[Mapserver-users] Distance Calculation
Ed McNierney
ed at topozone.com
Fri Mar 5 10:01:49 EST 2004
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Mr. Guy -
=20
1. If your output map images are to be in the UTM (or any other)
projection, then your EXTENT needs to be expressed in the native units
of that projection. So yes, you do need to express your EXTENT in
meters.
=20
2. You do NOT have to convert your data. Read the MapServer
documentation about projections; you can have source data layers in any
projection or as unprojected lat/lon degrees and MapServer will project
the data on the fly to produce the output map. This works very well.
=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: Friday, March 05, 2004 9:59 AM
To: Ed McNierney
Cc: mapserver-users at lists.gis.umn.edu
Subject: RE: [Mapserver-users] Distance Calculation
Thank you very much for your help so far.
=20
Up to this point we have been making all of calculations for our maps in
lat/long degrees. So, for instance, our map extent is given in lat/long
degrees. This acts as a window into our source geographic pjoection
layers. Based on what you have said, we now want our output to be
projected to UTM. Do we now need to convert all of our lat/long degrees
to meters for this to work (what is a good way to do this, if so)? An
example would be our map extent - does this now need to be in meters? Is
there no way to specify everything in lat/long degrees for accessing our
source data and then have the end result projected to UTM? Our map file
(simplified) is at the end of this email.=20
=20
NAME MAPREQUEST
SYMBOLSET "maprequest.sym"
EXTENT -178.226733 18.914590 -66.953248 71.381356 # Set dynamically
(Does this now need to be in meters?)
SIZE 586 586
UNITS DD # What should this be?
TRANSPARENT OFF
FONTSET "maprequest.fon"
IMAGECOLOR 204 255 255
IMAGETYPE PNG
SHAPEPATH /layers/
PROJECTION
"init=3Depsg:26915"
END
LAYER
NAME "Boundary"
DATA "cty90"
STATUS ON
TYPE POLYGON
TEMPLATE "foo"
CLASS
COLOR 255 255 204
OUTLINECOLOR 255 255 204
END
PROJECTION
"proj=3Dlatlong"
END
END
Thanks again!
=20
Ed McNierney <ed at topozone.com> wrote:
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
=09
=09
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
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=09
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<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>Mr. Guy -</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>1. If your output map images are to be in the =
UTM (or any=20
other) projection, then your EXTENT needs to be expressed in the native =
units of=20
that projection. So yes, you do need to express your EXTENT in=20
meters.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>2. You do NOT have to convert your data. =
Read the=20
MapServer documentation about projections; you can have source data =
layers in=20
any projection or as unprojected lat/lon degrees and MapServer will =
project the=20
data on the fly to produce the output map. This works very=20
well.</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004><FONT =
face=3DArial=20
color=3D#0000ff size=3D2></FONT></SPAN> </DIV>
<DIV dir=3Dltr align=3Dleft><SPAN =
class=3D578375914-05032004> <FONT=20
face=3DArial color=3D#0000ff size=3D2>- Ed</FONT></SPAN></DIV>
<DIV dir=3Dltr align=3Dleft><SPAN class=3D578375914-05032004>
<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> Friday, March 05, 2004 9:59 AM<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>Thank you very much for your help so far.</DIV>
<DIV> </DIV>
<DIV>Up to this point we have been making all of calculations for our =
maps in=20
lat/long degrees. So, for instance, our map extent is given in lat/long =
degrees.=20
This acts as a window into our source geographic pjoection layers. Based =
on what=20
you have said, we now want our output to be projected to UTM. Do we now =
need to=20
convert all of our lat/long degrees to meters for this to work (what is =
a good=20
way to do this, if so)? An example would be our map extent - does this =
now need=20
to be in meters? Is there no way to specify everything in lat/long =
degrees for=20
accessing our source data and then have the end result projected to UTM? =
Our map=20
file (simplified) is at the end of this email. </DIV>
<DIV> </DIV>
<DIV>NAME MAPREQUEST<BR>SYMBOLSET "maprequest.sym"<BR>EXTENT -178.226733 =
18.914590 -66.953248 71.381356 # Set dynamically (Does this now need to =
be in=20
meters?)<BR>SIZE 586 586<BR>UNITS DD # What should this =
be?<BR>TRANSPARENT=20
OFF<BR>FONTSET "maprequest.fon"<BR>IMAGECOLOR 204 255 =
255<BR>IMAGETYPE=20
PNG<BR>SHAPEPATH /layers/</DIV>
<DIV><BR>PROJECTION<BR> =20
"init=3Depsg:26915"<BR>END<BR></DIV>
<DIV>LAYER<BR> NAME=20
"Boundary"<BR> DATA=20
"cty90"<BR> STATUS=20
ON<BR> TYPE=20
POLYGON<BR> TEMPLATE=20
"foo"<BR> =20
CLASS<BR> &nbs=
p; =20
COLOR 255 255=20
204<BR> =
=20
OUTLINECOLOR 255 255 =
204<BR> =20
END<BR> =20
PROJECTION<BR>  =
; =20
"proj=3Dlatlong"<BR> END</DIV>
<DIV>END<BR><BR>Thanks again!</DIV>
<DIV> </DIV>
<DIV><BR><B><I>Ed McNierney <ed at topozone.com></I></B> 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=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. =20
It is simple, common, and easy to use. It is not an equidistant=20
projection and there will be a VERY small distortion, but for areas as =
small=20
as yours I think it will be completely invisible. There are =
other more=20
accurate projections (equidistant conic, for example) that require =
more work=20
on your 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=20
you 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=20
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> =20
<FONT 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=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 3:29 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>
<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=20
this we are drawing a ring usually less than 20 miles with the center =
being=20
the center of the map. Then we are mapping points given to us with =
lat/lon. In=20
the legend we put the distance of the point from the center of the =
map. It is=20
this distance which when done with Great Circle calc raised the red =
flag=20
because the distance would put the point outside the circle when it =
was being=20
drawn inside the circle. If we used Cartesian distance, then the =
distance=20
would be off. </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=20
in order for our distance shown in the legend to be accurate and also =
match=20
what we 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=20
scale", that's more detail (you didn't say that before). That =
helps=20
define the problem better and helps us suggest solutions. =
Please=20
describe the problem you're trying to solve and we'll=20
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=20
Chelmsford, 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=20
would be so different at such a small scale. Would you tell me if =
these=20
results seem 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=20
</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=20
first person to answer your question doesn't have to make a =
lifetime=20
commitment to 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=20
each has different properties, and therefore does some things well =
and=20
other things less well. The classic Mercator projection is =
excellent=20
if you are navigating with a compass; it is very poor if you're =
trying to=20
persuade 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=20
projections is popular in part because it allows fairly accurate =
distance=20
measurements, does not distort shapes badly, and is easy to =
use. =20
However, you need 10 different UTM zones to cover the lower 48=20
states. If you need even more accuracy a custom Transverse =
Mercator=20
projection centered on the longitude of your central point =
would be=20
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=20
do in an appealing way than mapping small areas, because spatial=20
distortion 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=20
you're 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=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></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=20
[mailto:mappington at yahoo.com] <BR><B>Sent:</B> Wednesday, March =
03, 2004=20
2:01 PM<BR><B>To:</B> Ed McNierney<BR><B>Subject:</B> RE:=20
[Mapserver-users] Distance 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=20
which includes the whole United States (TIGER data). Or, do I =
somehow have=20
to switch based on what I am mapping? If the latter is true, then =
do you=20
have a suggestion on how I would do that?</DIV>
<DIV> </DIV>
<DIV>Again, point me elsewhere if I am pushing in the wrong=20
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=20
a 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=20
fact, each row of pixels running across the image has a =
different width=20
in miles/feet/meters because they're all different distances =
from the=20
Equator. That's why the set of all points 8 miles from a =
given=20
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=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=20
class=3D606261418-03032004> <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 =20
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=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=20
mile extent, and we plot a point and calculate the distance =
from=20
the center using Great Circle we get 7.5 miles. Using a =
Cartesian=20
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=20
geographic projection (lat/lon) and display distances between =
points=20
using Great Circle without the distance placing the point =
incorrectly=20
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=20
these 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 =
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