[Mapserver-users] Distance Calculation

[Map Guy]

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We use TIGER data for our mapping data source. We use a geographic projection (lat/lon). When we draw an 8 mile ring on a map with a 20 mile extent, and we plot a point and calculate the distance from the center using Great Circle we get 7.5 miles. Using a Cartesian calculation we get 8.2 miles. 
 
2 questions:
 
1. Should we be seeing this much difference in distances?
 
2. What is the correct way to draw a ring on a map using a geographic projection (lat/lon) and display distances between points using Great Circle without the distance placing the point incorrectly inside or outside the ring?
 
Thanks in advance.
 
P.S. Point me somewhere else if this is the wrong place to ask these questions.


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<DIV>We use TIGER data for our mapping data source. We use a geographic projection (lat/lon). When we draw an 8 mile ring on a map with a 20 mile extent, and we plot a point and calculate the distance from the center using Great Circle we get 7.5 miles. Using a Cartesian calculation we get 8.2 miles. </DIV>
<DIV> </DIV>
<DIV>2 questions:</DIV>
<DIV> </DIV>
<DIV>1. Should we be seeing this much difference in distances?</DIV>
<DIV> </DIV>
<DIV>2. What is the correct way to draw a ring on a map using a geographic projection (lat/lon) and display distances between points using Great Circle without the distance placing the point incorrectly inside or outside the ring?</DIV>
<DIV> </DIV>
<DIV>Thanks in advance.</DIV>
<DIV> </DIV>
<DIV>P.S. Point me somewhere else if this is the wrong place to ask these questions.</DIV><p><hr SIZE=1>
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