ISSUE-31 relative points?

I think we need to have a discussion about what "relative" positioning means.
One example I have seen is in CityGML where building geometry is described in meters relative to an anchor point.
However, I'm not sure what such a system for describing polygons, etc. would buy us in the POI spec.
I tend to think that we intended relative positioning to facilitate things like the following:
<pois>
	<poi id="frame_of_reference">
		<location>
			<!-- some reference to a moving vehicle a building or any arbitrary frame of reference (not just coords but orientation would be nice) -->
		</location>
	</poi>
	<poi id="alex">
		<location>
			<!-- some value calculated within that frame of reference (WiFi tracking, etc.)  in something not WGS84 -->
			<point srsName="urn:ogc:def:crs:EPSG:6.6:4979" srsDimensions="3">
				<pos>10023.123 1234.123 666.66</pos> <!-- lets say this is mm for now -->
			</point>
		</location>
		<relation type="relative-to" id="#frame_of_reference"/>
	</poi>
</pois>

Is that appropriate, or do we want something where each individual georeference can specify a relative frame of reference?

<pois>
	<poi id="frame_of_reference">
		<location>
			<point relative_to="#frame_of_reference"  srsName="urn:ogc:def:crs:EPSG:6.6:4979" srsDimensions="3">
				<pos>10023.123 1234.123 666.66</pos> <!-- lets say this is mm for now -->
			</point>
			<!-- some reference to a moving vehicle a building or any arbitrary frame of reference (not just coords but oriented would be nice) -->
		</location>
	</poi>
	<poi id="alex">
		<location>
			<point relative_to="#frame_of_reference"  srsName="urn:ogc:def:crs:EPSG:6.6:4979">
		</location>
	</poi>
</pois>

Sorry if this is not "correct", but I'm more concerned about the spirit of this than the syntax.
Would this be a valuable addition to the spec?
Can we think of use cases that would benefit?
I suspect the AR advocates can.
What about indoor tracking of POIs?
What about vehicle/pedestrian entrances for the mapping crowd?
Can we put the relative information in the data model and then let systems back out the actual coordinates for efficiency?
Then if you find (don't know how yet) that an update is necessary you can re-calculate.
Perhaps what you have above ends up looking like:
		<location>
			<point>
				<pos>-72.123 34.1234 100.7</pos> <!-- the non-relative position of this point in WGS84 -->
			</point>
			<point relative_to="#frame_of_reference"  srsName="urn:ogc:def:crs:EPSG:6.6:4979" srsDimensions="3">
				<pos>10023.123 1234.123 666.66</pos> <!-- lets say this is mm for now -->
			</point>
			<!-- some reference to a moving vehicle a building or any arbitrary frame of reference (not just coords but oriented would be nice) -->
		</location>


Alex Hill Ph.D.
Postdoctoral Fellow
Augmented Environments Laboratory
Georgia Institute of Technology
http://www.augmentedenvironments.org/lab