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

Schee van der, Maurits M.vanderSchee at cso.nl
Thu Jun 22 03:26:05 EDT 2006

If you now the dotpitch (distance between two pixels) of your screen, you can accurately calculate the resolution and make the scale ratio correct.
If dotpitch is 0.25 mm/pixel then:
resolution = 1/dotpitch = 4 pixel/mm = 40 pixel/cm = 40*2,54 pixel/inch = 101,6 pixel/inch = 101,6 dpi
Set this resolution in your application.
TFT screens have at native resolution an exact dotpitch that can be found in there specifications.
Good luck!
[van der Schee, Maurits] 
 -----Oorspronkelijk bericht-----
Van: ka-map-users-bounces at lists.maptools.org [mailto:ka-map-users-bounces at lists.maptools.org]Namens Tim Schaub
Verzonden: woensdag 21 juni 2006 22:18
Aan: ka-Map-users at lists.maptools.org
Onderwerp: RE: [ka-Map-users] scalebar question 1cm = 1km?
Urgentie: Laag

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 Regan
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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