>> Q) shall the draft specify such uncertainty, or shall it leave that
>> to an external specification?
> The draft should specify an uncertainty radius in meters, either as a
> basic element or as a parameter.
And once again, I wonder, how uncertainty in meters is supposed
to be mapped to uncertainty in degrees?
Please consider the following examples.
* Assignment #1: drawing uncertainty on a world map
Given the world map drawn in (longitude, latitude) coordinate
system with WGS 84 as the datum (you may use the world.dat file
shipped with Gnuplot for a crude ``continents only'' world map),
draw the shape corresponding to a spherical circle of the given
radius, with the given point as its center.
You will get more credits if you use a Mercator-projection map
instead.
* point: 83.75N 53.36E, radius: 500000 m.
* Use case #1: omit insignificant digits
How do you transform a geo: URI specifying both location (using
`wgs84' as the coordinate system) and uncertainty (in meters) so
that the insignificant digits will be omitted?
* geo:53.36,83.75;crs=wgs84;uncertaintyradius=50000
* Use case #2: spatial relationship
The application is given a number of geo: URIs specifying both
location (`wgs84') and uncertainty (meters; varies.) Is there a
simple way to find how the locations given are related? Do
these URI specify completely distinct locations, or can some of
them, taking the uncertainty into account, indeed code the same
location?
As an example, could you please point the URIs out of the
following list that can be suspected of being related to the
same object, just expressed with different precision?
* geo:53.36,83.75;crs=wgs84;uncertaintyradius=30000
geo:53.3,83.7;crs=wgs84;uncertaintyradius=10000
geo:52.53,85.17;crs=wgs84;uncertaintyradius=20000
* Use case #3: mapping geo: URIs to mapping services
The current version of the draft reads:
--cut: http://www.ietf.org/id/draft-ietf-geopriv-geo-uri-01.txt --
<p>one of Vienna's popular sights is the <a href='geo:
48.198634,16.371648;crs=wgs84'>Karlskirche</a>.
A web brower could extract the coordinates from the HTML snippet, and
offer the user various options (based on configuration, context), for
example:
...
o switch to a mapping service of the user's choice once the link is
selected
--cut: http://www.ietf.org/id/draft-ietf-geopriv-geo-uri-01.txt --
The URL to switch to Google Maps could be constructed as
follows:
http://maps.google.com/maps?ll=LATITUDE,LONGITUDE&spn=LAT-SPAN,LON-SPAN
where LATITUDE and LONGITUDE are the coordinates, and LAT-SPAN,
LON-SPAN give the extent of the map (``spans''; in degrees) to
be displayed.
The uncertainty specified within the geo: URI could be related
to the scale (i. e., LAT-SPAN, LON-SPAN) of the map. How do you
calculate the spans given some uncertainty expressed in meters?
* Use case #4: travelling to Kemerovo
Before travelling to Kemerovo you have taken a look at its
Wikipedia page [1] and learned its geo: URI:
geo:55.36083,86.08889;crs=wgs84;uncertaintyradius=5000
[1] http://en.wikipedia.org/wiki/Kemerovo
Travelling to Kemerovo by car, you've glanced at your GPS and
read:
geo:55.38400,86.09200;crs=wgs84;uncertaintyradius=42
Do you expect to see the city around, or not yet?
* Use case #1 re-visited: absolute uncertainty
--cut: http://www.av8n.com/physics/uncertainty.htm --
1.2 How Should Uncertainty Be Expressed?
If you want to express the uncertainty, express it separately and
explicitly. For example, absolute uncertainty can be properly
expressed as 1.234(55) or equivalently 1.234+-0.055. Relative
uncertainty can be expressed as 2900+-13%.
--cut: http://www.av8n.com/physics/uncertainty.htm --
Given the following geo: URI, how could the absolute uncertainty
of the coordinates be calculated?
geo:53.36,83.75;crs=wgs84;uncertaintyradius=50000
* And finally, ...
... given how trivial the cases above become when the
uncertainty is expressed using the same units as used for the
coordinates, why should the obstacles be invented?
--
FSF associate member #7257
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