From: Jarkko Hietaniemi <jhi@iki.fi>
Date: Wed, 18 Aug 1999 08:27:22 +0000 (+0000)
Subject: Warn about small spherical distances.
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=41bd693c38b93b50972065296573f26c13cc43d1;p=p5sagit%2Fp5-mst-13.2.git

Warn about small spherical distances.

p4raw-id: //depot/cfgperl@4003
---

diff --git a/lib/Math/Trig.pm b/lib/Math/Trig.pm
index 924286d..d987b5c 100644
--- a/lib/Math/Trig.pm
+++ b/lib/Math/Trig.pm
@@ -409,7 +409,16 @@ To calculate the distance between London (51.3N 0.5W) and Tokyo (35.7N
         $km = great_circle_distance(@L, @T, 6378);
 
 The answer may be off by few percentages because of the irregular
-(slightly aspherical) form of the Earth.
+(slightly aspherical) form of the Earth.  The used formula
+
+	lat0 = 90 degrees - phi0
+	lat1 = 90 degrees - phi1
+	d = R * arccos(cos(lat0) * cos(lat1) * cos(lon1 - lon01) +
+                       sin(lat0) * sin(lat1))
+
+is also somewhat unreliable for small distances (for locations
+separated less than about five degrees) because it uses arc cosine
+which is rather ill-conditioned for values close to zero.
 
 =head1 BUGS