[Chameleon] Problem with Scale widget

Jacob Delfos jacob.delfos at maunsell.com
Thu Sep 23 04:19:37 EDT 2004


Which version of chameleon are you using? I'm using my own scaletool with a javascript calculation based on the current mapextents, and the printsize. It then zooms to the newly calculated map-extents. For latlong, I take into consideration the longitude. I use the same calculation in CWC 1.0.4 as I do in chameleon 1.99, but only in version 1.99 I managed to get to the same scale as the user enters. E.g. in CWC 1.0.4, my calculated extent resulted in a scale slightly different from what was 'predicted' (1:100005, in stead of 1:100000). I thought this was due to a ronding error).

But in chameleon 1.99, I always get the right scale, even if it is recalculating from 1:500 to 1:5000000. I use the same calculation, but for some reason the mapextents can be set more precisely in 1.99. Whatever has changed in chameleon 1.99 worked well for me, but did not find its way into the normal scale widget.

Paul, where can I see the calculation for the scale widget? Do I need to check the mapserver source code for this?




-----Original Message-----
From: chameleon-bounces at lists.maptools.org [mailto:chameleon-bounces at lists.maptools.org] On Behalf Of Berend Veldkamp
Sent: 23 September 2004 15:52
To: Chameleon Users
Subject: Re: [Chameleon] Problem with Scale widget


Apparently it depends on the values I use for the width and height of 
the MapWidget: with 780x537 the scale widget displays a value other than 
I typed, a change of one pixel in either width or height solves it.

BTW I'm not really concerned about the actual scale on screen, as long 
as the value I type in the scale editbox remains the same and scale 
dependent layers become visible at the right scale.


Paul Spencer wrote:
> Berend,
> I'm not sure that the scale widget can zoom to an exact scale.  The 
> scale is the relationship between units on the screen and units on the 
> ground.  The scale calculations in mapserver take into account several 
> factors, and the end result is an approximation of ground scale based on 
> several assumptions concerning the dpi of the display and the distance 
> of 1 degree at a given latitude (to name a couple).  So when zooming to 
> a particular scale, this calculation is, in effect, reversed to 
> determine how many ground units will fit inside a pixel and then the 
> extents are set to match some approximation of this.
> That being said, I don't think the discrepancy should be as big as you 
> are indicating here so perhaps there is a problem.  Please file a bug 
> about this and I will try to take a look at it.
> Cheers,
> PAul


Berend Veldkamp - ARIS
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Chameleon at lists.maptools.org
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