rdblue commented on code in PR #10981:
URL: https://github.com/apache/iceberg/pull/10981#discussion_r1940061312
##########
format/spec.md:
##########
@@ -205,15 +205,40 @@ Supported primitive types are defined in the table below.
Primitive types added
| | **`uuid`** | Universally unique identifiers
| Should use 16-byte fixed
|
| | **`fixed(L)`** | Fixed-length byte array of length L
|
|
| | **`binary`** | Arbitrary-length byte array
|
|
+| [v3](#version-3) | **`geometry(C)`** | Geometry features from [OGC – Simple
feature access][1001]. Edge-interpolation is always linear/planar. See
[Appendix G](#appendix-g-geospatial-notes). Parameterized by CRS C. If not
specified, C is `OGC:CRS84`. |
|
+| [v3](#version-3) | **`geography(C, A)`** | Geometry features from [OGC –
Simple feature access][1001]. See [Appendix G](#appendix-g-geospatial-notes).
Parameterized by CRS C and edge-interpolation algoritm A. If not specified, C
is `OGC:CRS84` and A is `spherical`. |
|
Notes:
1. Timestamp values _without time zone_ represent a date and time of day
regardless of zone: the time value is independent of zone adjustments
(`2017-11-16 17:10:34` is always retrieved as `2017-11-16 17:10:34`).
2. Timestamp values _with time zone_ represent a point in time: values are
stored as UTC and do not retain a source time zone (`2017-11-16 17:10:34 PST`
is stored/retrieved as `2017-11-17 01:10:34 UTC` and these values are
considered identical).
3. Character strings must be stored as UTF-8 encoded byte arrays.
-
For details on how to serialize a schema to JSON, see Appendix C.
+[1001]: <https://portal.ogc.org/files/?artifact_id=25355> "OGC Simple feature
access"
+
+##### CRS
+
+For `geometry` and `geography` types, the parameter C refers to the CRS
(coordinate reference system), a mapping of how coordinates refer to locations
on Earth.
+
+The default CRS value `OGC:CRS84` means that the objects must be stored in
longitude, latitude based on the WGS84 datum.
+
+Custom CRS values can be specified by a string of the format `$type:$content`,
where `type` is one of the following values:
Review Comment:
Nit: I think the idea is that this is an interpolated string, but we don't
use `$` elsewhere so people may embed the character. What about simply
"`type:identifer`, where `type` is ..."
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