tmp

example/CMakeLists.txt

@@ -22,4 +22,5 @@ add_example(mobser_tetra_ex OFF)
 add_example(mobser_tetra_ex2 OFF)
 add_example(mobser_tetra_sph_ex OFF)
 add_example(mobser_tesseroid_ex OFF)
-add_example(read_IGRF_ex OFF)
+add_example(read_IGRF_ex OFF)
+add_example(power_spectrum_ex OFF)

example/power_spectrum_ex.cpp

@@ -0,0 +1,65 @@
+/********************************************************
+ *  ██████╗  ██████╗████████╗██╗
+ * ██╔════╝ ██╔════╝╚══██╔══╝██║
+ * ██║  ███╗██║        ██║   ██║
+ * ██║   ██║██║        ██║   ██║
+ * ╚██████╔╝╚██████╗   ██║   ███████╗
+ *  ╚═════╝  ╚═════╝   ╚═╝   ╚══════╝
+ * Geophysical Computational Tools & Library (GCTL)
+ *
+ * Copyright (c) 2022  Yi Zhang (yizhang-geo@zju.edu.cn)
+ *
+ * GCTL is distributed under a dual licensing scheme. You can redistribute 
+ * it and/or modify it under the terms of the GNU Lesser General Public 
+ * License as published by the Free Software Foundation, either version 2 
+ * of the License, or (at your option) any later version. You should have 
+ * received a copy of the GNU Lesser General Public License along with this 
+ * program. If not, see <http://www.gnu.org/licenses/>.
+ * 
+ * If the terms and conditions of the LGPL v.2. would prevent you from using 
+ * the GCTL, please consider the option to obtain a commercial license for a 
+ * fee. These licenses are offered by the GCTL's original author. As a rule, 
+ * licenses are provided "as-is", unlimited in time for a one time fee. Please 
+ * send corresponding requests to: yizhang-geo@zju.edu.cn. Please do not forget 
+ * to include some description of your company and the realm of its activities. 
+ * Also add information on how to contact you by electronic and paper mail.
+ ******************************************************/
+
+#include "gctl/io.h"
+#include "gctl/geometry.h"
+#include "../lib/potential.h"
+
+using namespace gctl;
+
+int main(int argc, char const *argv[]) try
+{
+    std::string data_dir = "/Volumes/DataArchive/Earth/Global/MagneticModels/EMM2017_Linux";
+    text_content tc;
+    tc.head_num_ = 1;
+    tc.load_text(data_dir + "/EMM2017", ".COF");
+
+    int n, m;
+    _1d_array P, C, S;
+    std::stringstream tmp_ss;
+
+    C.resize(tc.lines_.size());
+    S.resize(tc.lines_.size());
+    for (size_t i = 0; i < tc.lines_.size(); i++)
+    {
+        str2ss(tc.lines_[i], tmp_ss);
+        tmp_ss >> n >> m >> C[i] >> S[i];
+    }
+
+    power_spectrum_shc(P, C, S, 6371200, 6371200, 1, 65);
+
+    for (size_t i = 0; i < P.size(); i++)
+    {
+        std::cout << i + 1 << " " << P[i] << "\n";
+    }
+    
+    return 0;
+}
+catch (std::exception &e)
+{
+    GCTL_ShowWhatError(e.what(), GCTL_ERROR_ERROR, 0, 0, 0);
+}

lib/potential/gm_data.cpp

@@ -239,4 +239,27 @@ void gctl::magnetic_tensors2deltaTs_sph(const array<tensor> &Mag_tensors, const
         deltaTs[i].z = za_f*Mag_tensors[i][0][2] - hay_f*Mag_tensors[i][1][2] + hax_f*Mag_tensors[i][2][2];
     }
     return;
+}
+
+void gctl::power_spectrum_shc(array<double> &P, const array<double> &C, const array<double> &S, double R, double r, int n, int N)
+{
+    if (n < 0 || n >= N || C.size() < (N + 1)*(N + 2)/2 || S.size() < (N + 1)*(N + 2)/2)
+    {
+        throw std::runtime_error("[gctl::power_spectrum_shc] Invalid input parameters.");
+    }
+
+    P.resize(N - n + 1, 0);
+
+    int id;
+    for (int i = n; i < N + 1; i++)
+    {
+        id = i*(i + 1)/2;
+        for (int j = 0; j < i + 1; j++)
+        {
+            P[i - n] += (C[id + j]*C[id + j] + S[id + j]*S[id + j]);
+        }
+
+        P[i - n] *= (n + 1)*pow(R/r, 2*n + 4);
+    }
+    return;
 }

lib/potential/gm_data.h

@@ -196,6 +196,20 @@ namespace gctl
      * @param T0_declina Declination degrees of the earth normal magnetic field.
      */
     void magnetic_tensors2deltaTs_sph(const array<tensor> &Mag_tensors, const _1d_array &T0_inclina, const _1d_array &T0_declina, array<point3dc> &deltaTs);
+
+    /**
+     * @brief Calculate the power spectrum of given spherical harmonic coefficients.
+     * 
+     * @param P Returned power spectrum
+     * @param C Cosine coefficients strats from 0 order 0 degree up to least N order N degree
+     * @param S Sine coefficients strats from 0 order 0 degree up to least N order N degree
+     * @param R Reference radius (usually is 6371.2 km)
+     * @param r Observation radius
+     * @param n Strat order
+     * @param N End order
+     */
+    void power_spectrum_shc(array<double> &P, const array<double> &C, const array<double> &S, 
+        double R, double r, int n, int N);
 }
 
 #endif // _GCTL_GM_DATA_H