Fig. 5

Differences between measured values from vessel survey of study area (see Additional file 1: Figure S1-4A) and coarse-scale mapped values vs. NAMAG anomaly magnitude value for each coarse-scale grid cell. a Raw differences between measured and mapped values. b Root mean square (rms) differences between measured and mapped within each coarse-scale grid cell. Blue line indicates predicted values from the GAM of map difference vs. anomaly level. Red dashed vertical lines indicate division into three groups based on NAMAG anomaly level: < 150 nT, 150–300 nT, and > 300 nT. Black horizontal lines indicate 80% quantile values of rms difference within each of these bins used to specify variance used by anomaly level (see Fig. 4d). Large negative anomalies were observed in the vicinity of the fuel dock near Bartlett cove