[Mapserver-users] Distance Calculation
Ed McNierney
ed at topozone.com
Wed Mar 3 11:19:44 PST 2004
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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
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
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
_____ =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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<META http-equiv=3DContent-Type content=3D"text/html; =
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<META content=3D"MSHTML 6.00.2800.1400" name=3DGENERATOR></HEAD>
<BODY>
<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 =
list. =20
That way everyone benefits from the conversation, and the first person =
to answer=20
your question doesn't have to make a lifetime commitment to further=20
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 your=20
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 has=20
different properties, and therefore does some things well and other =
things less=20
well. The classic Mercator projection is excellent if you are =
navigating=20
with a compass; it is very poor if you're trying to persuade someone =
that Brazil=20
is several times larger than 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 accuracy=20
you need for your measurements. The UTM family of projections is =
popular=20
in part because it allows fairly accurate distance measurements, does =
not=20
distort shapes badly, and is easy to use. However, you need 10 =
different=20
UTM zones to cover the lower 48 states. If you need even more =
accuracy a=20
custom Transverse Mercator projection centered on the longitude of =
your=20
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 in=20
an appealing way than mapping small areas, because spatial distortion =
becomes=20
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 =
Chelmsford,=20
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 =
switch=20
based on what I am mapping? If the latter is true, then do you have a =
suggestion=20
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 =
projection"=20
map it will not be a circle. If you create a map with a 20-mile =
extent=20
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 making=20
distance measurements; you simply cannot do them in any kind of =
sensible=20
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=20
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 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> =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 =
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 12:21 PM<BR><B>To:</B>=20
mapserver-users at lists.gis.umn.edu<BR><B>Subject:</B> [Mapserver-users] =
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=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=20
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=20
for faster.</A> </BLOCKQUOTE>
<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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