6 ms·
You are linking to the magnetic anomaly grid, which is primarily intended for geophysical research and modeling local variations. For a basic compass app, you
by xavieralexandre 8mo ago
You are linking to the magnetic anomaly grid, which is primarily intended for geophysical research and modeling local variations.
For a basic compass app, you can rely on the World Magnetic Model (WMM) instead:
https://www.ncei.noaa.gov/products/world-magnetic-model https://www.ncei.noaa.gov/products/world-magnetic-model
From that, you can pre-compute a low-resolution declination grid. NOAA even provides one here:
https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#igrfgrid https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#i.... That’s only a few KB of data and requires just a simple declination subtraction based on lat/lon, similar to how it’s handled on nautical charts. This works fine as long as you stay away from the poles (and Alaska).
- jacquesm 8mo agoNear ore deposits the map resolution needs to go up considerable beyond the 'few KB of data' to keep it working. Nautical charts are simpler because there is as a rule a bit more distance between the compass and any anomalies but in certain mineral rich areas of the world that definitely is not the case.
- xavieralexandre 8mo agoThanks. I was mostly basing my answer on my experience with nautical charts, where declination corrections are typically uniform over relatively large areas and local anomalies are less of a concern. I hadn’t considered how much more variable the field can be in terrestrial use.