estimating direction and distance of gibbon calls

["Marc Myers" primatemarc]

Anyone have experience locating animals by measuring distance and direction of 
their calls in natural environments? Gibbons often live in difficult terrain, 
at low densities and in small family groups. They can be hard to see in their 
native environment and are critically endangered through the majority of their 
range. Making population estimates is important in conservation planning, when 
advising local governments on forestry use and establishing risks to 
extinction. Gibbons have famously loud and expressive calls, typically sounded 
first thing in the morning. Populations are presently estimated based on the 
frequency, direction and apparent distance of the calls. Of course even 
experienced listeners are not very good at estimating distance and as a 
consequence population estimates vary wildly from environment to environment 
and researcher to researcher. Any thoughts?