[Proj] Are they six of one...
Mikael.Rittri at carmenta.com
Wed Apr 8 04:13:24 EST 2009
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."
So I'd say the choice is a matter of taste (or religion).
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
"+proj=tmerc +ellps=GRS80 +k=0.9996 +x_0=500000 +y_0=0
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?
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