# GISela Area & Length

> True area, perimeter and length of polygons and lines on the WGS 84 ellipsoid, per feature. Measures every polygon and line of GeoJSON, KML, GPX, CSV/WKT or shapefile data on the WGS 84 ellipsoid with Karney’s geodesic algorithms — the same numbers as GISela’s measure tool and attribute fields. Holes are subtracted; results in metric or imperial units.

Part of usefulweb.site (https://usefulweb.site/). Part of [GISela](https://gisela.usefulweb.site/) (ownerApp: gisela). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://giselaarealength.usefulweb.site/ (interactive tool at https://giselaarealength.usefulweb.site/tool)
- JSON API: POST https://giselaarealength.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://giselaarealength.usefulweb.site/api/v1/tool)
- OpenAPI: https://giselaarealength.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://giselaarealength.usefulweb.site/mcp - tool `gisela_area_length`

## Inputs
- data (textarea): Data - GeoJSON, TopoJSON, KML, GPX, CSV (lat/lon, x/y or a WKT column) or WKT lines. Or upload a file below.
- file (file): File (optional) - Used instead of the pasted text. Shapefiles: put .shp, .dbf and .prj into one ZIP.
- from (text): Coordinate system of the data - “auto” reads it from the file (.prj, GeoJSON crs); otherwise an EPSG code such as 2056, 25832 or 27700. Without information the data is read as WGS 84 longitude/latitude.
- units (select): Units

## Method
- Polygon area and perimeter: Karney’s geodesic polygon algorithm (edges are geodesics on WGS 84), outer rings minus holes. Line length: sum of geodesic distances (Karney inverse problem).
- Input in a projected coordinate system is first converted to WGS 84 longitude/latitude, so the result does not depend on the map projection.
- Units: 1 ha = 10,000 m², 1 acre = 4046.8564224 m², 1 mi = 1609.344 m, 1 ft = 0.3048 m.

## Limits
- Areas are on the ellipsoid surface (sea level), not along the terrain.