Re: Requirement for topological data?

Frans,

Just to be clear, TopoJSON uses the geometries of features that have particular edge-node topological relationships to one another in order to save space with encoding of overlapping features such as streets and blocks. GML (and SDWGEO) can already define topological properties / characteristics of features and how they are related. It’s a convenience to link their geometries together explicitly as a space saver. Not sure it’s a requirement.

Josh


> On Sep 23, 2016, at 6:55 AM, Frans Knibbe <frans.knibbe@geodan.nl> wrote:
> 
> Hello,
> 
> This is a message I intended to send to this group, but I just noticed I sent it to another (non-existent, as far as I can tell) address.
> 
> Regards,
> Frans
> 
> ---------- Forwarded message ----------
> From: Frans Knibbe <frans.knibbe@geodan.nl <mailto:frans.knibbe@geodan.nl>>
> Date: 31 August 2016 at 14:28
> Subject: Requirement for topological data?
> To: "sdwwg@lists.opengeospatial.org <mailto:sdwwg@lists.opengeospatial.org>" <SDWWG@lists.opengeospatial.org <mailto:SDWWG@lists.opengeospatial.org>>
> 
> 
> Hello all,
> 
> In general I do not like to assign myself more work, but I do wonder if there is yet another requirement that we have forgotten to identify. It could go something like this:
> 
> It should be possible to publish and use topological geometric data on the web.
> 
> To be clear, I mean something else than the ability to assert topological relationships between spatial things (e.g. A is contained by B) or to use topological functions in queries (e.g. give me all geometries touching geometry B). This is about maintaining the topological awareness in geometric datasets, as supported by data stores like PostGIS <http://postgis.net/docs/Topology.html> and Oracle Spatial <https://docs.oracle.com/cd/B19306_01/appdev.102/b14256/sdo_topo_concepts.htm>. As an example, a line geometry could be aware that it functions as a border between two municipalities and as a border between two provinces. And the line 'knows' which polygons are on its left and which polygons are on its right, and to which other lines it is connected. GIS data stores can have extended data models to deal with this extra level of information, next to the bare geometry. An interchange format exists for topological geometric data: TopoJSON <https://github.com/mbostock/topojson/wiki>. 
> 
> I think that whatever standards or recommendations we come up with, working with topological geometric data on the web should be possible and should at least not be hampered in any way. So that is why it would be good to have this extra requirement.
> 
> I stumbled upon the topic when I talked with a colleague about vector tiling (publishing vector geometry as a pyramid of tiles, just like is done with map images). For that to work, good generalization of geometry is required and for good generalization topological awareness is required, because in general you don't want generalisation to change topological relationships.
> 
> More in general, generalisation of geometry is necessary for limiting the bulk of geometry on the web and giving consumers the opportunity to request or filter data with an appropriate level of detail. So that at least is some sort of use case for the requirement.
> 
> Regards,
> Frans
> 
> 
>