Request for Guidance - Point in Polygon and S2

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Avaliação

Dificuldade
5/5
Tempo estimado
Mais de uma semana
Facilidade para iniciantes
15/100
Tipo de issue
Funcionalidade
Clareza
Precisa de esclarecimento
Status de atividade
Estagnada
Stack de tecnologia
android, java
Domínio
computer-graphics, mobile-dev

Direção de pesquisa

Nenhum arquivo do repositório, teste ou ponto de entrada é nomeado. Comece revisando as APIs referenciadas de S2Polygon, S2PolygonBuilder, S2Loop e S2Point e, em seguida, esclareça quais das seis questões sobre Android e point-in-polygon offline devem se tornar uma mudança concreta no repositório; o trabalho estará concluído quando o escopo e os critérios de aceitação tiverem sido acordados.

Escrita pelo modelo de indexação a partir do texto da issue.

Descrição

Would it be possible for the good folks here to please help me with a problem that I am trying to solve with the S2 geometry library? I am a complete novice, so please bear with me if I do not make sense.

Problem
I am trying to determine the best way to write an offline point-in-polygon check for the Android platform. Essentially, convert (lat,lng) to a country. In future, I plan to increase the granularity to regions, states, provinces, counties and cities/towns.

Research
My initial research landed me up on https://github.com/AReallyGoodName/OfflineReverseGeocode with http://download.geonames.org/export/dump/cities1000.zip. From what I understand, this is (lat,lng) to nearest neighbor. Now, I do get the country from this, but it relies on finding the nearest neighbor city with population, which may not be accurate.

Further research led me to S2. It is here that I am struggling, and request your guidance.

Current Approach
A) Computation at app startup:

  1. Get geojson co-ordinates from http://download.geonames.org/export/dump/shapes_simplified_low.zip.
  2. For each country,
    i) Convert geojson co-ordinates into S2 LatLng and then S2 Loops.
    ii) Build S2 Polygon using S2 Loops.
    iii) If MultiPolygon, use S2PolygonBuilder.addPolygon() to add additional polygons.

B) LatLng input during app functioning:

  1. Convert Lat Lng Input to S2Point.
  2. Iterate over country polygons, checking for S2Polygon.contains(S2Point).
  3. Return country.

Questions

  1. Some geojson countries are "multipolygon", whereas there is no multipolygon in S2. Are S2 polygons equivalent to multipolygons from geojson?
  2. Can the geojson to S2Polygon initial computation result be persisted, to avoid re-computation at app startup?
  3. Geonames geojson input does not seem to conform to counter clockwise (CCW) convention. Would S2PolygonBuilder be able to tackle this correctly?
  4. Is there a better way to search for point-in-polygon rather than iterating over each countries S2Polygons.contains function call? Does this mechanism also help avoid polygon re-computation?
  5. Is there a better way to achieve the offline point-in-polygon check?
  6. Is there a better (small, accurate) source for country boundaries for use with S2?

Thank you for your time.

@eengle @hagbard @kluever @cpovirk

Linguagem predominante
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