Atlas · Malawi / Methodologies
METHODOLOGIES

Geodesic area and perimeter

How hectares are computed from polygon geometry.

Intermediate 1 min read areaPostGISunits

Source layers report area in square degrees or in an unstated projection. Atlas ignores those fields and recomputes area from the stored polygon using PostGIS on the WGS 84 ellipsoid (ST_Area(geom::geography)), converted to hectares (1 ha = 10,000 m²). Perimeter is computed the same way in metres.

Method

  1. Repair invalid geometries with ST_MakeValid (the number repaired is recorded per import run).
  2. Cast to geography and compute area/length on the ellipsoid.
  3. Convert: m² ÷ 10,000 = ha.
  4. Store as area_ha_geodesic and perimeter_m next to the untouched original attributes.
Limits. Accuracy is limited by the source digitising scale. Very small slivers can be mapping artefacts.

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