"edges" field spec definition for version 0.1 (#27)

Author: Jesus89

examples/geoparquet/example.parquet

@@ -24,8 +24,8 @@
                                     '(Lon)",east],UNIT["degree",0.0174532925199433],USAGE[SCOPE["Horizontal '
                                     'component of 3D '
                                     'system."],AREA["World."],BBOX[-90,-180,90,180]],ID["EPSG",4326]]',
-                             'edges': 'planar',
-                             'encoding': 'WKB'}},
+                             'encoding': 'WKB',
+                             'edges': 'planar'}},
     'primary_column': 'geometry',
     'version': '0.1.0'}
 """

examples/geoparquet/example.py

@@ -53,6 +53,7 @@ Each geometry column in the dataset must be included in the columns field above
 | ---------- | ----------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------- |
 | crs       | string   | **REQUIRED** [WKT2](https://docs.opengeospatial.org/is/18-010r7/18-010r7.html) string representing the Coordinate Reference System (CRS) of the geometry.  |
 | encoding | string | **REQUIRED** Name of the geometry encoding format. Currently only 'WKB' is supported. |
+| edges | string | **OPTIONAL** Name of the coordinate system for the edges. Must be one of 'planar' or 'spherical'. The default value is 'planar'.  |
 | bbox   | \[number] | **OPTIONAL** Bounding Box of the geometries in the file, formatted according to [RFC 7946, section 5](https://tools.ietf.org/html/rfc7946#section-5) |
 
 #### crs
@@ -103,6 +104,14 @@ The axis order of the coordinates in WKB stored in a geoparquet follows the de f
 (x, y) where x is easting or longitude and y is northing or latitude. This ordering explicitly overrides the axis order as specified in the CRS. 
 This follows the precedent of [GeoPackage](https://geopackage.org), see the [note in their spec](https://www.geopackage.org/spec130/#gpb_spec). 
 
+#### edges
+
+This attribute indicates how to interpret the edges of the geometries: whether the line between two points is a straight cartesian line or the shortest line on the sphere (geodesic line). Available values are:
+- planar: use a flat cartesian coordinate system.
+- spherical: use a spherical coordinate system and radius derived from the spheroid defined by the coordinate reference system.
+
+If no value is set, the default value to assume is 'planar'.
+
 #### bbox
 
 Bounding boxes are used to help define the spatial extent of each geometry column.