[Proj] How to Scale a Mercator projection?

Frank Warmerdam warmerdam at pobox.com
Tue Jul 25 16:48:37 EDT 2006

Christopher Barker wrote:
> Hi all,
> I'm trying to figure out how to do a Mercator projection. I think I've 
> got how it's supposed to work, but I can't get the scale parameter to do 
> anything at all. On the proj4 command, it's either k=scale or k_0-scale, 
> neither does anything. Also, according to the docs:


I'm sorry I haven't responded yet to any of your email on gdal-dev and
elsewhere.  I have just checked, and I definately see an effect when
specifying +k_0:

warmerda at gdal2200[82]% proj +proj=merc +ellps=WGS84 +k_0=0.998
5 5                          (input)
555484.26       552476.68    (output)
warmerda at gdal2200[83]% proj +proj=merc +ellps=WGS84 +k_0=1.0
5 5                          (input)
556597.45       553583.85    (output)

Perhaps you could be more specific about how you determined it was having
no effect?

> """
> A more common formulation for Mercator is to drop the +k_0, and instead 
> to provide a latitude of true scale using the +lat_ts parameter, which 
> is the latitude at which the scale is 0
> """

Where exactly did you see this?  You are quite right that a scale of zero
is nonsensical.

Hopefully I'll have a chance to respond on the BSB issues on gdal-dev.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGF, http://osgeo.org

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