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

[Schee van der, Maurits]

Hi,
 
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.
 
Tim
 


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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?

Cheers,

Donal.





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