IVOA Footprint Service Protocol

[Roy Williams]

Keith (as DAL chair)

I am writing to discharge a task I have been given by the NVO Team 
Meeting, which is just finishing this afternoon. We have had a 
discussion of Footprint formats and Footprint services, and we would 
like you to start a new activity within DAL to standardize these 
services across all footprint servers.

Perhaps we could start by asking for proposals from DAL group members 
about how these protocols might look? Below are some notes and URLs that 
I hope will set the scope. I am assuming that everyone who wants to 
participate will be on the DAL mailing list.

Roy

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-- How should footprints be described? Presumably by a text 
serialization in the IVOA standard [1] or non-standard [2],[3] formats. 
Or through a reference to a footprint, by an IVO or http reference. Or 
as the footprint that belongs to a survey (eg. "the SDSS footprint"). 
What other ways are there to get a "handle" on a sky footprint?

-- What methods can be applied to a footprint? Intersection, Union I 
would think. Find the area on the celestial sphere. Does it contain a 
given point? Which of a set of points does it contain? Methods to 
serialize footprints, perhaps to make images of them, or to make 
polygons on Google Sky?

-- What services are already prototyped? I know of the Aladin system [4] 
and the JHU services [5]. Who else is involved in sky footprint work 
besides these?

[1] IVOA STC: http://www.ivoa.net/Documents/latest/STC-Model.html
[2] Linear STC (A. Rots): 
http://www.ivoa.net/Documents/latest/LinearSTC.html
[3] VOTable STC (Aladin group): 
http://www.ivoa.net/Documents/latest/Footprint.html
[4] Aladin: http://aladin.u-strasbg.fr/
[5] JHU footprint services: http://voservices.net/footprint/

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