Say you’re trolling a plain that ranges from 1,000 feet to 1,020 feet over a distance of several hundred yards. On a standard chart, that incline might look gradual, but as you pass over it with your sonar locked on the bottom, it records every ping and its corresponding GPS location to a micro-SD card. Upload the data to a compatible mapping system and, in less time than you can really guess, the system will churn out an incredibly detailed chart of the bathymetric features from your trip. Imagine looking it over and noticing that what you thought was a gradual incline from one depth to the other actually happened all at once, stepping from 1,000 to 1,020 feet somewhere in the middle of the plain. No wonder there’s an upwelling there and a weed line over it that produced a school of gaffer mahimahi!