Display & Interface Chart Overlay

Chart overlay is a display feature that superimposes sonar imaging data — typically side imaging or down imaging returns — directly onto your GPS chart. Instead of viewing sonar and map data on separate panels, chart overlay merges them into a single view that shows underwater structure positioned on the actual map at the GPS coordinates where it was scanned.

This feature is transformative for structure fishing. As you idle across a flat, the side imaging data paints onto the chart in real time, showing you exactly where brush piles, rock formations, ditches, and other fish-holding structure sit relative to depth contours, shoreline, and your saved waypoints. After a scanning pass, you have a visual map of underwater cover that you can reference and navigate back to with precision.

Humminbird pioneered this feature with their StructureMap technology. Garmin and Lowrance offer similar capabilities on their premium units. The quality of the overlay depends on GPS accuracy, boat speed consistency during the scan, and the resolution of your imaging transducer. Slow, steady passes at 3 to 5 mph produce the cleanest overlays.

Chart overlay data can be saved to memory cards and reloaded on future trips, building a comprehensive structural map over time. Some anglers dedicate entire outings to scanning and mapping, knowing that the structural intelligence gathered will pay dividends across many future fishing trips. This is especially valuable on large reservoirs and lakes where finding productive structure is the primary challenge.