Arrays of geometries?

Patrick Dowler pdowler.cadc at
Mon Sep 18 18:29:34 CEST 2017

The DALI polygon is simply a more comples datatype and like any other
(eg time values in a double column) the metadata for that column would
describe the coordinate system. So you can already use DALI polygon
for ICRS or galactic coordinates and the last paragraph (extra
restructions in spherical coordinates) at least hints that cartesian
polygons are also a valid use. It is true that it is limited to a 2-D
polygon... 3-D would require additional information about the number
of dimensions (could be new xtype like polygon3d or it could be a
something more subtle/complicated/flexible). Let's cross that bridge
when we come to it :-)


On 18 September 2017 at 01:59, alberto micol <amicol.ivoa at> wrote:
> 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,
> Alberto
>> 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

Patrick Dowler
Canadian Astronomy Data Centre
Victoria, BC, Canada

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