Atlas · Equatorial Guinea / Methodologies
METHODOLOGIES

Why Atlas totals are 2.3% higher than the official figures

Atlas measures polygons on the ellipsoid; the authors publish a "corrected area". The gap is a constant 2.3% in every country.

When Atlas compares its hectares with the national totals published with Global Mangrove Watch v4.1, every one of the 32 African countries is 2.3% higher in Atlas: the range is 2.3% to 2.3%.

What this tells us

  • A constant ratio is not a country-specific mistake. If borders or labels were wrong we would see different percentages for different countries.
  • The statistics file is named "corr_area", which indicates a corrected area statistic published by the authors. Atlas instead sums the geodesic area of the polygons (PostGIS ST_Area on the WGS 84 spheroid).

What it does not affect

  • Shares and rankings (each country is scaled the same).
  • Change between years (both are scaled the same).

What it may affect. If you compare an absolute number from Atlas with a number in the authors' tables, expect Atlas to be about 2.3% higher. For v3 maps the match is within 1%.

Recommendation. Quote the source of the area (Atlas measurement or official statistic) next to any number you publish.

Limits. Atlas has not confirmed the cause with the authors; the constant ratio is the evidence.

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