I am having problems when I started to use a zero line chip. The reading doesn't show up in various colors like when I was recording to the Helix built in memory. Now in shows a dark blue or black trail when recording to the zero line chip and the contour numbers are all over the place ( 500 to 900). The lake I am mapping is maybe 100 feet at the most. Everything was working fine before I began the zero line chip. Can the built in memory be erased?