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Re: What is the format for GeoShape properties?

From: Dan Brickley <danbri@danbri.org>
Date: Thu, 21 Jun 2012 14:04:10 +0200
Message-ID: <CAFfrAFq+S1MejDak5-fVLRad_K6HabYoc2pkSkadAtRa_aVsaw@mail.gmail.com>
To: Lin Clark <lin.w.clark@gmail.com>
Cc: public-vocabs <public-vocabs@w3.org>, Ramanathan Guha <guha@google.com>, Evan Sandhaus <sandhes@nytimes.com>, Andreas Gebhard <Andreas.Gebhard@gettyimages.com>
Copying Guha, Andreas and Evan who might recall more...

On 16 June 2012 23:11, Lin Clark <lin.w.clark@gmail.com> wrote:
> I'm currently helping Drupal's Geofield team figure out how to place
> microdata for their field.
>
> Is there a standard or examples that I can refer to for figuring out how to
> format the GeoShape properties? The descriptions aren't really precise
> enough.

Thanks for the gentle under-statement. You'd need to be telepathic to
understand them.

> box
> circle
> elevation
> line
> polygon

It seems that these (see also http://schema.org/GeoCoordinates ) came
from the IPTC rNews integration last September (before my time).
Digging around the rNews site I find this:

http://dev.iptc.org/rNews-10-The-Geo-Coordinates-Class

It seems the definitions are the same, except for the fundamental
'point' class which in schema.org is called GeoCoordinates which
provides a package with 'latitude', 'longitude', 'elevation' values
(presumably but not explicitly in WGS-84).

Let's run through the list.

0. point
rNews  "A point is a physical location on Earth. A point is expressed
as a latitude and longitude separated by a space character. Points
should expressed in decimal as laid out in the WGS84 specification."
http://schema.org/GeoCoordinates "The geographic coordinates of a
place or event."

1. polygon
"A polygon is the area enclosed by a point-to-point path for which the
starting and ending points are the same. A polygon is expressed as a
series of four or more spacedelimited points where the first and final
points are identical."

- this text is identical in schema.org and rNews. However in rNews,
points are always space separated textualised pairs of values, so a
polygon can be built up from a sequence of these unambiguously. In
schema.org because we use a structured object instead, which could
potentially have 3 values (or none) this is not so simple, and there
is no idiom indicated for composing a polygon via describing points
via http://schema.org/GeoCoordinates. The rNews spec has an example:
40.764161 -73.97283640.768062 -73.98141940.800296 -73.95841640.796918
-73.9494940.764161 -73.972836

This example is also unclear (even if you check the html src). I
suspect it is intended to be something like this, adding in a few
spaces:

40.764161 -73.972836 40.768062 -73.981419 40.800296 -73.958416
40.796918 -73.94949 40.764161 -73.972836



2. box
- schema.org copies the polygon text,
"A polygon is the area enclosed by a point-to-point path for which the
starting and ending points are the same. A polygon is expressed as a
series of four or more space-delimited points where the first and
final points are identical."
whereas rNews defines a box as,
"A box is the area enclosed by the rectangle formed by two points. The
first point is the lower corner, the second point is the upper corner.
A box is expressed as two points separated by a space character."

The rNews example is "38.920952 -94.64544338.951797 -94.680439" which
again I think might be intended as "38.920952 -94.645443 38.951797
-94.680439".

3. circle
schema.org has same text as rNews, i.e.
"A circle is the circular region of a specified radius centered at a
specified latitude and longitude. A circle is expressed as a pair
followed by a radius in meters."
rNews example, "38.920952 -94.645443500", perhaps meant as "38.920952
-94.645443 500"

4. line
both have,
"A line is a point-to-point path consisting of two or more points. A
line is expressed as a series of two or more point objects separated
by space."
rNews example, "38.92 -94.6442.71 -73.2", with added whitespace,
"38.92 -94.64 42.71 -73.2"

5. elevation
A different kind of concept, ... and a point of difference in the designs,
schema.org has "The elevation of a location." whereas rNews has
"The elevation is the height above sea level in meters of a specified
point. An elevation attribute must be specified in conjunction with a
point attribute."
rNews example: 807.79


What to do here?

* The simplest fix here would be to add something to each property
definition along lines of "...points (represented as space-separated
pairs of latitude and longitude decimal WGS84 values).", and add
something based on the rNews examples into a concrete markup sample.
Exact wording would vary case by case.
* I don't see a plausible way of building out box, circle, line and
polygon through a list of GeoCoordinate items. At some point pointing
off to KML becomes more cost effective.
* There is value in having a 'point' which packages the lat/long as a
single string; fwiw I got mail yesterday from someone asking to make
sure the comparable construct stays in http://www.w3.org/2003/01/geo/
and doesn't get removed. Knowing a latitude but not longitude or vice
versa is of limited value.
* We should likely mention WGS84.
* In the SWIG W3C BasicGeo namespace, for quite some time we forgot to
define 'altitude'. On realising this and asking around geo experts, we
ended up with "decimal meters above the local reference ellipsoid)".
I'm not sure how favourably elevation as "above sea level in meters"
compares here. The two concepts approximate each other, see detail at
http://www.esri.com/news/arcuser/0703/geoid1of3.html ... but with
quite some variation. In most cases, people looking for a restaurant,
hair dressers or even volcano will find it at ground level.

Lin - do the rNews definitions give you a working start here?

rNews folk - do you know of any major publishers of this markup?

There's obviously a bit of cleanup to do here, but hopefully the link
to the previous examples helps...

cheers,

Dan
Received on Thursday, 21 June 2012 12:04:42 GMT

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