Efficiency of CityGML

Or maybe should I be looking into 3DTiles instead?

@virtualcitymatt and @redgeographics can probably provide some insight here

Thanks for the ping @hkingsbury,

CityGML does have the concept of CityObjectGroups which would probably let you group your parts of pipe into a group of objects - I’ll be honest though, CityObjectGroups as a feature has pretty limited support among tools which support CityGML partly because it’s so seldom used. 

I’ve definitely seen Pipes created with relative geometries (e.g., the ones you can see here ) But indeed the issue is that it creates a single feature for each section of pipe which is on a different orientation. I think the geometry used was just a 1m length section of pipe with a 1m diameter. The transformation matrix was use to rotate/scale the sections as needed. In the linked map you can click on the sections of pipe to see how many there are.

For instancing I would also recommend that you include the offset of the transformation in the actual insert point and just use the matrix to transform the scale and rotation. this helps performance when doing spatial filters. 

Indeed 3DTiles makes sense for visualization - CityGML (or CityJSON) data are often the inputs for conversion to 3DTiles. And for sure if converting to 3DTiles (or any other format) then you would definitely benefit from relative geometries when it comes to rendering.  

You mentioned CityDB - by that do you mean the 3DCityDB? or something else. If you’re working with CityGML then I definitely would recommend looking into the 3DCityDB for CtiyGML storage. 

Cables and Pipes though are always a bit tricky because there’s no real definition of how to handle these in CityGML - No utility module. You’re just stuck with GenericCityObjects. If you’re really intersted you could look into creating a new Application Domain Extension (ADE) for CityGML. There might already be one but I don’t know of one off the top of my head.

Large file size just comes with the territory of CityGML I’m afraid - That’s why I’m a big pusher of the 3DCityDB. 


 

Hi @virtualcitymatt , thanks for your detailed reply.

Yes, I mean 3DCityDB. We are working on getting it up and running. I am aware of some ADE's for utilitynetworks but they don't seem very complete or active.

Could you explain why this issue would be resolved if I used 3DCityDB? What does it do differently (apart from being a database compared to a static written file of course) to make it better? If I understand correctly: if I would write my feature to a 3DCityDB instead of a CityGML, wouldn't I still have a separate record with attributes for each 2-vertex part of one pipe? Or can I aggregate all geometry-instances into 1 FME Aggregate which holds the attributes and push this to the CityDB?

Thanks for your tip on the offset of the transformation. The way I have it now it makes a referencePoint geometry; I believe this is also what you suggest(?).

I tried today to make CityObjectGroups from FME but I can't precisely figure out how to actually group them and have 1 group with a lot of groupmembers (2-vertex chopped part of pipes). I also can't find very much documentation on how to get it working.

Hi @virtualcitymatt, thanks for your again detailed response.

Interesting about the limitation of not being able to write directly to a 3DCityDB using FME. I did not expect that! If in essence 3DCityDB is based on a PgSQL with a custom schema, why is it not possible for FME?

Based on your help I'm not going to spend time on figuring out CityObjectGroups then, I will just accept my fate and produce very large static CityGML files where each straight part of a pipe is an instanced geometry with a lot of attributes 😀

Yeah - the database schema has about 60ish tables in there all linked in various ways.I guess in theory you could write directly to the database I just don’t think it’s really worth it. You would need to update a lot of tables and make sure all the keys are properly linked and all the sequences are correctly updated etc etc. the same is also true when it comes to reading form it. You can read from it with FME but it’s quite a task to properly support all the features. It’s not as easy as just connecting up a reader to PostGIS and getting the data. You have to access multiple tables and perform various joins. 

On that note though, the 3DCityDB team are working on a new version of the database which is much, much more simplified - The schema is pretty stable now but the client for still needs some work to be able to support all of the features which version 4 offers and there is currently no GUI for it. It’s still not as easy as just pointing a PostGIS reader at it but it’s closer.

Yes, skip the CityObjectGroups. If you ever get a solution to create the instanced geometries from the lines I’d love to take a look and what you were able to put together. 

Also, great that you are working on creating CityGML. If you have more questions about CityGML feel free to send me a PM on here and I will do my best to try and answer 

I 'finished’ my workspace. I am slightly disappointed with being unable to use relative geometries inside a multigeometry feature (aggregate). I wanted to include the pipes as 2D lines as LOD0/1 and the 3D pipes as LOD2. But this is not possible if LOD2 is the instanced geometry. 

I also used different shapes of pipes, in essence they are 1x1meter geometries. I add these as instance, and then scale it by the pipe segment length and the diameter (attribute driven). Then I offset it to the starting point (referencepoint). Then I use rotator to correct the horizontal (2D) orientation. Then I finish it by doing a 3D Rotation (with the reference point as its origin to rotate around). The 3D Rotation is used to offset it with the DEM; the pipes don't have individual Z values from the source data so I can only assume a standard depth per type of pipe in combination with DEM.

See image and sample GML (saved as TXT because GML cant be attached here) for your mentioned interest, @virtualcitymatt .