Given latitude and longitude, how to find in what country those coordinates are?

[clzola]

This is more of a question as I need help to figure out...

So as ti title says:

Given latitude and longitude, how to find in what country those coordinates are?

I have geojson file with country borders. Each country is represented with either Polygon or MultiPolygon. If I understand correctly, those coordinates that are inside geojson file are longitude and latitude in EPSG:4236 CRS. Because of that I cannot just check relation between Point (longitude, latitude) and Polygon because it might give error because operation/methods from this library and JTS does not take into account curvature of earth.

So my idea is first to convert all geometries from EPSG:4326 to Google Mercator, and then convert given longitude and latitude, and finally perform contains and intersect operations?

Am I on the right way? Or can I directly use geometries without transformation from one CRS to another (I am willing to tolerate some error, like some near border cases)?

Also for the optimization purposes, I would build quad tree and boundary box checks...

[dsmiley]

Most libraries don't take into consideration curvature of the earth in a direct sense, but if the polygons have sufficient detail and if the data isn't too close to the poles (where some projections lose accuracy), then this is a moot point. Also keep in mind that some polygons were plotted/authored in one projection but the gets re-projected to another afterwards and then the polygon could become technically invalid according to the 2 dimensional rules that JTS and most 2D systems expect. Basically a line that didn't intersect another vertex will after re-projection. But again, with sufficient detail, it won't happen. Another problem that can happen is that you think/hope regional polygons don't overlap but they may actually slightly on the edges if there were re-projection / translations.

So in summary, I wouldn't worry about it but if you see polygon errors, you may need more detail.