The OGC standard, which is the only currently active standard, and the original archived pages for GeoRSS Simple and GeoRSS GML do not define any specific element to describe a circle. If you want to represent a circle, you can do so using the <georss:radius> element, which will create a buffer around a point.
++<!-- GeoRSS Simple --> +<gml:point>-24.62759969859908 -70.40503541618583</gml:point> +<georss:radius>14.74038882</gml:radius> +<georss:featureName>Antu, VLT-UT1</georss:featureName> +<georss:featureTypeTag>telescope</georss:featureTypeTag> +<georss:relationshipTag>has-nothing-to-do-with</georss:relationshipTag> +<georss:elev>2635</georss:elev> + +<!-- GeoRSS GML --> +<georss:where> + <gml:CircleByCenterPoint> + <gml:pos>-24.62759969859908 -70.40503541618583</gml:pos> + </gml:CircleByCenterPoint> +</georss:where> +<georss:radius>14.74038882</gml:radius> +<georss:featureName>Antu, VLT-UT1</georss:featureName> +<georss:featureTypeTag>telescope</georss:featureTypeTag> +<georss:relationshipTag>has-nothing-to-do-with</georss:relationshipTag> +<georss:elev>2635</georss:elev> ++
However, the XSD defining the GeoRSS GML Profile and the one for GeoRSS Simple, both include ways to specify a circle separately. The OGC standard has links to the schemas and does not state that those schemas are not non-normative, as many other specifications do. That means that in theory, it is completely legal to use them.
+ +<georss:circle><gml:CircleByCenterPoint><georss:where> element. This should have a <gml:pos> element to specify the coordinates of the center point of the circle, and a <gml:radius> element to specify the radius.<gml:radius>uom attribute to specify the unit of measurement, which is by default m to represent meters.uomm for meters, cm for centimeters, [ft_i] for feet (international definition) or [ft_us] for U.S. feets. It is highly likely that most systems will only support meters.+<!-- GeoRSS Simple --> +<georss:circle>-24.62759969859908 -70.40503541618583 14.74038882</georss:circle> +<georss:featureName>Antu, VLT-UT1</georss:featureName> +<georss:featureTypeTag>telescope</georss:featureTypeTag> +<georss:relationshipTag>has-nothing-to-do-with</georss:relationshipTag> +<georss:elev>2635</georss:elev> + +<!-- GeoRSS GML --> +<georss:where> + <gml:CircleByCenterPoint> + <gml:pos>-24.62759969859908 -70.40503541618583</gml:pos> + <gml:radius uom="m">14.74038882</gml:radius> + </gml:CircleByCenterPoint> +</georss:where> +<georss:featureName>Antu, VLT-UT1</georss:featureName> +<georss:featureTypeTag>telescope</georss:featureTypeTag> +<georss:relationshipTag>has-nothing-to-do-with</georss:relationshipTag> +<georss:elev>2635</georss:elev> ++
The GeoRSS documentation on ArcGIS Online mentions supports for circles on GeoRSS Simple, but excludes them from GeoRSS GML. I would therefore advise against trying to use a CircleByCenterPoint. It is likely that the few GeoRSS implementations out there will only support <georss:circle>, if they support circles at all.
The W3C Feed Validation Service does not support circles in either its GeoRSS Simple validator nor its GeoRSS GML validator. +
Circles were probably either added before <georss:radius> was introduced, or added, then partially removed when someone noticed <georss:radius> could already do the job. Another possibility is that circles and curves are far less supported by GIS software than linear geometries, so they wouldn't be that usable anyway. This raises interesting questions: what happens if you use a circle, but also add a radius around it? Do you get a larger circle? Is the radius ignored? Does it become an approximation of a circle as a polygon, as is common with GIS software that doesn't support circles? Those questions will definitely remain unanswered, as with most things about RSS, the answer of most organizations nowadays will be "who cares?". This is why we can't have nice things.