Broken polygon geometries when using `postgis` provider, while `mvt_postgis` renders correctly

[NivGreenstein]

Summary

I am using Tegola with OSM data downloaded from Geofabrik and imported into PostGIS using osm2pgsql flex Lua.

When serving the data with the regular Tegola postgis provider, the polygon geometries are rendered broken / distorted / unusable in QGIS.

When serving the same table using mvt_postgis, the geometries render correctly.

This happens across the dataset, not only for a single isolated feature.

Expected behavior

Polygon and multipolygon building geometries should render correctly when using the postgis provider.

Actual behavior

When using:

type = "postgis"

the geometries are rendered incorrectly in QGIS.

When using:

type = "mvt_postgis"

against the same PostGIS table, the geometries render correctly.

This suggests that the source geometries are probably valid, and the issue may be related to Tegola's native postgis provider geometry handling / clipping / encoding path.

Environment

• Tegola version: latest from the repository
• PostgreSQL:

PostgreSQL 17.7 (Debian 17.7-3.pgdg12+1) on aarch64-unknown-linux-gnu, compiled by gcc (Debian 12.2.0-14+deb12u1) 12.2.0, 64-bit

• PostGIS:

POSTGIS="3.6.1 f533623" [EXTENSION] PGSQL="170" GEOS="3.11.1-CAPI-1.17.1" PROJ="9.1.1 NETWORK_ENABLED=OFF URL_ENDPOINT=https://cdn.proj.org USER_WRITABLE_DIRECTORY=/var/lib/postgresql/.local/share/proj DATABASE_PATH=/usr/share/proj/proj.db" (compiled against PROJ 9.1.1) LIBXML="2.9.14" LIBJSON="0.16" LIBPROTOBUF="1.4.1" WAGYU="0.5.0 (Internal)"

• Architecture: aarch64 / arm64
• Data source: Geofabrik Greece .osm.pbf
• Import tool: osm2pgsql flex Lua
• Geometry projection: EPSG:3857
• Client: QGIS
• Example coordinate observed in QGIS:

2626966.8, 4645618.6

This coordinate is consistent with EPSG:3857 / WebMercator meters.

Tegola config using postgis

tile_buffer = 64

[webserver]
port = ":8080"

[cache]
type="file"
basepath="/tmp/tegola"

[[providers]]
name = "test_postgis"
type = "postgis"
uri = "postgres://postgres:postgres@localhost:5432/tegola"

    [[providers.layers]]
    name = "buildings"
    tablename = "public.buildings"
    geometry_fieldname = "geom"
    id_fieldname = "id"

[[maps]]
name = "zoning"
tile_buffer = 64

    [[maps.layers]]
    provider_layer = "test_postgis.buildings"
    min_zoom = 14
    max_zoom = 18

osm2pgsql Lua config

local SRID = 3857

local tables = {}

local counter = 1

tables.buildings = osm2pgsql.define_table({
    name = 'buildings',
    ids = { type = 'area', id_column = 'osm_id' },
    columns = {
        { column = 'id', type = 'int8' },
        { column = 'geom', type = 'polygon', projection = SRID },
        { column = 'relative_feature_height', type = 'real' },
        { column = 'all_entity_names', type = 'text' },
        { column = 'building_type', type = 'text' },
        { column = 'entity', type = 'text' },
        { column = 'date', sql_type = 'timestamp' }
    }
})

function insert_safely(table_key, insert_object)
    if not tables or not tables[table_key] then
        error("Table '" .. tostring(table_key) .. "' does not exist.")
    end

    local inserted, message, column, object = tables[table_key]:insert(insert_object)

    if not inserted then
        print("Insert failed: ", message)
        print("column ", column)
        print("object ", object)
    end

    return true
end

function random_double(min, max)
    return min + math.random() * (max - min)
end

function unix_to_pg_timestamp(unix_time)
    return os.date("%Y-%m-%d %H:%M:%S", unix_time)
end

function osm2pgsql.process_way(object)
    local buildingType = object.tags['building']
    local name = object.tags['name']

    if object.is_closed and buildingType then
        insert_safely("buildings", {
            id = counter,
            entity = name,
            geom = object:as_polygon(),
            building_type = buildingType,
            all_entity_names = name,
            relative_feature_height = random_double(0, 1000),
            date = unix_to_pg_timestamp(object.timestamp)
        })
    end

    counter = counter + 1
end

Notes about the data

The data is polygonal OSM building data imported with:

object:as_polygon()

The table may contain both POLYGON and MULTIPOLYGON geometries.

Reproduction steps

1. Download Greece OSM data from Geofabrik.
2. Import the data into PostGIS using osm2pgsql flex and the Lua file above.
3. Start Tegola using the postgis provider config above.
4. Open the Tegola MVT layer in QGIS.
5. Observe that the building geometries are broken / distorted.
6. Switch the provider to mvt_postgis using the same PostGIS table.
7. Observe that the geometries render correctly.

Screenshots

[ARolek]

@NivGreenstein this is a known issue. I appreciate the detailed report. The fix is to use mvt_postgis.

[NivGreenstein]

@NivGreenstein this is a known issue. I appreciate the detailed report. The fix is to use mvt_postgis.

Thanks!
I used the example configuration from the docs. I think you should update the docs.

Do you know the root cause?

[ARolek]

I used the example configuration from the docs. I think you should update the docs.

I agree! Which docs specifically were you looking at? The ones on tegola.io?

Do you know the root cause?

The root cause is the geo processing algo on polygons. This is something @gdey started work on years ago but hasn't officially cracked. The processing routine is called "Make Valid" and relies on Delaney triangulation. Getting the triangulation to work reliably has been the main blocker (as well as finding time to allocate to the R&D) You can explore some of the implementation at https://github.com/go-spatial/geom/tree/master/planar/makevalid.

[NivGreenstein]

I used the example configuration from the docs. I think you should update the docs.

I agree! Which docs specifically were you looking at? The ones on tegola.io?

Yea - https://tegola.io/documentation/configuration/#full-config-example
I would assume that the "Full Config Example" provides one that works. 😆
Also, a disclaimer for type = "postgis" would be nice.

I would have submitted a PR to enhance the docs, but I’m still no expert with Tegola.

Do you know the root cause?

The root cause is the geo processing algo on polygons. This is something @gdey started work on years ago but hasn't officially cracked. The processing routine is called "Make Valid" and relies on Delaney triangulation. Getting the triangulation to work reliably has been the main blocker (as well as finding time to allocate to the R&D) You can explore some of the implementation at https://github.com/go-spatial/geom/tree/master/planar/makevalid.

Thanks for the context! I'll try to dig into it myself. Any additional context you have would be helpful.

Also, I'm working on a fork to support WorldCRS84Quad tile requests, since my users only use EPSG:4326.
Let me know if that's something you'd be willing to accept into Tegola.

[ARolek]

@NivGreenstein good find! I will get that documentation updated.

Also, I'm working on a fork to NivGreenstein#2, since my users only use EPSG:4326.
Let me know if that's something you'd be willing to accept into Tegola.

Did you see the example we put together for handling 4326 tiles? Let me know if that's what you're looking to achieve.

[NivGreenstein]

@NivGreenstein good find! I will get that documentation updated.

Also, I'm working on a fork to NivGreenstein#2, since my users only use EPSG:4326.
Let me know if that's something you'd be willing to accept into Tegola.

Did you see the example we put together for handling 4326 tiles? Let me know if that's what you're looking to achieve.

Our users work exclusively with EPSG:4326, including in their front-end map components such as Cesium, OpenLayers, etc.
So when they query :z/:x/:y, they expect those coordinates to be in the WorldCRS84Quad tile matrix set, not Web Mercator :z/:x/:y.
You support the 4326 projection on the data side with PostGIS, but not on the client side.