ADQL 2.1 geometry shape semantics

[Mark Taylor]

Dear DAL,

The current ADQL PR (PR-ADQL-2.1-20180112) defines the geometry functions
like this (e.g. section 3.6.2):

   "The CIRCLE datatype maps to the corresponding type defined in the DALI
    specification."

without any other discussion of the semantics.  The relevant section
in DALI 1.1 (sec 3.3.6) says:

   "Circle values serialised in VOTable or service parameters must have
    the following metadata in the FIELD element: datatype="double" or
    datatype="float", arraysize="3", xtype="circle". For circles in a
    spherical coordinate system, the values are ordered as: longitude
    latitude radius; longitude values must fall within [0,360], latitude
    values within [-90,90], and radius values in (0,180]. For example:

        12.3 45.6 0.5

    In spherical coordinates, all longitude values must fall within
    [0,360] and all latitude values within [-90,90]."

This seems to leave open the question of whether an ADQL 2.1 CIRCLE
is (sometimes? always?) to be understood in spherical coordinates,
or whether it can be understood to have Cartesian semantics.
In ADQL 2.0 this was explicit ("This function expresses a circular
region on the sky (a cone in space)...").  I guess the answer is
probably that CIRCLE is always to be understood in sky coordinates;
in that case it should be made explicit in the text.

The same point applies to POLYGON, and maybe to POINT.

Mark

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/