1 files changed, 36 insertions, 0 deletions
diff --git a/src/auth.cpp b/src/auth.cpp
new file mode 100644
index 00000000..cde60a29
--- /dev/null
+++ b/src/auth.cpp
@@ -0,0 +1,36 @@
+/* determine latitude from authalic latitude */
+
+#include <math.h>
+#include <stddef.h>
+
+#include "projects.h"
+
+# define P00 .33333333333333333333 /*   1 /     3 */
+# define P01 .17222222222222222222 /*  31 /   180 */
+# define P02 .10257936507936507937 /* 517 /  5040 */
+# define P10 .06388888888888888888 /*  23 /   360 */
+# define P11 .06640211640211640212 /* 251 /  3780 */
+# define P20 .01677689594356261023 /* 761 / 45360 */
+#define APA_SIZE 3
+
+	double *
+pj_authset(double es) {
+	double t, *APA;
+
+	if ((APA = (double *)pj_malloc(APA_SIZE * sizeof(double))) != nullptr) {
+		APA[0] = es * P00;
+		t = es * es;
+		APA[0] += t * P01;
+		APA[1] = t * P10;
+		t *= es;
+		APA[0] += t * P02;
+		APA[1] += t * P11;
+		APA[2] = t * P20;
+	}
+	return APA;
+}
+	double
+pj_authlat(double beta, double *APA) {
+	double t = beta+beta;
+	return(beta + APA[0] * sin(t) + APA[1] * sin(t+t) + APA[2] * sin(t+t+t));
+}