The poly_grid from imagico

Shapefiles SRSs, +over or not?

I'm having issues rendering osm-carto, oceans and seas do not render. I remembered this comment and took a look:

$ ogrinfo -al data/water-polygons-split-3857/water_polygons.shp
INFO: Open of `data/water-polygons-split-3857/water_polygons.shp'
    using driver `ESRI Shapefile' successful.

Layer name: water_polygons
Metadata:
DBF_DATE_LAST_UPDATE=2019-05-28
Geometry: Polygon
Feature Count: 14488
Extent: (-20037508.340000, -14773407.419294) - (20037508.340000, 20037508.340000)
Layer SRS WKT:
PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["X",EAST],
    AXIS["Y",NORTH],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"],
    AUTHORITY["EPSG","3857"]]
FID: Integer64 (11.0)

The SRS seem to still lack the +over parameter. Now, I read again all that thread and @imagico mentions that mapnik back then was reprojecting to some other projection and then back, ruining everything.

In my case, the issue is that no seas or oceans are rendered, using either the water polygons or the simplified version. Turning on Mapnik's debug messages I see that the issue is that for some reason, mapnik is not finding things to render for those layers:

agg_renderer: Start processing layer=ocean-lz
agg_renderer: -- datasource=0x1f03ee0
agg_renderer: -- query_extent=box2d(460456.6583899010438472,5080310.6479459535330534,1086628.7941020664293319,5706482.7836581189185381)
agg_renderer: Start processing style
shape_index_featureset: 0 features
agg_renderer: End processing style
agg_renderer: End layer processing

I'm writing an email to the mapnik google group to try to figure all this out (I'm definitely not an expert on projections, and I can't even understand in what projection those coordinates are), but maybe you can shed a light too. Remember, osm-carto's SRS is srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over".

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Shapefiles SRSs, +over or not?

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