I want to begin fishing a reservoir that I haven't fished since I got a couple of Helix 7 G2 Chirps (GPS/Sonar). The water level varies as much as 30 feet over the course of a year. My question is...will the units show the actual current water level each time I'm on the water without any adjustments? Thanks (BTW, I have Navionics Western Region cards in both units, and this lake is a mapped lake)