# GISela Coordinate Reprojection

> Convert GeoJSON, KML, GPX, CSV or shapefiles between WGS 84, UTM, LV95, British Grid and more. Reprojects vector data between coordinate systems with GISela’s own projection maths: WGS 84, ETRS89, Web Mercator, every UTM zone, ETRS89-LAEA, German Gauss–Krüger zones, Swiss LV95/LV03 and the British National Grid. Reads the coordinate system from .prj files and GeoJSON, and shows sample coordinates before and after.

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

## 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.
- to (select): Convert to - Other zones: type the code below.
- toOther (text): Or another EPSG code - For example 32633 (WGS 84 / UTM 33N) or 25831. Overrides the list when filled.
- format (select): Output format

## Method
- The data is read into WGS 84 longitude/latitude, then projected to the target system. Readers: GeoJSON (also the old “crs” member), TopoJSON (arcs decoded with the transform), KML/KMZ, GPX, CSV with coordinate or WKT columns, WKT and zipped shapefiles with .prj detection by EPSG authority, name or Transverse Mercator parameters.
- Transverse Mercator (UTM, Gauss–Krüger, British National Grid) uses Karney’s 6th-order Krüger series on the system’s own ellipsoid; ETRS89-LAEA uses the ellipsoidal Lambert azimuthal equal-area formulas of IOGP Guidance Note 7-2; LV95/LV03 use swisstopo’s rigorous oblique Mercator formulas; Web Mercator uses the spherical formulas on the WGS 84 radius.
- Datum changes (OSGB 1936, DHDN, CH1903/CH1903+) go through geocentric coordinates with a published 7-parameter Helmert transformation. ETRS89 is treated as equal to WGS 84 (well under 1 m in Europe).
- Outputs are rounded to 8 decimals for degrees and 3 decimals (millimetres) for metres.

## Limits
- Helmert datum shifts are good to about 1–5 m. For survey work use the national grid transformations (OSTN15, BeTA2007, FINELTRA).