Arrays of geometries?

alberto micol amicol.ivoa at
Mon Sep 18 10:59:37 CEST 2017

Let me intrude with a question/curiosity/use case:

Is it useful/needed to distinguish between geometry on a plane 
and geometry on a sphere?

(SQLServer calls geometry the first, geography the latter.)

How can I serialise a polygon (or a point, or an array of points, …)
defined not on the sky (3d), but e.g. on the focal plane (2d)?
How would a client recognise the difference?

Many thanks,

> On 06 Sep 2017, at 20:05, Dave Morris <dave.morris at> wrote:
> Hiya,
> In an earlier discussion Markus suggested using a language feature to indicate whether a service supported arrays.
>    <languageFeature type="ivo://"/>
> I'd like to build on that and define two features within that URI to indicate support for numeric and geometric arrays.
>    <languageFeature type="ivo://">
>        <feature>
>            <form>numeric</form>
>        </feature>
>        <feature>
>            <form>geometric</form>
>        </feature>
>    </languageFeature>
> This would enable services to declare support one or the other or both.
> If this is a useful approach I'll add a section about it in the next draft of the ADQL specification.
> -- Dave
> On 2017-09-05 12:24, Markus Demleitner wrote:
>> Dear DAL,
>> While implementing DALI 1.1 with a view to TAP 1.1, one of my test
>> cases lead to the encoding of a 2-array of points to a
>> <FIELD datatype="float" arraysize="2x2" xtype="point"...>
>> This made me realise that, as far as I can see, it's unclear what
>> that means by DALI.  And it leads to the more fundamental question of
>> whether xtype amends the datatype -- which would support such usage
>> -- or whether it amends (datatype, arraysize) -- which probably would
>> outlaw things like these.
>> So:
>> Is anyone strongly in favour of such usage?  Note that another,
>> perhaps more common, use case is arrays of timestamps, which *could*
>> play a role in certain time series serialisations?
>> ...or is anyone strongly opposed to it?
>> Either way, I think it's one of these corner cases where we should
>> somehow agree upon a policy.  Should xtype+array be allowed?
>> Forbidden?  Undefined at this point?
>>          -- markus

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