Relationship between Unit dd and MAX/MINSCALE

[Ed McNierney]

Paul -

Thanks; I don't think it's worth attempting to fix something that's
unfixable.  As one moves further from the equator, not only does the
size of a degree of longitude change, but any actual measurement becomes
more and more dependent on the angle of the line measured, since the
difference between the size of a degree of latitude and a degree of
longitude grows.

We've had map projections for about two millenia or so, and there's a
reason for that <g>.

        - 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 Paul Spencer
Sent: Wednesday, April 27, 2005 12:53 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Relationship between Unit dd and
MAX/MINSCALE

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
>
>

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