Relationship between Unit dd and MAX/MINSCALE

[Ed McNierney]

Tim -

I do not know how MapServer calculates a scalebar in linear units (feet,
meters, miles, etc.) from data that's in ellipsoidal coordinates.  I
have no idea whether it's doing the right thing for your particular
case.

You might be able to easily test it by viewing data at different
latitudes - if the scalebar doesn't change, it's not working.

        - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com 

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Tim
Sent: Wednesday, April 27, 2005 11:35 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Relationship between Unit dd and
MAX/MINSCALE

On Wed, Apr 27, 2005 at 09:02:56AM -0400, Ed McNierney wrote:
> If your units are DDs, you cannot express them in miles - it's 
> meaningless, since the distance between any two points on such a map 
> (in
> miles) depends on the latitude of the map and the angle of the 
> distance being measured.

  Hmmm, does that mean my scalebar is meaningless?  Note that in this
case, I am referring to latitude only so technically I am only talking
about the horizontal distance and the distortion should be minimal.

> This is one of the reasons using "geographic" output projections for 
> maps is, IMHO, a very poor practice that seems to be infesting the Web

> mapping world.

  Should I be using an equidistance projection then?  That's normally OK
except I am overlaying some information outside of MapServer that's
unprojected.

  Thanks,

  Tim