Table 1 Comparison of geolocations based on irradiance, magnetic intensity, and sea surface temperature

Sensor cost

Low

Medium–high

Low–medium

Sensor calibration

Available from the factory

Requires special equipment and magnetically undisturbed site or facility

Available from the factory

Field characteristics

Naturally smooth, varies with time of year, less of gradient with latitude at equinox than solstice

Smooth main field changes slowly over years (intensity, inclination, and declination) with overlaid crustal and man-made anomalies

Highly dynamic over time. High gradients in mid-latitudes, low gradients near equator and high latitudes.

Main disturbance

Tag fouling, water clarity, diving depth, sea state, tag orientation if directional; cosine correction or wrap-around collector avoids directionality

Crustal anomalies associated with magnetic lineations and volcanic seamounts and islands, man-made anomalies of wrecks, rare solar storms

Diurnal variations and vertical stratification. Errors arising from small, hot or cold regions or uncertain depth profiles not captured by the model but influencing the tag measurement

Availability at depth

Within the euphotic zone, <200 of meters depending on water clarity

Present at all depths. Main field uniform at surface, anomalies increase with depth or in proximity to magnetized bodies

Mixed layer to a depth captured by model

Typical model error

Civil day length computed using astronomical equations. Negligible modeling error

Mean error of 152 nT for WMM model. This paper uses site-specific maximum anomaly in 0.25° × 0.25° cells per WDMAM

Error of 0.6 °C mean for OSTIA foundation Sea surface (temperature at the indeterminate depth free of diurnal variations)

Typical measurement error

±4 min typical error for measuring equivalent points on the sunrise and sunset irradiance curve. This corresponds to approx. +100/−50% irradiance disturbance due to factors such as weather changes, turbidity changes, animal depth changes and in situ bias compensation

A 300-nT residual error after in situ bias compensation

0.1 °C after in situ bias compensation