[mapserver-users] Map Scale
Palmer, Steve
Steve_Palmer@lpsg.com
Tue, 12 Nov 2002 09:08:52 -0600
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That's about as succinct an explanation as I've heard/seen.
So if the map units were in meters, would one plug in a resolution value of
182.88 pixels per centimeter (72 * 2.54) to derive the scale?
-----Original Message-----
From: Ed McNierney [mailto:ed@topozone.com]
Sent: Monday, November 11, 2002 4:58 PM
To: vinko@cprsig.cl; mapserver-users@lists.gis.umn.edu
Subject: RE: [mapserver-users] Map Scale
Vinko -
The scale number has no units - a scale ratio, as in "1:25,000" scale.
Scale ratios basically make no sense with a digital image, unless you
specify the physical output (display or print) resolution of that image in
pixels per some physical unit. By default, MapServer assumes that the
output device has a resolution of 72 pixels per inch, and computes the scale
ratio that way.
For example, if MapServer generated a line that it knows is 12 miles long
(because MapServer knows such things), and the resulting image is 144 pixels
long (which MapServer also knows since it generated the image), then at 72
pixels per inch the image will be two inches long, which is a scale ratio of
12 miles = 2 inches or 6 miles = 1 inch or 380,160 inches = 1 inch. That's
a scale of 1:380,160, which is what MapServer will report.
You can use the RESOLUTION keyword in the MAP file to change the presumed
output resolution. Unless you control the output device completely,
reporting the scale as a ratio will never be accurate - but it's what most
users expect, so it's worth doing.
- Ed
Ed McNierney
President and Chief Mapmaker
TopoZone.com
ed@topozone.com
-----Original Message-----
From: Vinko Vrsalovic [mailto:vinko@cprsig.cl]
Sent: Mon 11/11/2002 7:32 PM
To: mapserver-users@lists.gis.umn.edu
Cc:
Subject: [mapserver-users] Map Scale
Hello,
I've been looking at the archives, but didn't find a clear
explanation
about this issue: The scale number shown by Mapserver is measured in
what? and in what ways it can be useful for output?
Because it doesn't make sense viewing the image, compared to what is
'normal' in real paper maps.
Thanks,
--
Vinko Vrsalovic <el[|-@-|]vinko.cl>
http://www.cprsig.cl
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<TITLE>RE: [mapserver-users] Map Scale</TITLE>
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<P><FONT SIZE=3D2>That's about as succinct an explanation as I've =
heard/seen.</FONT>
</P>
<P><FONT SIZE=3D2>So if the map units were in meters, would one plug in =
a resolution value of 182.88 pixels per centimeter (72 * 2.54) to =
derive the scale?</FONT></P>
<P><FONT SIZE=3D2>-----Original Message-----</FONT>
<BR><FONT SIZE=3D2>From: Ed McNierney [<A =
HREF=3D"mailto:ed@topozone.com">mailto:ed@topozone.com</A>]</FONT>
<BR><FONT SIZE=3D2>Sent: Monday, November 11, 2002 4:58 PM</FONT>
<BR><FONT SIZE=3D2>To: vinko@cprsig.cl; =
mapserver-users@lists.gis.umn.edu</FONT>
<BR><FONT SIZE=3D2>Subject: RE: [mapserver-users] Map Scale</FONT>
</P>
<BR>
<P><FONT SIZE=3D2>Vinko -</FONT>
<BR><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2>The scale number has no units - a scale ratio, as in =
"1:25,000" scale.</FONT>
<BR><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2>Scale ratios basically make no sense with a digital =
image, unless you specify the physical output (display or print) =
resolution of that image in pixels per some physical unit. By =
default, MapServer assumes that the output device has a resolution of =
72 pixels per inch, and computes the scale ratio that way.</FONT></P>
<P><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2>For example, if MapServer generated a line that it =
knows is 12 miles long (because MapServer knows such things), and the =
resulting image is 144 pixels long (which MapServer also knows since it =
generated the image), then at 72 pixels per inch the image will be two =
inches long, which is a scale ratio of 12 miles =3D 2 inches or 6 miles =
=3D 1 inch or 380,160 inches =3D 1 inch. That's a scale of =
1:380,160, which is what MapServer will report.</FONT></P>
<P><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2>You can use the RESOLUTION keyword in the MAP file =
to change the presumed output resolution. Unless you control the =
output device completely, reporting the scale as a ratio will never be =
accurate - but it's what most users expect, so it's worth =
doing.</FONT></P>
<P><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2> - Ed</FONT>
<BR><FONT SIZE=3D2> </FONT>
<BR><FONT SIZE=3D2>Ed McNierney</FONT>
<BR><FONT SIZE=3D2>President and Chief Mapmaker</FONT>
<BR><FONT SIZE=3D2>TopoZone.com</FONT>
<BR><FONT SIZE=3D2>ed@topozone.com</FONT>
</P>
<P> <FONT =
SIZE=3D2>-----Original Message----- </FONT>
<BR> <FONT SIZE=3D2>From: =
Vinko Vrsalovic [<A =
HREF=3D"mailto:vinko@cprsig.cl">mailto:vinko@cprsig.cl</A>] </FONT>
<BR> <FONT SIZE=3D2>Sent: Mon =
11/11/2002 7:32 PM </FONT>
<BR> <FONT SIZE=3D2>To: =
mapserver-users@lists.gis.umn.edu </FONT>
<BR> <FONT SIZE=3D2>Cc: =
</FONT>
<BR> <FONT SIZE=3D2>Subject: =
[mapserver-users] Map Scale</FONT>
<BR> =20
<BR> =20
</P>
<P> <FONT =
SIZE=3D2>Hello,</FONT>
<BR> =20
<BR> <FONT SIZE=3D2>I've been =
looking at the archives, but didn't find a clear explanation</FONT>
<BR> <FONT SIZE=3D2>about =
this issue: The scale number shown by Mapserver is measured in</FONT>
<BR> <FONT SIZE=3D2>what? and =
in what ways it can be useful for output?</FONT>
<BR> =20
<BR> <FONT SIZE=3D2>Because =
it doesn't make sense viewing the image, compared to what is</FONT>
<BR> <FONT SIZE=3D2>'normal' =
in real paper maps.</FONT>
<BR> =20
<BR> <FONT =
SIZE=3D2>Thanks,</FONT>
<BR> <FONT SIZE=3D2>--</FONT>
<BR> <FONT SIZE=3D2>Vinko =
Vrsalovic <el[|-@-|]vinko.cl></FONT>
<BR> <FONT SIZE=3D2><A =
HREF=3D"http://www.cprsig.cl" =
TARGET=3D"_blank">http://www.cprsig.cl</A></FONT>
<BR> =20
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