s2geography

Google's s2geometry is a spherical geometry engine providing accurate and performant geometry operations for geometries on the sphere. This library provides a compatability layer on top of s2geometry for those more familiar with simple features, GEOS, and/or the GEOS C API.

The s2geography library was refactored out of the s2 package for R, which has served as the backend for geometries with geographic coordinates in the popular sf package for R since version 1.0.0. The library is currently under construction as it adapts to suit the needs of more than just a single R package. Suggestions to modify, replace, or completely rewrite this library are welcome!

Example

A quick example (see also the examples/ directory):

#include <stdio.h>

#include <iostream>

#include "s2geography.h"

using namespace s2geography;

int main(int argc, char *argv[]) {
  WKTReader reader;
  std::unique_ptr<Geography> geog1 = reader.read_feature("POINT (-64 45)");
  std::unique_ptr<Geography> geog2 = reader.read_feature(
      "GEOMETRYCOLLECTION (POINT (30 10), LINESTRING (30 10, 10 30, 40 40), "
      "POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10)))");

  ShapeIndexGeography geog1_index(*geog1);
  ShapeIndexGeography geog2_index(*geog2);

  double dist = s2_distance(geog1_index, geog2_index);

  printf("distance result is %g\n", dist);

  WKTWriter writer;
  std::cout << "geog1: " << writer.write_feature(*geog1) << "\n";
  std::cout << "geog2: " << writer.write_feature(*geog2) << "\n";
}

Overview

The basic unit in s2geography is the Geography class. The three main subclasses of this wrap std::vector<S2Point>, std::vector<std::unique_ptr<S2Polyline>>, std::unique_ptr<S2Polygon>, and std::vector<std::unique_ptr<Geography>>; however, the Geography class is parameterized as zero or more S2Shape objects that also define an S2Region. This allows a flexible storage model (although only the four main subclasses have been tested).

Many operations in S2 require a S2ShapeIndex as input. This concept is similar to the GEOS prepared geometry and maps to the ShapeIndexGeography in this library. For indexing a vector of features, use the GeographyIndex (similar to the GEOS STRTree object).

The s2geography library sits on top of the s2geometry library, and you can and should use s2 directly!

Build and Installation (from source)

There is no s2geography package available yet. If you want to use it, you need to build it from source. You can download the source by cloning this repository:

git clone https://github.com/paleolimbot/s2geography.git

Requirements

CMake
s2geometry
Abseil
OpenSSL (via s2geometry)

Conda

All the required dependencies above are available on conda-forge. You can install them using conda (or mamba):

conda install cmake libabseil s2geometry openssl -c conda-forge

Homebrew (MacOS)

Alternatively, you can install the required dependencies on MacOS with Homebrew:

brew install cmake abseil s2geometry openssl

Build using CMake

s2geography uses CMake to build the library. You first need to configure the build, e.g, using the following commands (from where the source has been downloaded or cloned):

mkdir build
cd build
cmake .. -DS2GEOGRAPHY_S2_SOURCE=AUTO -DCMAKE_CXX_STANDARD=17
cmake --build .

The CMake option S2GEOGRAPHY_S2_SOURCE specifies the method to use for acquiring s2geometry:

AUTO: try to find s2geometry on the system default locations or download and build it from source if not found (default)
BUNDLED: download and build s2geometry automatically from source
SYSTEM: use s2geometry installed on one of the system default locations
CONDA: use s2geometry installed in a conda environment (automatically selected when the environment is active)
BREW: use s2geometry (and OpenSSL) installed with Homebrew

Note: s2geography does not support automatically acquiring and building Abseil and OpenSSL from source. If you don't have installed those libraries with conda or Homebrew, you might need to manually specify their location using the CMake optionsabsl_DIR and OPENSSL_ROOT_DIR.

The CMake option CMAKE_CXX_STANDARD should be set according to the standard used to build Abseil and s2geometry (C++17 is set by default).

The project is structured such that the VSCode CMake integration is triggered when the folder is open (if the default build doesn't work, consider adding CMakeUserPresets.json to configure things like the install directory, absl_DIR, or the location of OpenSSL).

Install

After building the library, you can install it using:

cmake --install . --prefix ../dist

When building and installing in a conda environment, you can specify the conda environment's prefix location:

cmake --install . --prefix $CONDA_PREFIX

Or alternatively configure this by passing -DCMAKE_INSTALL_PREFIX=$CONDA_PREFIX when invoking cmake to configure the project.

Development

s2geography provides units tests that can be built and run using GTest. To enable it, use the CMake option S2GEOGRAPHY_BUILD_TESTS=ON (GTest will be downloaded and built automatically):

cmake .. -DS2GEOGRAPHY_BUILD_TESTS=ON
cmake --build .

You can then run the tests using ctest:

ctest -T test --output-on-failure .

You can also run specific tests using ctest's -R flag:

ctest -T test . -R "Distance$"

s2geography also provides some examples that can be build using:

cmake .. -DS2GEOGRAPHY_BUILD_EXAMPLES=ON
cmake --build .

For VSCode users (with the C/C++ and CMake extensions), the CMakeUserPresets.json.example file shows a possible test/configuration preset that will build and run the tests and the examples.