Simplification with respect to topology

[skinkie]

At this moment a layer each object is simplified individually.

Is it possible to simplify with respect to topology? Hopfully enable this on specific layers.

As workaround I use now: TEGOLA_OPTIONS=DontSimplifyGeo ./tegola_linux_amd64.3 serve

But how could I enable DontSimplifyGeo for a specific layer?

[ARolek]

@skinkie we have configurable simplification on a per layer basis planned for v0.6.0. You can track #165. v0.6.0 is not far away.

[ARolek]

@skinkie v0.6.0 is out now. You can add dont_simplify to any of the map layer configs and that layer will not have simplification applied. Give it a run and let me know the results for your use case. Example config:

[[maps.layers]]
name = "land"
provider_layer = "osm.land_8-20"
dont_simplify = true
min_zoom = 8
max_zoom = 20

[skinkie]

@ARolek that doesn't solve the problem, that would just be a workaround for the problem. Simplification with respect for topology would still be a valuable asset for any layer that has adjacent surfaces. Please reopen the ticket.

[gdey]

@skinkie Sorry, could you please explain what do you mean with respect to topology? Do you mean try to keep as much of the original topology as possible? I'm planning on changing out the simplification algo, to one that is based on area. This is said to preserve topological aspects better than the current algo we are using.

[skinkie]

What I mean is that if two surfaces match on nodes. The DP algorithm now used will consider each surface individually. What would be required is the node simplification to be applied on multiple objects at once, if the nodes would match at the same position.

I can make a drawing later.

[gdey]

I going to repeat just to make sure I understand. If two polygons share a border, the simplification applied to that border should be the same. Say they share the partial linestring ( 10,10 10,20 15,30 ) and the simplification was one could end up being ( 10,10 15,30 ) the other could end up being (10,10 15,20 ) depending on the point coming after on each of the features. What would be better is if both simplified that section to the same thing, either ( 10,10 15,30 ) or ( 10,10 15,20 ).

Would it be okay, if this was only done for features from the same data provider and layer?

[skinkie]

I think you got it right, and I doubt it is feasible to do it taking multiple layers in account.

I am also looking if something like https://postgis.net/docs/TP_ST_Simplify.html could assist to make a manual simplification.

[gdey]

Okay, would be a more involved task. For now (as a workaround) I would create a separate table with simplified polygons, and then use the dont_simplify option for that layer.

I will have to think about this and come up with a deeper plan. I think this is going to be a lower priority for us, as we want to get the new simplification algo out and solve the some of the other clipping issues. After that, we can take a look at this. But, for now, I think using TP_ST_Simplify and the "don't simplify" option on a layer is a good workaround.

I'll keep this issues open.

[skinkie]

I would like to report back that using the PostGIS topology extension I am able to simplify the structure with respect to topology, which gives good results. I have not yet used this in combination with tegola, but expect this could work as well.

select CreateTopology('zones_topology', 28992, 2);
create table zones_topology.zones_prod (like zones_prod including all);
select addtopogeometrycolumn('zones_topology','zones_topology','zones_prod','topo','polygon');

insert into zones_topology.zones_prod(gid, zone_id, zonenr, zonenaam, nulzones, area, perimeters, hectares, geom, topo) select gid, zone_id, zonenr, zonenaam, nulzones, area, perimeters, hectares, geom, zones_topology.totopogeom(geom, 'zones_topology', 1) from zones_prod;

create view zones_filter_225 as select zone_id, nulzones, ST_Simplify(topo, 225) as geom from zones_topology.zones_prod where nulzones in ('overlap', 'grens') or nulzones is null;

[ARolek]

@skinkie thanks for the follow up. I'm interested in how the results look with tegola. Please let us know when you have a chance.