ADD: new track message, Entity class and Position class

libs/geographiclib/examples/example-MagneticCircle.cpp

@@ -0,0 +1,43 @@
+// Example of using the GeographicLib::MagneticCircle class
+// This requires that the wmm2010 magnetic model be installed; see
+// https://geographiclib.sourceforge.io/C++/doc/magnetic.html#magneticinst
+
+#include <iostream>
+#include <exception>
+#include <GeographicLib/MagneticModel.hpp>
+#include <GeographicLib/MagneticCircle.hpp>
+
+using namespace std;
+using namespace GeographicLib;
+
+int main() {
+  try {
+    MagneticModel mag("wmm2010");
+    double lat = 27.99, lon0 = 86.93, h = 8820, t = 2012; // Mt Everest
+    {
+      // Slow method of evaluating the values at several points on a circle of
+      // latitude.
+      for (int i = -5; i <= 5; ++i) {
+        double lon = lon0 + i * 0.2;
+        double Bx, By, Bz;
+        mag(t, lat, lon, h, Bx, By, Bz);
+        cout << lon << " " << Bx << " " << By << " " << Bz << "\n";
+      }
+    }
+    {
+      // Fast method of evaluating the values at several points on a circle of
+      // latitude using MagneticCircle.
+      MagneticCircle circ = mag.Circle(t, lat, h);
+      for (int i = -5; i <= 5; ++i) {
+        double lon = lon0 + i * 0.2;
+        double Bx, By, Bz;
+        circ(lon, Bx, By, Bz);
+        cout << lon << " " << Bx << " " << By << " " << Bz << "\n";
+      }
+    }
+  }
+  catch (const exception& e) {
+    cerr << "Caught exception: " << e.what() << "\n";
+    return 1;
+  }
+}