Triangulated irregular network (TIN) JSON format

The TIN JSON format defines the format for a triangulation model stored in a JSON file.

That file format may be used by the tinshift operation since PROJ 7.2

Note: the TIN GeoPackage file format can also be used since PROJ 9.9. It can handle arbitrarily large triangulations.

Below a minimal example, from the KKJ to ETRS89 transformation, with just a single triangle:

{
  "file_type": "triangulation_file",
  "format_version": "1.0",
  "name": "Name",
  "version": "Version",
  "publication_date": "2018-07-01T00:00:00Z",
  "license": "Creative Commons Attribution 4.0 International",
  "description": "Test triangulation",
  "authority": {
    "name": "Authority name",
    "url": "http://example.com",
    "address": "Address",
    "email": "test@example.com"
  },
  "links": [
    {
      "href": "https://example.com/about.html",
      "rel": "about",
      "type": "text/html",
      "title": "About"
    },
    {
      "href": "https://example.com/download",
      "rel": "source",
      "type": "application/zip",
      "title": "Authoritative source"
    },
    {
      "href": "https://creativecommons.org/licenses/by/4.0/",
      "rel": "license",
      "type": "text/html",
      "title": "Creative Commons Attribution 4.0 International license"
    },
    {
      "href": "https://example.com/metadata.xml",
      "rel": "metadata",
      "type": "application/xml",
      "title": " ISO 19115 XML encoded metadata regarding the deformation model"
    }
  ],
  "transformed_components": [ "horizontal" ],
  "vertices_columns": [ "source_x", "source_y", "target_x", "target_y" ],
  "triangles_columns": [ "idx_vertex1", "idx_vertex2", "idx_vertex3" ],
  "vertices": [ [2,49,2.1,49.1], [3,50,3.1,50.1], [2, 50, 2.1,50.1] ],
  "triangles": [ [0, 1, 2] ]
}

So after the generic metadata, we define the input and output CRS (informative only), and that the transformation affects horizontal components of coordinates. We name the columns of the vertices and triangles arrays. We defined the source and target coordinates of each vertex, and define a triangle by referring to the index of its vertices in the vertices array.

More formally, the specific items for the triangulation file are:

input_crs: String identifying the CRS of source coordinates in the vertices. Typically EPSG:XXXX. If the transformation is for vertical component, this should be the code for a compound CRS (can be EPSG:XXXX+YYYY where XXXX is the code of the horizontal CRS and YYYY the code of the vertical CRS). For example, for the KKJ->ETRS89 transformation, this is EPSG:2393 (KKJ / Finland Uniform Coordinate System). The input coordinates are assumed to be passed in the "normalized for visualisation" / "GIS friendly" order, that is longitude, latitude for geographic coordinates and easting, northing for projected coordinates.
output_crs: String identifying the CRS of target coordinates in the vertices. Typically EPSG:XXXX. If the transformation is for vertical component, this should be the code for a compound CRS (can be EPSG:XXXX+YYYY where XXXX is the code of the horizontal CRS and YYYY the code of the vertical CRS). For example, for the KKJ->ETRS89 transformation, this is EPSG:3067 ("ETRS89 / TM35FIN(E,N)"). The output coordinates will be returned in the "normalized for visualisation" / "GIS friendly" order, that is longitude, latitude for geographic coordinates and easting, northing for projected coordinates.
transformed_components: Array which may contain one or two strings: "horizontal" when horizontal components of the coordinates are transformed and/or "vertical" when the vertical component is transformed.
fallback_strategy: String identifying how to treat points that do not fall into any of the specified triangles. This item is available for format_version >= 1.1. Possible values are none, nearest_side and nearest_centroid. The default is none and signifies, that points that fall outside the specified triangles are not transformed. This is also the behavior for format_version before 1.1. If fallback_strategy is set to nearest_side, then points that do not fall into any triangle are transformed according to the triangle closest to them by euclidean distance. If fallback_strategy is set to nearest_centroid, then points that do not fall into any triangle are transformed according to the triangle with the closest centroid (intersection of its medians).
vertices_columns: Specify the name of the columns of the rows in the vertices array. There must be exactly as many elements in vertices_columns as in a row of vertices. The following names have a special meaning: source_x, source_y, target_x, target_y, source_z, target_z and offset_z. source_x and source_y are compulsory. source_x is for the source longitude (in degree) or easting. source_y is for the source latitude (in degree) or northing. target_x and target_y are compulsory when horizontal is specified in transformed_components. (source_z and target_z) or offset_z are compulsory when vertical is specified in transformed_components
triangles_columns: Specify the name of the columns of the rows in the triangles array. There must be exactly as many elements in triangles_columns as in a row of triangles. The following names have a special meaning: idx_vertex1, idx_vertex2, idx_vertex3. They are compulsory.
vertices: An array whose items are themselves arrays with as many columns as described in vertices_columns.
triangles: An array whose items are themselves arrays with as many columns as described in triangles_columns. The value of the idx_vertexN columns must be indices (between 0 and len(vertices-1) of items of the vertices array.

A JSON schema is available for this file format.