Transform geojson data different coordinate reference systems in Go using go-geom and go-proj

crs.go

package main

import (
	"encoding/json"
	"fmt"

	"github.com/twpayne/go-geom"
	"github.com/twpayne/go-geom/encoding/geojson"
	"github.com/twpayne/go-proj/v10"
)

const outCRS = "EPSG:4326"

var data = `{
  "type": "FeatureCollection",
  "crs": {
    "type": "name",
    "properties": {
      "name": "EPSG:3857"
    }
  },
  "features": [
    {
      "type": "Feature",
      "geometry": {
        "type": "Point",
        "coordinates": [0, 0]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [4e6, -2e6],
          [8e6, 2e6]
        ]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [4e6, 2e6],
          [8e6, -2e6]
        ]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [-5e6, -1e6],
            [-3e6, -1e6],
            [-4e6, 1e6],
            [-5e6, -1e6]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "MultiLineString",
        "coordinates": [
          [
            [-1e6, -7.5e5],
            [-1e6, 7.5e5]
          ],
          [
            [1e6, -7.5e5],
            [1e6, 7.5e5]
          ],
          [
            [-7.5e5, -1e6],
            [7.5e5, -1e6]
          ],
          [
            [-7.5e5, 1e6],
            [7.5e5, 1e6]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "MultiPolygon",
        "coordinates": [
          [
            [
              [-5e6, 6e6],
              [-3e6, 6e6],
              [-3e6, 8e6],
              [-5e6, 8e6],
              [-5e6, 6e6]
            ]
          ],
          [
            [
              [-2e6, 6e6],
              [0, 6e6],
              [0, 8e6],
              [-2e6, 8e6],
              [-2e6, 6e6]
            ]
          ],
          [
            [
              [1e6, 6e6],
              [3e6, 6e6],
              [3e6, 8e6],
              [1e6, 8e6],
              [1e6, 6e6]
            ]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "geometry": {
        "type": "GeometryCollection",
        "geometries": [
          {
            "type": "LineString",
            "coordinates": [
              [-5e6, -5e6],
              [0, -5e6]
            ]
          },
          {
            "type": "Point",
            "coordinates": [4e6, -5e6]
          },
          {
            "type": "Polygon",
            "coordinates": [
              [
                [1e6, -6e6],
                [3e6, -6e6],
                [2e6, -4e6],
                [1e6, -6e6]
              ]
            ]
          }
        ]
      }
    }
  ]
}`

type Properties struct {
	Name string `json:"name"`
}

type CRS struct {
	Type       string `json:"type"`
	Properties Properties
}

type FeatureCollection struct {
	Type     string            `json:"type"`
	CRS      CRS               `json:"crs"`
	Features []geojson.Feature `json:"features"`
}

func transform(g geom.T, pj *proj.PJ) {
	switch g.(type) {
	case *geom.Point, *geom.LineString, *geom.LinearRing, *geom.Polygon, *geom.MultiPoint, *geom.MultiLineString, *geom.MultiPolygon:
		geom.TransformInPlace(g, func(c geom.Coord) {
			coords, err := pj.Forward(proj.Coord{c[0], c[1]})
			if err != nil {
				panic(err)
			}
			c[0], c[1] = coords.X(), coords.Y()
		})
	case *geom.GeometryCollection:
		gc := g.(*geom.GeometryCollection)
		for _, g := range gc.Geoms() {
			transform(g, pj)
		}
	default:
		panic("unsupported geometry type")
	}
}

func main() {
	var fc FeatureCollection
	err := json.Unmarshal([]byte(data), &fc)
	pj, err := proj.NewCRSToCRS(fc.CRS.Properties.Name, outCRS, nil)
	if err != nil {
		panic(err)
	}
	for _, f := range fc.Features {
		transform(f.Geometry, pj)
	}
	fc.CRS.Properties.Name = outCRS
	out, err := json.Marshal(fc)
	if err != nil {
		panic(err)
	}
	fmt.Println(string(out))
}