1 files changed, 15 insertions, 10 deletions
diff --git a/src/adjlon.c b/src/adjlon.c
index 09b3b14b..784a90aa 100644
--- a/src/adjlon.c
+++ b/src/adjlon.c
@@ -1,15 +1,20 @@
 /* reduce argument to range +/- PI */
 #include <math.h>
-#include <projects.h>
-
-#define SPI     3.14159265359
-#define TWOPI   6.2831853071795864769
-#define ONEPI   3.14159265358979323846
+#include "projects.h"
 
 double adjlon (double lon) {
-    if (fabs(lon) <= SPI) return( lon );
-    lon += ONEPI;  /* adjust to 0..2pi rad */
-    lon -= TWOPI * floor(lon / TWOPI); /* remove integral # of 'revolutions'*/
-    lon -= ONEPI;  /* adjust back to -pi..pi rad */
-    return( lon );
+    /* Let lon slightly overshoot, to avoid spurious sign switching at the date line */
+    if (fabs (lon) < M_PI + 1e-12)
+        return lon;
+
+    /* adjust to 0..2pi range */
+    lon += M_PI;
+
+    /* remove integral # of 'revolutions'*/
+    lon -= M_TWOPI * floor(lon / M_TWOPI);
+
+    /* adjust back to -pi..pi range */
+    lon -= M_PI;
+
+    return lon;
 }