[Proj] wgs84 northings and eastings
Mikael.Rittri at carmenta.com
Fri May 27 05:45:26 EST 2011
> 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
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:
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.
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
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
Following that link eventually led me to:
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:
perhaps this more suited to my original goal?
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