IRSA Image Server - SIA Queries
- Available data-sets
- Metadata queries
- Spatial queries
- Extensions and non-standard behavior
- Getting Help
|WISE All-Sky Release|
|Data-set||SIA base URL||METADATA||Max SIZE|
|4-band Atlas Coadds||<server><root>/sia/wise/allsky/4band_p3am_cdd||<root>/sia/wise/allsky/4band_p3am_cdd?FORMAT=METADATA||5 deg|
|4-band Level 1B Frames||<server><root>/sia/wise/allsky/4band_p1bm_frm||<root>/sia/wise/allsky/4band_p1bm_frm?FORMAT=METADATA||5 deg|
|WISE Preliminary Release|
|Data-set||SIA base URL||METADATA||Max SIZE|
|Atlas Coadds||<server><root>/sia/wise/prelim/p3am_cdd||<root>/sia/wise/prelim/p3am_cdd?FORMAT=METADATA||5 deg|
|Level 1B Frames||<server><root>/sia/wise/prelim/p1bm_frm||<root>/sia/wise/prelim/p1bm_frm?FORMAT=METADATA||5 deg|
An overview of how to perform SIA queries is provided below - for full details, please see the IVOA SIA specification. Note that the SIA services provided here fall under the Atlas/Pointed Image Archive categories (as defined by the SIA standard). That is, no facilities for performing image cutouts or mosaicing are provided.
An SIA metadata query returns a VOTable describing both the SIA service input parameters and the image metadata columns that are returned by actual queries. The output of a metadata query is exactly identical to the output of an SIA query returning no images.
To perform a metadata query, simply append the FORMAT argument with a value of METADATA to the SIA service URL. For example, <root>/sia/wise/allsky/4band_p3am_cdd?FORMAT=METADATA is a metadata query against the WISE All-Sky Release Atlas Coadds.
An SIA query returns a VOTable of images satisfying a spatial constraint. The constraint involves a search region, specified via the POS and SIZE parameters, and a spatial predicate, specified via the INTERSECT parameter.
The POS parameter value must consist of an ICRS right ascension and declination in decimal degrees. It identifies the point which returned images must contain, or the center of the search region. The coordinate values must be separated by a comma; no embedded whitespace is allowed.
This parameter is required.
The SIZE parameter consists of one or two (comma separated) values in decimal degrees. The first SIZE value is taken to be the full-width of the search region along the east axis at POS, and the second is taken to be the full-height along the north axis. Taken together, POS and SIZE define a convex spherical polygon on the sky with great circle edges - the search region. During a query, this region is compared against the convex spherical polygons formed by connecting the 4 corners of each image in a data-set to determine which images should be returned.
If only one size value is specified, it is used as both the full-width and full-height. Negative sizes are illegal, and a width and height of zero indicate that the search region is a point.
SIZE is an optional parameter. If unspecified, the default search region width and height is 0 (i.e. the search region is just the point POS). Note also that if INTERSECT=CENTER (see below), SIZE is ignored.
The INTERSECT parameter specifies a spatial predicate that specifies how the search region S and the spatial extents of a candidate image X must be related. Four values are allowed:
- COVERS: X must completely contain S. Equivalent to CENTER and OVERLAPS if S is a point.
- ENCLOSED: S must completely contain X. If S is a point, the query will always return an empty image table.
- CENTER: X must contain the center of S. If S is a point, this is equivalent to COVERS and OVERLAPS.
- OVERLAPS: The intersection of S and X is non-empty. If S is a point, this is equivalent to CENTER and COVERS.
The INTERSECT parameter is optional; if unspecified, it defaults to OVERLAPS.
The spatial extent of an image is approximated by connecting its corners with great circles to form a spherical convex polygon. The search region as defined by this service is also a spherical convex polygon (or a point). Spatial predicate evaluation is performed on pairs of spherical convex polygons without recourse to any performance motivated short-cuts. Nevertheless, the implementation uses floating point arithmetic and results are therefore not guaranteed to be mathematically exact. Note also that the spatial extent of an image with a non-gnomonic projection (or large distortion terms) may not be well approximated by the technique employed. However, the images currently being served have gnomonic or orthographic projections, are of small area and have small distortion terms - hence, we believe our approximations are accurate enough to be useful.
Extensions and non-standard behavior
As an extension to the SIA standard, one can specify the mcen parameter. This parameter requires no value; if one is nevertheless specified, it is ignored. The presence of mcen in a query indicates that only the most centered image/image set (with respect to POS) should be returned, rather than all images/image sets containing POS. If the SIZE parameter is specified and non-zero, the mcen parameter is ignored (except when INTERSECT=CENTER).
Note that the definition of the SIZE parameter differs from the SIA standard. The latter defines the SIZE values as widths/heights in coordinate angle instead of true angle, leading to search regions that are boxes in spherical coordinate space rather than polygons on the sphere.
Here is an SIA query returning all WISE All-Sky Release Level 1b Frames containing (ra,dec) = (20,40):
And here is a query returning all WISE Preliminary Release Atlas coadds overlapping a search rectangle centered on (ra,dec) = (20,40) with width and height of 1.5 degrees:
If you've run into a problem, think you've found a bug, or simply have questions, please contact IRSA User Support.