Relationship between Unit dd and MAX/MINSCALE

[Paul Spencer]

I believe that MapServer uses an approximation based on the length of a
degree at the equator when using a geographic projection, which means
that the latitude will not affect your scalebar and it will be
increasing inaccurate the further south/north you get.

I believe some tests were done by Daniel to use a great circle
calculation to get a more accurate number based on current latitude but
the results were inconclusive as to whether it was actually more or less
accurate this way.

Paul

Ed McNierney wrote:
> 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
>
>

--
+-----------------------------------------------------------------+
|Paul Spencer                           pspencer at dmsolutions.ca   |
+-----------------------------------------------------------------+
|Applications & Software Development                              |
|DM Solutions Group Inc                 http://www.dmsolutions.ca/|
+-----------------------------------------------------------------+