GPS, compass and a hybrid map made from topo maps and aerial photographs.
So what do you do when you run out of people to ask? You go out and you seek more information. I was very fortunate to have as mentors Gary and Kay Walten who did a lot of the original exploration of the caves we dive every day (much of Ponderosa, the Pit, Chac Mool, Yax Chen, the list goes on). Aside from teaching me the 'ropes' (really, the lines) of exploring and surveying underwater they taught me the importance of seeking out information. Topographical maps and aerial photos of this area are easily available, and before the advent of Google Earth they were all that we had to show us where potential cenotes might be. I didn't know this until Gary and Kay told me. This knowledge transformed the way we look for cenotes.
The topo maps for this area are in fact derived from the aerial photographs by someone who has probably never set foot on the ground here. The topo maps are referenced to real space with UTM or Lat Long coordinates, yet they lack the detail of the aerial photographs which have no reference what-so-ever. So in the beginning I had to make hybrid maps that combined both the reference system of the topo maps and the detail of the aerial photographs. I could then derive theoretical waypoints for where cenotes 'should' be and then hike out to find them. I could also use a scale ruler (1:75,000) and compass on an aerial photograph to measure distance and azimuth from a known point to point I wanted to travel to. I started with big targets that were hard to miss and then progressively sought smaller and smaller targets. I really put this to the test on a large tract of land directly behind Playa del Carmen in the early 2000's. The system worked very well, we were able to find @90% of the target cenotes we were looking for.
Making a hybrid map from a 1:20,000 scale aerial photograph.
In the back of my mind though I was always wondering how could I combine topo maps, aerial photographs and my GPS into one solid system? Next time I will tell you what I did...
Good to see it actually works!