Another thing to consider is that ice won't be the same thickness throughout the whole pond. If there is any moving water (even so slight that you don't see it moving) you will have thinner ice.
The only way to know for sure is to drill test holes and measure the ice.
A buddy last year called me over 'cause he was chopping thru the ice and said that it was 4" thick. I came over during the evening of the next day, bringing my ice auger. We scattered like a covy of quail for shore when I cut thru the ice with only 3-4 turns of the auger........
The only thing that we can figure is that due to some slow water movement in his pond, the ice was thinned out in a day in that area. Since we were in a bout 8' of water at the time, we would have been in trouble if we had gone thru.
Lesson learned: Don't take anybody's word on how thick the ice is, test it for yourself!
Right Mike??
