[ka-Map-users] scalebar question 1cm = 1km?

Tim Schaub tim at commenspace.org
Wed Jun 21 16:17:48 EDT 2006

Hey Donal-
MapServer assumes a monitor resolution of 72 dpi by default (so this is
what the scale bar assumes by default).  This is seldom correct.  The
upshot is that the scale ratio (1:1000000) is seldom correct.  However,
this doesn't really matter unless you advertise the scale ratio and a
client with a non 72 dpi monitor takes out a ruler.
There are two things that you can do if this bothers you.
1) Guess at your client's resolution and modify the scale bar's
resolution property (myScaleBar.resolution = 96).
2) Don't tell anybody what the scale ratio of your map is, and rely on
the scale bar for relating display distances to map distances.
Since people always toss around numbers like 1:250000 for map scales, it
might be disappointing to hear that these numbers aren't generally
accurate for monitor displays (since there is not a reliable way for the
server to know the client's screen resolution ahead of time).  This is
why I think it's better to rely on a scale bar than a numeric scale.


	From: ka-map-users-bounces at lists.maptools.org
[mailto:ka-map-users-bounces at lists.maptools.org] On Behalf Of Donal
	Sent: Wednesday, June 21, 2006 1:48 AM
	To: ka-Map-users at lists.maptools.org
	Subject: [ka-Map-users] scalebar question 1cm = 1km?
	Hello all.
	I am displaying a map that has amongst it's scales 1:100000. I
would expect my scalebar to show 1cm = 1km, i.e if  I physically measure
the scalebar with a ruler, 1cm on the ruler would correspond to 1km on
the scalebar. However, I'm finding that 0.9cm = 1km , 1.85cm = 2km  up
to 5.7cm = 6km.
	 Is this just the way it is or am I misunderstanding the way the
scalebar should be working?


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