[Proj] wgs84 northings and eastings

[Mikael Rittri]

Hello Roger.

> I would like to project WGS84 latitude and longitudes on 
> to a transverse mercator surface, without any datum shift. 

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

The +to definition you suggest is not correct for WGS84 Web Mercator, though.
This is because the datum shift you use, 

   +towgs84=0,0,0,0,0,0,0

is not the same as unmodified lat/long, or "without any datum shift"
as you say. It means "no datum shift" only if the +from and +to definitions
use the same ellipsoid, and in your case the +from definition uses the
WGS84 ellipsoid, while the +to definition uses a sphere instead.  

So, you have to use a trick: use 

   +nadgrids=@null 

instead. See 

http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator

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 24 maj 2011 19:30
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] wgs84 northings and eastings

So, perhaps I should re-phrase my original question:

I would like to project WGS84 latitude and longitudes on to a transverse mercator surface, without any datum shift. How should that be specified in proj? I use the C interface. Using ED50 as an example, I divide the proj string into a 'from':

	from = pj_init_plus("+proj=latlong +datum=WGS84");

and a 'to':

	to = pj_init_plus("+proj=utm "
			"+ellps=intl "
			"+k=0.999600 "
			"+x_0=500000.0 "
			"+y_0=0.0 "
			"+zone=30 "
			"+units=m "
			"+towgs84=-125.098545,-76.000054,-156.198703,0.0,0.0,-1.129,8.30463103 "
			"+no_defs");

I then use pj_transform(from, to, ...) to do the transform. (Switching the 'from' and 'to' allows a reverse transform).

Would this be correct for the transform?

	from = pj_init_plus("+proj=latlong +datum=WGS84");

	to = pj_init_plus("+proj=merc " 
                		"+k=1.0 "
				"+x_0=0 " 
				"+y_0=0 "
				"+lon_0=0.0 "
				"+lat_0=0.0 "
				"+towgs84=0,0,0,0,0,0,0 "
				"+a=6378137 "
				"+b=6378137 "
				"+no_defs");

It is the entire 'from', and the +towgs= in the 'to' that have me confused. This is based on http://spatialreference.org/ref/sr-org/45/

All comments welcome!



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


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