Gehajake , I always say " believe the graph " .. there might be a way to calibrate them and I am 7 years under my belt with a live scope and to this day have never had a reason to question it , I am maybe 60 miles south of you and Friday was out on my 2.5 acre pond and i did have 49 degree water on the lower side and 53 on the north west corner ( about 7 feet deep all the way ) , its been WINDY AND SUNNY the last week or so so depending on your depth and or mud bottom / rock or whatever I would believe that graph