geo-extension-nonWGS84

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Geo Extension Straw-Man Proposal

Further to proposals for luna and mars equivalents to geo, the following is a "straw-man" proposal, to incorporate those ideas (and likewise for other bodies) into geo, and to make Geo available for other terrestrial schema than WGS84, in order that further debate may take place. Please feel free to critique it harshly but fairly!

Author

Andy Mabbett

Straw-Man

<span class="geo">

  <span class="body">
    Mars [1]
  </span>

  <span class="reference frame">
    [name of mapping schema] [2]
  </span>

  <span class="latitude">37.386013</span>, 
  <span class="longitude">-122.082932</span> [3]
</span>

Notes

  1. A list of acceptable, case-insensitive, values for 'body' would need to be drawn up (e.g. "Earth", "Mars", "Moon", "Venus", etc.) with "Earth" being assumed if none is specified.
  2. A list of acceptable values for 'reference frame' would need to be drawn up, for each body, with one being declared the default, to be used if no value is present (geo for Earth uses the datum of WGS84 by default. This extension would also allow for other terrestrial schema, of which there are many, such as OSGB36).
  3. As currently with geo, if the "latitude" and "longitude" classes are omitted, the two values MUST be separated by a semi-colon and latitude MUST be first:<span class=geo">37.386013;-122.082932</span>
Also:
  • If latitude is present, so MUST be longitude, and vice versa.
  • The same number of decimal places SHOULD be used in each value; zeros are significant
  • The Gazetteer of Planetary Nomenclature has coordinates for other planets, e.g. Venus and moons, e.g. Io.

Issues

Planetocentric longitude

Geographical longitude on, for example, Mars is not 180 degrees west to 180 degrees east: 18N,226E is a legal Martian location.

This is from Timekeeping on Mars ([2])

Note that the modern standard for measuring longitude on Mars s "planetocentric longitude", which is measured from 0°–360° East and measures angles from the center of Mars. The older "planetographic longitude" was measured from 0°–360° West and used coordinates mapped onto the surface.

You can also check the description in JPL Horizons document, which uses planetographic longitude rather than planetocentric longitude, and gives much more longitude ranges used for different planets and moons, at http://ssd.jpl.nasa.gov/horizons_doc.html#sitecoord

Comments

thare

Comments from thare

Brent A. Archinal

(reproduced from e-mail to Andy Mabbett, by kind permission)

Brent A. Archinal, Geodesist, Astrogeology Team, U.S. Geological Survey

multiple representations of the same location

I assume that someday, some other reference frame will replace the WGS84 reference frame. During the changeover, I expect many people to post 2 descriptions for a particular location (giving slightly different lat/long coordinates for each reference frame).

Even today, I can imagine someone wanting to tag a single location with WSG84, UTM (universal transverse Mercator), MGRS (military grid reference system), and the Maidenhead locator system. So readers can use whichever system they find most convenient, and ignore the others.

<span class="geo"> ... </span> span? If so, what is the best way to make sure that each set of lat/long numbers gets associated with the correct reference frame?

<span class="geo"> ... </span> span? In that case, how do we indicate that they all (are indended to) indicate the same location, rather than a series of locations ?

--DavidCary 17:06, 5 Apr 2007 (PDT)

--Richard Flapper 23:46, 21 May 2008 (MET)

Other

I'm no expert on these matters, and am not sure if this is the right place for such comments, but these are are a few ideas/issues that came to mind after reading this page:

Multiple (Image) geo-tagging

A single XHTML document might make reference to (or include images of) multiple locations. It is not clear from the documents on this site how thee are to be dealt with. Should an <IMG ... > be child of the < div class=geo > or should both tags be on the same level in a parent < div >?

Geo-tagging areas and images of landscapes Geo-tags specify a single point which occupies no space. It might be desirable to be able to define areas, or single locations in a more fuzzy manner. Sometimes, as in the case of pictures of landscapes, you might want to indicate the Point of View (i.e. the location of the viewer)

To accomplish all of this would require three additions to the geo microformat.

  1. a precision attribute: This can be generally useful as even GPS devices have a margin of error, which you might want to indicate. Otherwise it can be used to give an idea of the bounding sphere of a large object like a building.
  2. an area attribute: This could use a system similar to that used by SVG for defining paths, but using lat and long for defining points. It is rather odd that, for example on wikipedia, entire countries or cities are identified by a single point.
  3. some sort of vector attribute: While the only use I can think of for this if for photos of landscapes, it would be good to be able to indicate that the Geographic information relates to the position of the viewer rather than the subject. The vector would indicate where the viewer (camera) is looking

I'm not sure whether the area and vector features are well suited to be extensions of geo or should be independent microformats. Particularly the vector might be better suited for inclusion in some 'EXIF-data equivalent micro format'. Unless of course it you can think of a use beyond photos of landscapes.

--Inkwina 01:54, 30 Nov 2007 (PST)

Astronomical Coordinates

Comments from Stuart

Rather than limit the format to the surfaces of planets and moons, would it be possible to include astronomical coordinates for objects in celestial coordinate frames? This would be very useful for tagging astronomical objects (e.g. M41 or NGC 7027).

If you do not want to break the existing geo tag might I suggest the more generic term coord?

As suggested already it is important to define a reference frame. The term coordinate-frame (or coord-frame) might also avoid ambiguities that exist for the word reference. In astronomical applications it is necessary to provide both a frame (e.g. FK5) and an equinox/epoch (e.g. 2000.0). Astronomical coordinate frames that are widely supported are: ICRS, FK5, FK4, ECL, GAL, SGAL.

Related pages

See also

NASA shares space with Google (2006-12-20)

geo-extension-nonWGS84 was last modified: Wednesday, December 31st, 1969

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