[Mapserver-users] Distance Calculation
Ed McNierney
ed at topozone.com
Wed Mar 3 15:55:11 EST 2004
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Yes, that helps a lot. First, you MUST project your data if you're
going to create a raster image of it, unless your users have ellipsoidal
monitor screens <g>. Using lat/lon coordinates as X/Y coordinates is a
projection, too - it's just not a very useful one for most purposes. If
you're going to measure distances you need appropriately projected maps.
=20
You might do best with the UTM projection family. It is simple, common,
and easy to use. It is not an equidistant projection and there will be
a VERY small distortion, but for areas as small as yours I think it will
be completely invisible. There are other more accurate projections
(equidistant conic, for example) that require more work on your part.
=20
You do, however, need to choose the appropriate UTM zone for each map
you make, based on the longitude of the center point. If you try to
make a map that is very small scale (showing a large portion of the
country) it will look strange around the edges. But if you make maps of
less than a few hundred miles in extent no one will notice.
=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 3:29 PM
To: Ed McNierney
Cc: mapserver-users at lists.gis.umn.edu
Subject: RE: [Mapserver-users] Distance Calculation
We are trying to map underlying layers like streets, parks, cities, etc.
taken from TIGER for the United States. Then on top of this we are
drawing a ring usually less than 20 miles with the center being the
center of the map. Then we are mapping points given to us with lat/lon.
In the legend we put the distance of the point from the center of the
map. It is this distance which when done with Great Circle calc raised
the red flag because the distance would put the point outside the circle
when it was being drawn inside the circle. If we used Cartesian
distance, then the distance would be off.=20
=20
So, based on your answers, we MUST project our data in order for our
distance shown in the legend to be accurate and also match what we draw
on the map, right?
=20
Thanks and let me know if you need more detail to provide suggestions.
Ed McNierney <ed at topozone.com> wrote:=20
I don't know what a "Cartesian" distance in miles means when
your units are in degrees and the conversion between degrees and miles
depends on the length and the orientation of the line.
=20
As I said, if you can provide more detail we can offer more
advice. For example, if all your distances are at a "small scale",
that's more detail (you didn't say that before). That helps define the
problem better and helps us suggest solutions. Please describe the
problem you're trying to solve and we'll help!
=20
- Ed
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 3:15 PM
To: Ed McNierney
Cc: mapserver-users at lists.gis.umn.edu
Subject: RE: [Mapserver-users] Distance Calculation
=09
=09
I know it is a very simple question, but I didn't think the
results would be so different at such a small scale. Would you tell me
if these results seem right:
=20
Point 1 (Lon,Lat): -87.6552,41.9148
Point 2 (Lon,Lat): -87.8081,41.8884
=20
Cartesian Distance between Point 1 and 2 in miles: 11.2
Great Circle (assuming sphere) Distance between Point 1 and 2:
8.1=20
=20
Thanks again.
=09
Ed McNierney <ed at topozone.com> wrote:
First, please ALWAYS reply to the entire list. That way
everyone benefits from the conversation, and the first person to answer
your question doesn't have to make a lifetime commitment to further
questions!
=20
Yes, of course you can choose one projection for your
entire set of data. Or you could switch based on what you're mapping.
The reason there are so many map projections is because each has
different properties, and therefore does some things well and other
things less well. The classic Mercator projection is excellent if you
are navigating with a compass; it is very poor if you're trying to
persuade someone that Brazil is several times larger than Greenland.
=20
You also need to have a feel for what level of accuracy
you need for your measurements. The UTM family of projections is
popular in part because it allows fairly accurate distance measurements,
does not distort shapes badly, and is easy to use. However, you need 10
different UTM zones to cover the lower 48 states. If you need even more
accuracy a custom Transverse Mercator projection centered on the
longitude of your central point would be better.
=20
Mapping large areas in a single map is harder to do in
an appealing way than mapping small areas, because spatial distortion
becomes more obvious.
=20
If you can provide a bit more detail on what you're
trying to do we can offer more advice.
=20
- Ed
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 2:01 PM
To: Ed McNierney
Subject: RE: [Mapserver-users] Distance Calculation
=09
=09
Thank you for your quick response.
=20
Is it possible to choose one projection for my entire
set of data which includes the whole United States (TIGER data). Or, do
I somehow have to switch based on what I am mapping? If the latter is
true, then do you have a suggestion on how I would do that?
=20
Again, point me elsewhere if I am pushing in the wrong
direction.
=20
Thank you,
=20
Map Guy
=09
Ed McNierney <ed at topozone.com> wrote:
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
=09
=09
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.
=09
_____ =20
Do you Yahoo!?
Yahoo! Search - Find what you're looking for
faster. <http://search.yahoo.com/?fr=3Dad-mailsig-home> =20
=09
_____ =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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_____ =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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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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<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>Yes, that helps a lot. First, you MUST =
project your=20
data if you're going to create a raster image of it, unless your users =
have=20
ellipsoidal monitor screens <g>. Using lat/lon coordinates =
as X/Y=20
coordinates is a projection, too - it's just not a very useful one for =
most=20
purposes. If you're going to measure distances you need =
appropriately=20
projected maps.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>You might do best with the UTM projection =
family. It=20
is simple, common, and easy to use. It is not an equidistant =
projection=20
and there will be a VERY small distortion, but for areas as small as =
yours I=20
think it will be completely invisible. There are other more =
accurate=20
projections (equidistant conic, for example) that require more work on =
your=20
part.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>You do, however, need to choose the appropriate =
UTM zone=20
for each map you make, based on the longitude of the center point. =
If you=20
try to make a map that is very small scale (showing a large portion of =
the=20
country) it will look strange around the edges. But if you make =
maps of=20
less than a few hundred miles in extent no one will =
notice.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN =
class=3D284524120-03032004> <FONT=20
face=3DArial color=3D#0000ff size=3D2>- Ed</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D284524120-03032004>
<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 3:29 PM<BR><B>To:</B> Ed=20
McNierney<BR><B>Cc:</B> =
mapserver-users at lists.gis.umn.edu<BR><B>Subject:</B> RE:=20
[Mapserver-users] Distance Calculation<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial size=3D2><SPAN=20
class=3D577011319-03032004>We are trying to map underlying layers like =
streets,=20
parks, cities, etc. taken from TIGER for the United States. Then on top =
of this=20
we are drawing a ring usually less than 20 miles with the center being =
the=20
center of the map. Then we are mapping points given to us with lat/lon. =
In the=20
legend we put the distance of the point from the center of the map. It =
is this=20
distance which when done with Great Circle calc raised the red flag =
because the=20
distance would put the point outside the circle when it was being drawn =
inside=20
the circle. If we used Cartesian distance, then the distance would be =
off.=20
</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial size=3D2><SPAN=20
class=3D577011319-03032004>So, based on your answers, we MUST project =
our data in=20
order for our distance shown in the legend to be accurate and also match =
what we=20
draw on the map, right?</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial size=3D2><SPAN=20
class=3D577011319-03032004>Thanks and let me know if you need more =
detail to=20
provide suggestions.</SPAN></FONT></DIV><BR><BR><B><I>Ed McNierney=20
<ed at topozone.com></I></B> wrote:=20
<BLOCKQUOTE class=3Dreplbq=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px =
solid">
<META content=3D"MSHTML 6.00.2800.1400" name=3DGENERATOR>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D336382120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>I don't know what a "Cartesian" distance in =
miles means=20
when your units are in degrees and the conversion between degrees and =
miles=20
depends on the length and the orientation of the =
line.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D336382120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D336382120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>As I said, if you can provide more detail we =
can offer=20
more advice. For example, if all your distances are at a "small =
scale",=20
that's more detail (you didn't say that before). That helps =
define the=20
problem better and helps us suggest solutions. Please describe =
the=20
problem you're trying to solve and we'll help!</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D336382120-03032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN =
class=3D336382120-03032004> =20
<FONT face=3DArial color=3D#0000ff size=3D2>- Ed</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D336382120-03032004>
<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=20
</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 3:15 PM<BR><B>To:</B> Ed=20
McNierney<BR><B>Cc:</B> =
mapserver-users at lists.gis.umn.edu<BR><B>Subject:</B>=20
RE: [Mapserver-users] Distance Calculation<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>I know it is a very simple question, but I didn't think the =
results would=20
be so different at such a small scale. Would you tell me if these =
results seem=20
right:</DIV>
<DIV> </DIV>
<DIV>Point 1 (Lon,Lat): <FONT size=3D2>-87.6552,41.9148</FONT></DIV>
<DIV>Point 2 (Lon,Lat): <FONT size=3D2>-87.8081,<FONT=20
size=3D2>41.8884</FONT></FONT></DIV>
<DIV> </DIV>
<DIV>Cartesian Distance between Point 1 and 2 in miles: 11.2</DIV>
<DIV>Great Circle (assuming sphere) Distance between Point 1 and 2: =
8.1 </DIV>
<DIV> </DIV>
<DIV>Thanks again.<BR><BR><B><I>Ed McNierney =
<ed at topozone.com></I></B>=20
wrote:</DIV>
<BLOCKQUOTE class=3Dreplbq=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px =
solid">
<META content=3D"MSHTML 6.00.2800.1400" name=3DGENERATOR>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>First, please ALWAYS reply to the entire=20
list. That way everyone benefits from the conversation, and =
the first=20
person to answer your question doesn't have to make a lifetime =
commitment to=20
further questions!</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>Yes, of course you can choose one =
projection for=20
your entire set of data. Or you could switch based on what =
you're=20
mapping. The reason there are so many map projections is =
because each=20
has different properties, and therefore does some things well and =
other=20
things less well. The classic Mercator projection is excellent =
if you=20
are navigating with a compass; it is very poor if you're trying to =
persuade=20
someone that Brazil is several times larger than=20
Greenland.</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>You also need to have a feel for what =
level of=20
accuracy you need for your measurements. The UTM family of =
projections=20
is popular in part because it allows fairly accurate distance =
measurements,=20
does not distort shapes badly, and is easy to use. However, =
you need=20
10 different UTM zones to cover the lower 48 states. If you =
need even=20
more accuracy a custom Transverse Mercator projection centered =
on the=20
longitude of your central point would be better.</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>Mapping large areas in a single map is =
harder to do=20
in an appealing way than mapping small areas, because spatial =
distortion=20
becomes more obvious.</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>If you can provide a bit more detail on =
what you're=20
trying to do we can offer more advice.</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004></SPAN></FONT> </DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004> - =
Ed</SPAN></FONT></DIV>
<DIV dir=3Dltr align=3Dleft><FONT face=3DArial color=3D#0000ff =
size=3D2><SPAN=20
class=3D577011319-03032004>
<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=20
Chelmsford, MA 01863<BR>ed at topozone.com<BR>(978) 251-4242=20
</FONT></P></SPAN></FONT></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 2:01 PM<BR><B>To:</B> Ed=20
McNierney<BR><B>Subject:</B> RE: [Mapserver-users] Distance=20
Calculation<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>Thank you for your quick response.</DIV>
<DIV> </DIV>
<DIV>Is it possible to choose one projection for my entire set of =
data which=20
includes the whole United States (TIGER data). Or, do I somehow have =
to=20
switch based on what I am mapping? If the latter is true, then do =
you have a=20
suggestion on how I would do that?</DIV>
<DIV> </DIV>
<DIV>Again, point me elsewhere if I am pushing in the wrong =
direction.</DIV>
<DIV> </DIV>
<DIV>Thank you,</DIV>
<DIV> </DIV>
<DIV>Map Guy<BR><BR><B><I>Ed McNierney =
<ed at topozone.com></I></B>=20
wrote:</DIV>
<BLOCKQUOTE class=3Dreplbq=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff =
2px solid">
<META content=3D"MSHTML 6.00.2800.1400" name=3DGENERATOR>
<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=20
projection" map it will not be a circle. If you create a map =
with a=20
20-mile extent it will 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=20
making distance measurements; you simply cannot do them in any =
kind of=20
sensible way. The width of the top edge (east-west) of your =
"20=20
mile" image is DIFFERENT than the width of the bottom edge. =
In fact,=20
each row of pixels running across the image has a different width =
in=20
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=20
is not a circle or ring; it's not even an =
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=20
order 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> =20
<FONT 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=20
Mapmaker<BR>TopoZone.com / Maps a la carte, Inc.<BR>73 Princeton =
Street,=20
Suite 305<BR>North Chelmsford, MA =
01863<BR>ed at topozone.com<BR>(978)=20
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=20
[mailto:mappington at yahoo.com] <BR><B>Sent:</B> Wednesday, March =
03, 2004=20
12:21 PM<BR><B>To:</B>=20
mapserver-users at lists.gis.umn.edu<BR><B>Subject:</B> =
[Mapserver-users]=20
Distance 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 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=20
Circle without the distance placing the point incorrectly inside =
or=20
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>
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