CrowdMag is a free iPhone app developed by the National Oceanic and Atmospheric Administration. It allows users to measure the local magnetic field using their smartphones. With CrowdMag, you can view the magnetic field data as a graph or a map in nanotesla units. The app measures the Z (downward component), H (horizontal intensity), and F (total intensity) magnetic field components.

One of the standout features of CrowdMag is its Flight Mode, which enables users to contribute to scientific research by measuring the magnetic field while flying. By providing your itinerary on the app, you can turn your flight into a scientific expedition. The app also provides a step-by-step tutorial for measuring data during flights.

CrowdMag is not limited to flying; it can be used for measuring magnetic data during outdoor activities or for conducting your own experiments. You can save your measurements as "magtivities" and export them as a CSV file for further analysis. The app also features a magnetic calculator that provides magvar, the magnetic field's dip angle, total magnetic field, and other magnetic field components based on the latest World Magnetic Model (WMM2020).

Additionally, CrowdMag allows users to customize recording frequency and location accuracy, view generalized crowdsourced magnetic data from other users, and export data via email or Google Drive. The app also includes a live magnetic compass with both true and magnetic north displays, as well as a 3D rendering option.

Overall, CrowdMag is a versatile and user-friendly app that not only allows users to measure the magnetic field but also contributes to scientific research in understanding Earth's magnetic field. Whether you're an outdoor enthusiast, a curious individual, or a scientist, CrowdMag offers valuable tools for exploring and analyzing the magnetic field.