Not self intersecting geojson becomes self intersecting as a result of simplify

[tiokmu1731]

First, I thank everyone participating in this project.

I guess it's not a bug, but I now found the case that

• The original geojson is not self intersecting.
• As a result of simplify in converting to vector-tile, generated vector-tile becomes self intersecting.

The original data I used is data of the Amami Islands from National land numerical information, Japan. The geojson I attached is clipped from the original. This clipped geojson is not self intersecting.

A20-070402_46_AmamiIslands_kasari.json

But after I converting this geojson to vector-tile using geojson-vt in the code below, generated vector-tile becomes self intersecting as a result of simplify.

import geojsonvt from 'geojson-vt';
import geojson from "./geojson/A20-070402_46_AmamiIslands_kasari.json";

const tileIndex = geojsonvt(geojson);

const z = 10;
const x = 880;
const y = 427;

// request a particular tile
const features = tileIndex.getTile(z, x, y).features;

const amami_geometry = features[12].geometry[0];

// get WKT
let cood = "POLYGON((";

for (let i = 0; i < amami_geometry.length; i++) {
    cood+= amami_geometry[i][0] + (' -') + amami_geometry[i][1] + ',';
}

cood += "))";
console.log(cood);

Left: Original geojson Right: Geometry made into vector tiles using geojson-vt

I'm using Maplibre and it seems to use geojson-vt in converting to vector-tile. In Maplibre project, an issue that self intersecting vector tile causes displaying unexpected form has been raised before. I tried to display the geojson I attached in Maplibre, but it is also not displayed as I expected. *An acute-angled triangle is displayed in particular zoom level:9.0-10.9. (At the point of blue arrow)

The result I expect is the generated vector tile from geojson-vt is not self intersecting if the original geo-json is as well. Is it possible to fix this case in geojson-vt? *As I search similar cases, Tippecanoe corrects the geometry using mapbox/wagyu libraly.

[mourner]

Yes, basically this is still an unsolved problem, and I still don't know how to address it. You can read more context / history on the issue in https://github.com/mapbox/mapbox-gl-js/issues/7023

[tiokmu1731]

@mourner I was very surprised to receive a reply from the original author of Leaflet. Our team often uses Leaflet, and we appreciate your development of this wonderful map library.

I learned that this issue has already existed in mapbox-gl-js since 2018, and I understood that it would be technically difficult to resolve. Your quick reply was very helpful. Thank you for letting us know about this issue!