Comparing MODIS to VIIRS harmonization methods

Nine ways to join the MODIS and VIIRS fire records compared on real data: annual totals, leave-one-year-out and forward tests, and the months held out against Aqua.

One fire record from 2001 to now: NASA MODIS and VIIRS active-fire detections harmonized into a single monthly calendar of Aqua-equivalent fire-days.

Calibrated regions

Key facts

Why can't MODIS and VIIRS fire counts be joined directly?

VIIRS has 375 m pixels against MODIS's 1 km, so it reports 2–4 times as many detections. Joining the records makes fire activity appear to jump in 2012, when VIIRS starts; the error of a direct join is 163–355% per month.

What is a fire-day?

A 2 km × 2 km square with at least one fire detection on a given local day, counted once no matter how many rows, passes or satellites saw it. It removes most of the pixel-size difference between the sensors.

How are VIIRS counts converted to MODIS units?

For each region, the average of two estimates: a per-month Aqua/VIIRS ratio and a seasonal curve of the chance Aqua also sees a square given the fire radiative power (FRP) VIIRS measured, learned from 2012–2021 with recent years weighted more.

How accurate is the harmonized calendar?

About 10% average monthly error (WAPE) when Aqua is hidden and predicted from VIIRS, against about 7% between two identical VIIRS satellites, the floor this data allows. A fixed multiplier gives about 19%.

Why does the calendar use VIIRS after 2022?

Terra and Aqua began drifting in 2022: Aqua now passes near 15:15 instead of 13:30, when fires burn more, so its counts are no longer comparable with its own past. VIIRS stayed at about 13:20.

Where does the data come from?

NASA FIRMS (Fire Information for Resource Management System): the yearly archives for MODIS and VIIRS, and the FIRMS API for recent days.

Data: NASA FIRMS. Open source (MIT). Made by feeef.