Data Interpolation:

The GPS position and depth data are collected in the laptop computer controlling the sonar system. Each data string is tagged with the computer time. The 'GL' or geographic location string is tagged at 20:52:01.30 in this example while the 'D1' or depth string has a slightly later time at 20:52:01.60, a difference of 0.30 seconds. The 'Q1' line begins the series of record which indicate strength of each recorded echo return in the sonar pulse.

The time offset is manifested as a position error since the boat continues to travel forward through the water throughout the data collection. The GPS position is interpolated along the track to match up with the 'position' of the depth reading. Another error, not indicated here, is the offset due to the lag time for the receiver to calculate and update its position after receiving the GPS satellite transmission.