[Mapserver-users] Distance Calculation

Wed Mar 3 10:25:08 PST 2004

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Dear Map Guy -
=20
If you draw an 8-mile ring on a "geographic projection" map it will not
be a circle.  If you create a map with a 20-mile extent it will not be a
square.
=20
Maps using "geographic projection" are useless for making distance
measurements; you simply cannot do them in any kind of sensible way.
The width of the top edge (east-west) of your "20 mile" image is
DIFFERENT than the width of the bottom edge.  In fact, each row of
pixels running across the image has a different width in
miles/feet/meters because they're all different distances from the
Equator.  That's why the set of all points 8 miles from a given point is
not a circle or ring; it's not even an ellipse.
=20
You need to choose a projected coordinate system in order to do
reasonable distance measurements.
=20
    - Ed
=20
Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242=20

  _____ =20

From: Map Guy [mailto:mappington at yahoo.com]=20
Sent: Wednesday, March 03, 2004 12:21 PM
To: mapserver-users at lists.gis.umn.edu
Subject: [Mapserver-users] Distance Calculation

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.=20
=20
2 questions:
=20
1. Should we be seeing this much difference in distances?
=20
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?
=20
Thanks in advance.
=20
P.S. Point me somewhere else if this is the wrong place to ask these
questions.

  _____ =20

Do you Yahoo!?
Yahoo! Search - Find what you're looking for faster.
<http://search.yahoo.com/?fr=3Dad-mailsig-home>=20

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<BODY>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>Dear Map Guy -</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>If you draw an 8-mile ring on a "geographic =
projection" map=20
it will not be a circle.  If you create a map with a 20-mile extent =
it will=20
not be a square.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>Maps using "geographic projection" are useless =
for making=20
distance measurements; you simply cannot do them in any kind of sensible =

way.  The width of the top edge (east-west) of your "20 mile" image =
is=20
DIFFERENT than the width of the bottom edge.  In fact, each row of =
pixels=20
running across the image has a different width in miles/feet/meters =
because=20
they're all different distances from the Equator.  That's why the =
set of=20
all points 8 miles from a given point is not a circle or ring; it's not =
even an=20
ellipse.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>You need to choose a projected coordinate =
system in order=20
to do reasonable distance measurements.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN =
class=3D606261418-03032004>    <FONT=20
face=3DArial color=3D#0000ff size=3D2>- Ed</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D606261418-03032004>
<P><FONT size=3D2>Ed McNierney<BR>President and Chief =
Mapmaker<BR>TopoZone.com /=20
Maps a la carte, Inc.<BR>73 Princeton Street, Suite 305<BR>North =
Chelmsford,=20
MA  01863<BR>ed at topozone.com<BR>(978) 251-4242 =
</FONT></P></SPAN></DIV><BR>
<DIV class=3DOutlookMessageHeader lang=3Den-us dir=3Dltr align=3Dleft>
<HR tabIndex=3D-1>
<FONT face=3DTahoma size=3D2><B>From:</B> Map Guy =
[mailto:mappington at yahoo.com]=20
<BR><B>Sent:</B> Wednesday, March 03, 2004 12:21 PM<BR><B>To:</B>=20
mapserver-users at lists.gis.umn.edu<BR><B>Subject:</B> [Mapserver-users] =
Distance=20
Calculation<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>We use TIGER data for our mapping data source. We use a geographic=20
projection (lat/lon). When we draw an 8 mile ring on a map with a 20 =
mile=20
extent, and we plot a point and calculate the distance from the=20
center using Great Circle we get 7.5 miles. Using a Cartesian =
calculation=20
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=20
projection (lat/lon) and display distances between points using Great =
Circle=20
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=20
questions.</DIV>
<P>
<HR SIZE=3D1>
Do you Yahoo!?<BR>Yahoo! Search - <A=20
href=3D"http://search.yahoo.com/?fr=3Dad-mailsig-home">Find what =
you’re looking for=20
faster.</A></BODY></HTML>

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