I apologize if this has been already covered in other discussions or publications, but it seemed like a cheap and novel way to defog you camera's lens/LCD screen/viewfinder- or your survey instruments.
We were working in a very hot cave in Puerto Rico last week(hot, like 90F), and we had trouble with instrument fogging in the past. A friend pointed out to me that the root cause of lens fogging was that the lens was cooler than the surrounding air.
I had a brief flash of inspiration(or perhaps it was gas), and realized that a heat pack could bring the instrument(or lens, etc.) up to a an equal or slightly higher temp than the surrounding air.
I tried two different heat packs- "Toasty Toes", and a generic hand warmer. The Toasty Toes have an adhesive backing that allowed me to attach it to the lenscap, as well as the back of my Suunto Tandem. The Toasty Toes get up to about 100F, and the other hand warmers claim to get up to about 150F.
The Toasty Toe on instrument method worked perfectly. I still had fogging on my camera lens, though. To solve that, I activated one of the generic hand warmers and held the lens surface over it for several seconds- voila! No more fog.
As to keeping comfortable in a 90F cave- I recommend a Swaygo pack filled with ice, and a ziploc bag of ice in your helmet, replenished every 10-15 minutes from the stash in your Swaygo. Ahhhh....