Atlas · Ghana
/
Methodologies
METHODOLOGIES
Geodesic area and perimeter
How hectares are computed from polygon geometry.
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
- Repair invalid geometries with
ST_MakeValid(the number repaired is recorded per import run). - Cast to
geographyand compute area/length on the ellipsoid. - Convert: m² ÷ 10,000 = ha.
- Store as
area_ha_geodesicandperimeter_mnext to the untouched original attributes.
Limits. Accuracy is limited by the source digitising scale. Very small slivers can be mapping artefacts.
Related reading
More in Methodologies
- Field plots versus the map
- Repairing invalid geometry
- Diversity: Shannon index and evenness
- How Global Mangrove Watch maps are made
- Versions and cumulative updates
- Spatial statistics: Moran's I and Getis-Ord Gi*
- How recommendations and briefs are generated
- Why Atlas totals are 2.3% higher than the official figures
Still have a question?
The AI analyst answers from these articles and from the data.
Ask Atlas