[EHPweb] Change to the bearing function in the Nearest Cities

[Matthew J Donnelly]

The Problem:  It was found that the bearing function used in nearest 
cities incorrectly calculated bearing if it required going over the 180 
degree meridian.   Also on further inspection, the algorithm it was using 
has great distortion of the bearing when at high latitudes. 

The Solution:  I developed a simple methodology for determining if the 
bearing crosses 180 and the way to handle bearing in that case.   I also 
replaced the existing algorithm that calculates bearing by using Rhumb 
line (basically the planar bearing) with one that calculates it using a 
Great Circle calculation.  By using the Great Circle it seems to have 
fixed the distortion we saw at high latitudes. 

Matthew

-------------------------
Matthew J Donnelly
Information Technology Specialist
National Earthquake Information Center
US Geological Survey
Golden, CO 80401
Ph: 303-273-8590
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