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<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Weems, to me, like radius is a distance. As such, wouldn’t it be represented in whatever units the datum uses for measuring distance?
For WGS84, that’s meters. For other non-planar coordinate systems it varies, but it is always defined.<o:p></o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Arlo<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> geojson-bounces@lists.geojson.org [mailto:geojson-bounces@lists.geojson.org]
<b>On Behalf Of </b>andy e<br>
<b>Sent:</b> Sunday, November 06, 2011 12:43 PM<br>
<b>To:</b> geojson@lists.geojson.org<br>
<b>Subject:</b> Re: [Geojson] circles (was: Toward consensus on proposals)<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">I was a little hesitant to use those examples, because I knew it would bring up the "isn't this just a point with a buffer?" argument.<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">The GPS example was probably bad. Let's go back to the CEP:<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">" It is defined as the radius of a circle, centered about the mean, whose boundary is expected to include 50% of the population within it.<sup>"</sup><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">In the work we do, it's not always treated as a point + error. It's treated as "any point within this area, defined by a point and axis/radius". <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">Yes, totally agree on the conversion. But again, what does conversion/implementation have to do with the spec? I may convert all lines/polygons I come
across into an infinite number of points, but that doesn't mean all shapes should just be points, right?<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">Re: units, I'm fine with defaulting to CRS, hence the option for specifying something other than that. If the default CRS uses the WGS-84 datum, I
guess I'm not clear on why using "units" : "meters" isn't appropriate.<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">Overall, I know this is a rare use case for the spec, but I can assure you we'd make good use of it. :)<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:"Arial","sans-serif"">Thanks,<o:p></o:p></span></p>
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<p class="MsoNormal">On Sun, Nov 6, 2011 at 1:00 PM, <<a href="mailto:christopher.schmidt@nokia.com">christopher.schmidt@nokia.com</a>> wrote:<o:p></o:p></p>
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On Nov 6, 2011, at 1:05 PM, ext andy e wrote:<br>
<br>
> Tim,<br>
><br>
> Nope, totally understand about wanting examples. Correct, ellipses/circles on Earth. Unfortunately, can't give concrete examples in this setting, but think along the lines of Circular Error (<a href="http://en.wikipedia.org/wiki/Circular_error_probable" target="_blank">http://en.wikipedia.org/wiki/Circular_error_probable</a>),
where, while you have a center point, the "point" may be anywhere within the ellipse/circle. A la polygons.<br>
> GPS error/accuracy is another valid use case, I would think.<o:p></o:p></p>
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<p class="MsoNormal">But neither of these are really 'geometries' in the classical sense; a point<br>
with an error radius around it could be easily represented as a point, with<br>
a property describing the attribute that it is representing. (So,<br>
<br>
{"type":"Feature", "geometry": {"type":"Point"}, {"properties": {"uncertainty": 5, "uncertainyUnits": "km"}}<br>
<br>
This would leave the interpretation up to the client -- where it seems it's<br>
going to have to belong no matter what, I believe.<br>
<br>
If we felt this was a very common use case, I'd even be willing to say that<br>
a point geometry can have a radius indication:<br>
<br>
{"type":"Point", "coordinates": [], "radius": 5, "radiusUnits": "km"}<br>
<br>
(Or some such.)<br>
<br>
But I think that it's important to recognize that for most clients, you're<br>
not going to be able to round trip that information through almost any<br>
conventional GIS system if you expect them to be treated as circles -- because<br>
the first thing anyone does with a circle is convert it to a Polygon :)<br>
<br>
Treating this as a common extension of point falls into a couple things I<br>
don't think the current spec does:<br>
<br>
- Compatibility (treating things as 'just points') with any application<br>
that doesn't understand circles; this seems valuable to me<br>
- Not providing a sane approach to ellipses -- I don't know if this is<br>
important or not. (I've had needs where a radius of a circle might make<br>
sense, but nothing I can think of that would be more accurately described<br>
by an ellipse than a polygon).<br>
<br>
In general, I think that circles -- because they fall outside of commonly implemented<br>
geo features -- will quickly be converted to either points, or polygons. I think<br>
that making it an optional extension of point to deliver a radius-like property<br>
which applications can take advantage of helps maintain compatibility and<br>
round-tripping for people who use this data, and lets applications which *can*<br>
do something with circles do something meaningful with them.<br>
<br>
(Then my problems are just reduced to the ones that I think Tim has already mentioned<br>
and I don't think are adequately covered: If the units are anything other than<br>
'coordinate system units', I'm not sure how clients are meant to interpret them,<br>
and if they are coordinate system units, I'm not sure that they'll typically have<br>
meaning.)<br>
<br>
-- Chris<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
> Thanks,<br>
><br>
> Andy<br>
><br>
><br>
> On Fri, Nov 4, 2011 at 6:03 PM, Tim Schaub <<a href="mailto:tschaub@opengeo.org">tschaub@opengeo.org</a>> wrote:<br>
> Hey Andy,<br>
><br>
> Maybe it would help sway the curve-haters if you could provide some<br>
> ellipse examples. Are we talking about ellipses on Earth? Any links<br>
> to pictures?<br>
><br>
> Perhaps this one:<br>
> <a href="http://maps.google.com/maps?ll=38.893956,-77.036503&z=18" target="_blank">
http://maps.google.com/maps?ll=38.893956,-77.036503&z=18</a> (though I<br>
> think you'd get more accuracy with a polygon)<br>
><br>
> I'm not trying to be crass, I'm honestly interested. I also know we<br>
> didn't make this same request to proponents of the other geometry<br>
> types. Guess we all just bought into the point, line, poly (and<br>
> multi-part cousins) without question.<br>
><br>
> Tim<br>
><br>
><br>
><br>
> On Fri, Nov 4, 2011 at 5:39 PM, andy e <<a href="mailto:virtualandy@gmail.com">virtualandy@gmail.com</a>> wrote:<br>
> > Thanks for the discussion/feedback on circles/ellipses. Sorry for not<br>
> > responding sooner (been busy trying to work up a proposal that will get us<br>
> > more time with these very circles/ellipses :) ). Also wanted to talk with<br>
> > coworkers about this.<br>
> > To reiterate what Sean has stated, the inclusion of circles and ellipses is<br>
> > to support raw geometries and not queries/relationships/features. The data<br>
> > that we are handling is purely just that - a circle or ellipse with a<br>
> > radius/axii (typically in meters) and in the case of ellipses, a rotation<br>
> > (typically in degrees). To be honest, Ellipses are much more important to us<br>
> > than Circles, but I threw that in as "Just use a even Ellipse for a Circle"<br>
> > seemed like a cop-out. I did not realize GML supported Ellipses, so I will<br>
> > try to make the change to the spec to line that up.<br>
> > In regards to units, in other specs, if a "unit" is not widely standardized,<br>
> > it is defined by the requestor/provider. Take HTTP and Content-Type<br>
> > (<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html" target="_blank">http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html</a>), where they specify<br>
> > "Content-Type" : " text/html ..."<br>
> > That was my intention here - to clear up any possible confusion, specify the<br>
> > units. As mentioned (by Tim S and others, sorry, forgetting right now), the<br>
> > problem with keeping units in the CRS occurs when you start specifying geoms<br>
> > in lat/lon. Radians and CRS conversions are not very intuitive for those<br>
> > without strong GIS backgrounds (like myself).<br>
> > I would be okay with defaulting to CRS if no units were specified (or vice<br>
> > versa with meters). Hence the option of using that field.<br>
> > I realize it is somewhat repetitive/complex, but given that no one seems to<br>
> > be using Circles/Ellipses anyway, maybe it just won't show up that often. :)<br>
> > With regards to GEOS and JTS, I understand the concern, but wonder if we<br>
> > should even think about implementation when defining the spec. As previously<br>
> > mentioned, it is likely that even we will just store Ellipses/Circles this<br>
> > way and convert to approximate polygons when processing.<br>
> > Thanks to my co-worker Nathan for input here.<br>
> > Andy<br>
> > On Fri, Nov 4, 2011 at 7:29 AM, <<a href="mailto:christopher.schmidt@nokia.com">christopher.schmidt@nokia.com</a>> wrote:<br>
> >><br>
> >> On Nov 4, 2011, at 5:13 AM, ext Martin Daly wrote:<br>
> >><br>
> >> >> I propose that we accept or reject the Circle/Ellipse proposal without<br>
> >> >> getting too caught up in the units details right now. If it can't be<br>
> >> >> solved and stalls out we can terminate it then. How does that sound?<br>
> >> ><br>
> >> > I hate curves as much as, if not more than, the next person, but I can<br>
> >> > see that a compact representation of a circle would be useful for some<br>
> >> > situations.<br>
> >> ><br>
> >> > Also, while not exactly a precedent, we already have "bbox" as a compact<br>
> >> > representation of a rectangular polygon, albeit restricted to the "Feature"<br>
> >> > and "FeatureCollection" objects.<br>
> >><br>
> >> BBOX is not a representation of a geometry. It's a piece of metadata about<br>
> >> a feature<br>
> >> or FeatureCollection. It doesn't replace a geometry.<br>
> >><br>
> >> -- Chris<br>
> >><br>
> >> ><br>
> >> > Martin<br>
> >> ><br>
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> Tim Schaub<br>
> OpenGeo <a href="http://opengeo.org/" target="_blank">http://opengeo.org/</a><br>
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