Very interesting comment on data. Here are 2 examples of other difficulties
one may encounter.
For instance: a label says: collected on Rd 18 from X to Z going southwards
at KM 7 in 1980(no GPS available)In the meantime the road has been rebuild
and does not follow the same path. How can anyone spot exactly the place at
a later date without carefully checking where the old rd was going tru?
Another example about self collecting snails in a Filippino village. Once
the kids see you pick up something, they will help you collect some more.
Since they have to walk (but they could also take a tricycle without your
knowing it)you can therefore assume more or less the collecting area,
considering the time they have been out of sight.One problem of locality
was raised the next day when some other or same kid brought me a bag of
same helicostyla species. I imagined wrongly they were from the same
place.No , they were from another island. I found out after noticing that
the average size of said helicostyla presented an average much smaller size
and it is only after questionning the kid in Tagalog that I learned about
the name of the island which was only a few miles away.
Another example: while doing some island hopping in the Calamian
Archipelago, do not expect the natives that carry you on their boat to tell
you exactly where they are . They could not even show you on a map where
you are.For those of you who know such archipelagos with tenths of
different islands, I hope you will agree there is definitely a problem
(communication possibilities)to  know exactlyw where you collect if you
have no GPS.
As far as PI localities are concerned, do not expect any local diver to
tell you exactly where his leucodon exactly come from. And certainly do not
expect him to take you to the place. Plenty of localities are just guesses.