geoContains function doesn't seem to handle MultiPolygons

Question

geoContains function doesn't seem to handle MultiPolygons

secondl1ght opened this issue 2 years ago · comments

I am using the geoContains function to determine if a point is inside a polygon. It works fine for simple polygons, but it does not seem to work for multi polys. Is this expected behavior? Thanks

Philippe Rivière · Answer 1 · Thu Jan 19 2023 16:10:18 GMT+0800 (China Standard Time)

It should work. Please make sure the polygon has the expected winding order. https://observablehq.com/@fil/rewind
I'm going to close the issue, but if you think this is a bug, please share the data which triggers it, and I'll reopen.

secondl1ght · Answer 2 · Thu Jan 19 2023 23:33:33 GMT+0800 (China Standard Time)

Thanks for the quick response @Fil, I have tested to make sure the polygon is in the expected winding order by using this GeoJSON linter at https://geojsonlint.com/. It will notify you if you need to rewind the data and also the polygon is displaying correctly on my map and I am not seeing this behavior you described:

Alas when it happens, it’s painful: a shape that represents a tiny speck of land becomes inflated to represent the whole globe minus that tiny speck of land, the map fills with a uniform color, the local projection explodes.

Here is an example of one of the GeoJSON files that does not seem to be working correctly for me: https://gist.github.com/secondl1ght/a902b36dac2379fead3452af49a77973

The same code I wrote is working for other polygons so your theory does make sense that it would be certain GeoJSONs that I am using causing issues. I thought I had it narrowed down to a multipoly issue because those seemed to be the problem ones from when I looked. Maybe you can see if something is wrong with this Dublin file I shared?

Thanks for the help!

Philippe Rivière · Answer 3 · Fri Jan 20 2023 02:29:00 GMT+0800 (China Standard Time)

If you rewind it it works
https://observablehq.com/@recifs/dublin-geojson--support

secondl1ght · Answer 4 · Fri Jan 20 2023 04:06:15 GMT+0800 (China Standard Time)

Wow ok thanks! I guess I should have tested that the polygon drew correctly using D3. I am trying to find your package to use but I don't see it on NPM when I search for @fil/rewind, do you mind sharing a link to it please?

Philippe Rivière · Answer 5 · Fri Jan 20 2023 04:38:27 GMT+0800 (China Standard Time)

For now it's just a function in a notebook. https://observablehq.com/@fil/rewind

More discussion at #138

secondl1ght · Answer 6 · Fri Jan 20 2023 12:14:43 GMT+0800 (China Standard Time)

Thanks, I had a look and it's not super clear to me which part of the code I need to copy over into my app. There is a lot of code there, do I need all of it to complete the rewind that I require?

Philippe Rivière · Answer 7 · Fri Jan 20 2023 13:07:12 GMT+0800 (China Standard Time)

Consider fixing the geojson outside your application. If that's not possible, then you should copy 3 cells: geoRewindFeature and its dependencies geoRewindStream and geoProjectSimple. Also make sure you agree with the license (which I had not set yet, now ISC).

secondl1ght · Answer 8 · Sat Jan 21 2023 00:57:36 GMT+0800 (China Standard Time)

Thanks, is there also a way to check which polygons need to be rewound? I don't want to rewind ones that are already following the required spec.

secondl1ght · Answer 9 · Sat Jan 21 2023 00:59:01 GMT+0800 (China Standard Time)

I tried fixing the geojsons outside the application using https://geojsonlint.com/ to check for the 'right-hand rule'. But for some reason the problem GeoJSONs are not being picked up with errors on the lint website but they are still not working with D3.