ADD: new track message, Entity class and Position class

libs/geographiclib/examples/example-NormalGravity.cpp

@@ -0,0 +1,25 @@
+// Example of using the GeographicLib::NormalGravity class
+
+#include <iostream>
+#include <exception>
+#include <GeographicLib/NormalGravity.hpp>
+#include <GeographicLib/Constants.hpp>
+
+using namespace std;
+using namespace GeographicLib;
+
+int main() {
+  try {
+    NormalGravity grav(Constants::WGS84_a(), Constants::WGS84_GM(),
+                       Constants::WGS84_omega(), Constants::WGS84_f());
+    // Alternatively: const NormalGravity& grav = NormalGravity::WGS84();
+    double lat = 27.99, h = 8820; // Mt Everest
+    double gammay, gammaz;
+    grav.Gravity(lat, h, gammay, gammaz);
+    cout << gammay << " " << gammaz << "\n";
+  }
+  catch (const exception& e) {
+    cerr << "Caught exception: " << e.what() << "\n";
+    return 1;
+  }
+}