[Proj] wgs84 northings and eastings

[Mikael Rittri]

Roger,
you wrote:

> Would I then remove the +towgs84=0,0,0,0,0,0,0 as well? Or do I need both?

You would remove +towgs84=0,0,0,0,0,0,0 and only use the nadgrids parameter. 

> which it calls World Mercator. ... perhaps this more suited to my original goal?

Maybe. First you need to know that World Mercator
is a different projection than Web Mercator. 
The projected coordinates for the same position
differs a lot (tens of kilometers). So don't confuse
them if you exchange data in Mercator. 

WGS 84 / World Mercator uses the ellipsoidal projection 
formulas based on the WGS 84 ellipsoid. Nothing strange
there. 

WGS 84 / Web Mercator uses the simpler spherical projection 
formulas, but the long/lat on the WGS 84 ellipsoid are 
interpreted as if they were long/lat on a sphere, without 
any conversion.  This interpretation is wrong, technically, 
and has the consequence that WGS 84 / Web Mercator is not an 
exactly conformal mapping from the WGS 84 ellipsoid. But it 
is nearly conformal, and projection can be done more efficiently. 

Noel Zinn has made a detailed presentation about Web Mercator:

http://www.hydrometronics.com/downloads/Web%20Mercator%20-%20Non-Conformal,%20Non-Mercator%20(full).pdf

My personal opinion is that Web Mercator isn't so bad if you
don't need exact conformality.  Noel Zinn dislikes the Web
Mercator more than I do, but I don't think there is any 
concrete fact that we disagree about, so I certainly 
recommend his presentation.  

Regards,
Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Roger Oberholtzer
Sent: den 26 maj 2011 16:58
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] wgs84 northings and eastings

On Thu, 2011-05-26 at 13:27 +0000, Mikael Rittri wrote:

> You write "transverse mercator", but it seems you mean ordinary 
> mercator (Web Mercator, to be specific).

My mistake. I was doing other work at the same time with transverse mercator. Parallel work is not always a good thing.

> So, you have to use a trick: use
> 
>    +nadgrids=@null

I remember the discussion a while back. Would I then remove the
+towgs84=0,0,0,0,0,0,0 as well? Or do I need both?

> instead. See
> 
> http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfr
> omWGS84toGoogleEarthVirtualGlobeMercator

Following that link eventually led me to:
http://spatialreference.org/ref/epsg/3395/

which it calls World Mercator. The definition is:

	+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs

Since my source data is defined as:

	+proj=latlong +datum=WGS84

perhaps this more suited to my original goal?



Yours sincerely,

Roger Oberholtzer

OPQ Systems / Ramböll RST

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer at ramboll.se
________________________________________

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se


_______________________________________________
Proj mailing list
Proj at lists.maptools.org
http://lists.maptools.org/mailman/listinfo/proj