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Navionics Boating: GPS and Cellular Data Usage
Location Services allows location-dependent apps and websites to use information from cellular, Wi-Fi, and GPS1 networks to determine your approximate location. If your device does not have a GPS fix or does not include a GPS receiver, it can determine your location using crowd-sourced Wi-Fi (Hotspot) and cell towers (if available).
The GPS itself does not use data, it merely receives satellite signals independent of cell service. The device actually uses a GPS, meaning it receives help from the cell carrier - but that help uses the voice network, and you aren't charged for it. So, there's no charge for the GPS localization alone.
Viewing the maps and navigating does not use up cellular data. Certain features, such as wind overlay or Sync My Data will use cellular data if a Wi-Fi connection is not present.
1 Some mobile devices do not have an internal GPS receiver.