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Clarify geographic vs non-geographic bbox values #145

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Nov 18, 2022
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Clarify geographic vs non-geographic bbox values #145

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17 changes: 12 additions & 5 deletions format-specs/geoparquet.md
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Expand Up @@ -168,11 +168,18 @@ Bounding boxes are used to help define the spatial extent of each geometry colum
Implementations of this schema may choose to use those bounding boxes to filter
partitions (files) of a partitioned dataset.

The bbox, if specified, must be encoded with an array containing the minimum
and maximum values of each dimension: `[<xmin>, <ymin>, <xmax>, <ymax>]` for 2D
or `[<xmin>, <ymin>, <zmin>, <xmax>, <ymax>, <zmax>]` for 3D geometries.
This follows the GeoJSON specification ([RFC 7946, section 5](https://tools.ietf.org/html/rfc7946#section-5)),
but is expressed in the same CRS as the geometry.
The bbox, if specified, must be encoded with an array representing the range of values for each dimension in the
geometry coordinates. For geometries in a geographic coordinate reference system, longitude and latitude values are
listed for the most southwesterly coordinate followed by values for the most northeasterly coordinate. This follows the
GeoJSON specification ([RFC 7946, section 5](https://tools.ietf.org/html/rfc7946#section-5)), which also describes how
to represent the bbox for a set of geometries that cross the antimeridian.

For non-geographic coordinate reference systems, the items in the bbox are minimum values for each dimension followed by
maximum values for each dimension. For example, given geometries that have coordinates with two dimensions, the bbox
would have the form `[<xmin>, <ymin>, <xmax>, <ymax>]`. For three dimensions, the bbox would have the form
`[<xmin>, <ymin>, <zmin>, <xmax>, <ymax>, <zmax>]`.

The bbox values are in the same coordinate reference system as the geometry.

### Additional information

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