Add FlatGeoBuf encoding

[m-mohr]

Proposed by @StefanBrand in fiboa/specification#24 and fiboa/specification#32.

All the following has been written by @StefanBrand and more can be found in fiboa/specification#24:

Some related resources:

• https://guide.cloudnativegeo.org/flatgeobuf/intro.html and linked therein:
• https://worace.works/2022/03/12/flatgeobuf-implementers-guide/

Here are some experiments based on the Austrian GSA dataset.

Tools

• GDAL/OGR 3.9
• flatc binary from https://github.com/google/flatbuffers/releases/tag/v24.3.25
• header.fbs file from https://github.com/flatgeobuf/flatgeobuf

Steps

1. Convert to FlatGeobuf:

   ogr2ogr GSA_Austria_2023-2_sample.fgb INSPIRE_SCHLAEGE_2023-2_POLYGON.gpkg \
     -limit 2 \
     -mo 'fiboa={"fiboa_version": "0.2.0", "fiboa_extensions": []}' \
     -nomd \
     -nln parcels

2. Find length of Header: hexdump --skip 8 -n 4 -d GSA_Austria_2023-2_sample.fgb -> 2084
3. Dump header into file: dd skip=12 count=2084 bs=1 if=GSA_Austria_2023-2_sample.fgb > header.bin
4. Deserialize into JSON: flatc --json --strict-json --raw-binary src/fbs/header.fbs -- header.bin

Result

This is the resulting JSON:

{
  "name": "parcels",
  "envelope": [
    672737.349999999977,
    476317.109999999986,
    675804.430000000051,
    477193.350000000035
  ],
  "geometry_type": "Polygon",
  "columns": [
    {
      "name": "FS_KENNUNG",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "SNAR_BEZEICHNUNG",
      "type": "String",
      "width": 240
    },
    {
      "name": "SL_FLAECHE_BRUTTO_HA",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "GEO_ID",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "INSPIRE_ID",
      "type": "String",
      "width": 500
    },
    {
      "name": "GML_ID",
      "type": "String",
      "width": 500
    },
    {
      "name": "GML_IDENTIFIER",
      "type": "String",
      "width": 500
    },
    {
      "name": "SNAR_CODE",
      "type": "String",
      "width": 30
    },
    {
      "name": "GEO_PART_KEY",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "LOG_PKEY",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "GEOM_DATE_CREATED",
      "type": "DateTime",
      "width": 0
    },
    {
      "name": "FART_ID",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "GEO_TYPE",
      "type": "String",
      "width": 50
    },
    {
      "name": "GML_GEOM",
      "type": "Binary",
      "width": 0
    },
    {
      "name": "GML_LENGTH",
      "type": "Double",
      "precision": 0
    },
    {
      "name": "KZ_BIO_OEPUL_JN",
      "type": "String",
      "width": 1
    }
  ],
  "features_count": 2,
  "crs": {
    "org": "EPSG",
    "code": 31287,
    "name": "MGI / Austria Lambert",
    "wkt": "PROJCRS[\"MGI / Austria Lambert\",BASEGEOGCRS[\"MGI\",DATUM[\"Militar-Geographische Institut\",ELLIPSOID[\"Bessel 1841\",6377397.155,299.1528128,LENGTHUNIT[\"metre\",1]]],PRIMEM[\"Greenwich\",0,ANGLEUNIT[\"degree\",0.0174532925199433]],ID[\"EPSG\",4312]],CONVERSION[\"Austria Lambert\",METHOD[\"Lambert Conic Conformal (2SP)\",ID[\"EPSG\",9802]],PARAMETER[\"Latitude of false origin\",47.5,ANGLEUNIT[\"degree\",0.0174532925199433],ID[\"EPSG\",8821]],PARAMETER[\"Longitude of false origin\",13.3333333333333,ANGLEUNIT[\"degree\",0.0174532925199433],ID[\"EPSG\",8822]],PARAMETER[\"Latitude of 1st standard parallel\",49,ANGLEUNIT[\"degree\",0.0174532925199433],ID[\"EPSG\",8823]],PARAMETER[\"Latitude of 2nd standard parallel\",46,ANGLEUNIT[\"degree\",0.0174532925199433],ID[\"EPSG\",8824]],PARAMETER[\"Easting at false origin\",400000,LENGTHUNIT[\"metre\",1],ID[\"EPSG\",8826]],PARAMETER[\"Northing at false origin\",400000,LENGTHUNIT[\"metre\",1],ID[\"EPSG\",8827]]],CS[Cartesian,2],AXIS[\"northing (X)\",north,ORDER[1],LENGTHUNIT[\"metre\",1]],AXIS[\"easting (Y)\",east,ORDER[2],LENGTHUNIT[\"metre\",1]],USAGE[SCOPE[\"Topographic mapping (medium and small scale).\"],AREA[\"Austria.\"],BBOX[46.4,9.53,49.02,17.17]],ID[\"EPSG\",31287]]"
  },
  "metadata": "{\"fiboa\":\"{\\\"fiboa_version\\\": \\\"0.2.0\\\", \\\"fiboa_extensions\\\": []}\"}"
}

Note that OGR already seems to quote the JSON, which results in too many quoted characters.