# GISela Buffer

> Buffer zones around points, lines or polygons at a distance in metres, merged or one per feature. Creates buffer polygons at a true distance in metres around every feature of GeoJSON, KML, GPX, CSV or shapefile data: circles around points, corridors around lines, grown or shrunk polygons. Overlapping zones can be merged into one area. Returns GeoJSON, the ellipsoidal area of each zone and a map preview.

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://giselabuffer.usefulweb.site/ (interactive tool at https://giselabuffer.usefulweb.site/tool)
- JSON API: POST https://giselabuffer.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://giselabuffer.usefulweb.site/api/v1/tool)
- OpenAPI: https://giselabuffer.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://giselabuffer.usefulweb.site/mcp - tool `gisela_buffer`

## 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.
- distance (number): Distance (metres) - Negative values shrink polygons.
- segments (integer): Segments per circle - More segments = rounder curves, bigger files.
- dissolve (boolean): Merge overlapping zones into one

## Method
- Each feature (or all features when merging) is projected to a local Transverse Mercator plane centred on the data, so distances are in metres with a scale error below 0.03 % within 250 km of the centre.
- Points become circles; every line segment and polygon edge becomes a stadium (two half circles joined by straight sides); the stadiums are merged with GISela’s polygon overlay (exact edge splitting on a snapped grid, even-odd classification). A positive polygon buffer is the polygon united with its edge band, a negative one the polygon minus the band.
- Results are converted back to WGS 84. Areas and perimeters are ellipsoidal (Karney’s geodesic polygon algorithm on WGS 84).

## Limits
- Circles are polygons with the chosen number of segments, so the area is slightly smaller than the true disc (0.6 % at 32 segments).