[Proj] Are they six of one...

[Mikael Rittri]

Roger Oberholtzer wrote about +proj=utm versus +proj=tmerc 
when defining SWEREF99 TM:

> If no difference in results, could there be any 
> possible difference in calculation speed?

No. In the file src/PJ_tmerc.c, you can see the 
initialization code for +proj=utm, and it just 
translates the zone and hemisphere parameters 
into ordinary tmerc parameters. After this 
initialization is done, the computations 
should be exactly the same. 

> Or any other non-religious reason to choose 
> one over the other? 

Human readability, perhaps? If you use +proj=utm, it 
is obvious from the command line that SWEREF99 TM 
uses the same projection as UTM Zone 33 North. 
That's less obvious with the +proj=tmerc definition. 

On the other hand, maybe you _want_ to obscure this
fact, to avoid confusion, since SWEREF99 TM has not 
the same area of use as WGS84 UTM Zone 33 North.  
Its area of use is wider to cover Sweden, and not
as tall (if "tall" is the right word).  The Swedish
Land Survey do not say that SWEREF99 TM _is_ UTM,
they just claim 
   "consistency with UTM in the major part of the country."
   (http://www.lantmateriet.se/templates/LMV_Page.aspx?id=4219&lang=EN)  

So I'd say the choice is a matter of taste (or religion). 

Best regards,
--
Mikael Rittri
Carmenta AB
SWEDEN
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 7 april 2009 11:37
To: proj at lists.maptools.org
Subject: [Proj] Are they six of one...

I am calculating northings/eastings in SWEREF99 TM, from WGS84 lat/longs. I see that I can specify the calculation in two ways:

"+proj=utm +zone=33 +ellps=GRS80
	+towgs84=0,0,0,0,0,0,0 +no_defs"

"+proj=tmerc +ellps=GRS80 +k=0.9996 +x_0=500000 +y_0=0
	+lon_0=15.0  +lat_0=0.0
	+towgs84=0,0,0,0,0,0,0 +no_defs"

and I get identical results. So I am wondering if either specification has merit over the other. If no difference in results, could there be any possible difference in calculation speed? Or any other non-religious reason to choose one over the other? 

--
Roger Oberholtzer

OPQ Systems / Ramböll RST

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

Office: Int +46 8-615 60 20
Mobile: Int +46 70-815 1696

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