Atlas · Lesotho / Methodologies
METHODOLOGIES

Net change versus gross loss and gain

Net change is the difference in total area between two years; gains and losses inside it can hide each other.

Net change = area in the later year − area in the earlier year, in hectares or percent.

Example. A country has 1,000 ha in 1996 and 1,020 ha in 2025: net change is +20 ha (+2%). That could mean nothing happened, or it could mean 200 ha were lost and 220 ha regrew elsewhere.

Gross loss and gross gain need an overlay of the two maps to see which pixels changed. The Global Mangrove Watch authors publish "change from 1996" products with gain and loss separately; Atlas has archived the time series and will add overlay analysis later.

Why net still helps

  • It is robust: it does not depend on exact alignment between years.
  • It is comparable between countries.
  • It is what the authors' official country statistics report for extent.

Why you must be careful

  • Real loss is usually larger than net loss.
  • Very small net changes (under a percent or two) are within the uncertainty of 30 m mapping.
  • Atlas never mixes v3 and v4.1 areas in one net-change figure.
Limits. Net figures cannot say where change happened.

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