Dimension property: returned value for geography collections

[benbovy]

In the case where a collection contains features of different dimensions, the default value returned via s2geography (-1) is not consistent with the value returned by shapely (max dimension value found).

Which one do we want?

Originally posted by @jorisvandenbossche in #51 (comment)

[jorisvandenbossche]

I am not entirely sure why s2geography returns -1 (cc @paleolimbot?), because AFAIK R's s2 returns the max dimension of the parts (except for an empty collection): https://r-spatial.github.io/s2/reference/s2_is_collection.html (see the examples).
And I suppose this is then also consistent with bigquery (I assume, based on the description, but didn't test that)

[jorisvandenbossche]

It seems that in s2geography, there is a difference between the Geography::dimension() attribute and the s2_dimension() function.

The attribute is used in code where you want to know if you have points or polylines or polygons (and if not one of those three, it returns -1), and then the logic to calculate the max dimension in case of collections is handled inside the s2_dimension function (https://github.com/paleolimbot/s2geography/blob/1c67ddc62c30977fabca291fb68fd8cb62f3beaa/src/s2geography/accessors.cc#L55-L69)

[benbovy]

Yes I think we rather want the result from the s2geography::s2_dimension() function here, which returns values that are consistent with shapely.

IIUC bigquery's ST_DIMENSION always returns -1 for empty geography objects? Whereas shapely.get_dimensions() and s2geography::s2_dimension() both return 0, 1, 2 for empty point, linestring and polygon geography objects, respectively.