end of the Tua river and headed north from there. The highest altitude we saw on the altimeter that day was 4800'....and that was not until crossing that ridge between Menyamya and Tsile tsile. After that trip he packed away his new fangled gadget and said he might leave it packed away for 6 mths or so.