I own a garmin etrex venture hc and whilst out looking for a cache it was counting down 50,40,30 and all off a sudden it started to count upwards,30,40,50 etc etc,so i turned around and walked back,50,40,30 so i stopped at the point where it would of counted up and searched around until i found the cache.There was no tree cover directly over my head but a few trees 5 metres to my side,could these trees have affected my satellite reception?.A friend of mine has a facility on his garmin to calibrate it,do i need or can i calibrate mine?