[Proj] Geotrans, PCTrans and cs2cs - different angle signs?

[Mikael Rittri]

Jan wrote:
> Yes, the angles can be given either in "right-hand" (American) 
> or "left-hand" (European) convention.

The two rotation sign conventions have formal names, which are 
useful to know since they are used by EPSG.

* The "American" sign convention is formally known as 
  "Coordinate Frame Rotation", and it is actually used
  by many European countries too (although a minority).  

* The "European" sign convention is formally known as 
  "Position Vector Transformation", and it is the convention 
  used by cs2cs.  It is also the NATO standard, and (I think)
  the Eurogeographics standard.  

EPSG hasn't standardized on one sign convention. For each 7-parameter
datum shift, they use the sign convention of the original information
source, and then they document which one it was. When you look up a 
7-parameter datum shift at www.epsg-registry.org, you must remember
to scroll down, since the sign convention is given below the parameters. 

Jan also wrote:
> Mind that angles can be given either in (micro)degrees (PROJ) or radians (EPSG).  

Not quite. In cs2cs (PROJ), the rotation angles must be given in 
arc seconds (1 arc second = 1/3600 degree.)  

As for EPSG, it is true that the formulas in Guidance Note 7.2 assumes 
that the rotation angles are in radians. However, for a 7-parameter
datum shift published in the EPSG database, the angle unit isn't
standardized.  Instead, it is the angle unit used by the information 
source.  
   This means that the rotation angles in EPSG are usually in arc seconds,
but there are some datum shifts for the Netherlands and for Jan Mayen 
where they are in microradians, and a datum shift for Canada where they
are in radians. 

Regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com
 
________________________________

From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Jan Hartmann
Sent: Tuesday, January 18, 2011 9:00 AM
To: misha at mail.geometrics.com; PROJ.4 and general Projections Discussions
Cc: Mikhail Tchernychev
Subject: Re: [Proj] Geotrans, PCTrans and cs2cs - different angle signs?


Yes, the angles can be given either in "right-hand" (American) or "left-hand" (European) convention. See the thread on:

http://lists.maptools.org/pipermail/proj/2010-April/005198.html

Mind that angles can be given either in (micro)degrees (PROJ) or radians (EPSG). You can find a discussion about the conversion from EPSG numbers to PROJ, with both sign and radians conversion in:

http://trac.osgeo.org/geotiff/ticket/22

with an attached spreadsheet to perform the computation for the Dutch coordinate system (EPSG:28992)

Jan

On 01/18/11 06:24, Mikhail Tchernychev wrote: 

	Hello List,
	
	It came to my attention that 7-parameters values should be different for 
	cs2cs and
	programs like Geotrans  and PCTrans. It seems all 3 angles should have 
	different
	sign to match cs2cs results. Shifts stay the same, as well as scaling 
	(well, for
	geotrans it is not in ppm).  I wonder if it is something well known or 
	am I missing
	something?
	
	Thank you,
	
	Mikhail
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