[Proj] Re: Proj Digest, Vol 51, Issue 2 - axis rotation issue

Gerald I. Evenden geraldi.evenden at gmail.com
Wed Aug 13 11:06:26 EDT 2008

On Wednesday 13 August 2008 4:58:50 am you wrote:
> I think it's my fault. I'd had latitudes as S20;  S30; etc. and
> should've put the negative in.<br>

My goodness, that make a lot of difference!

> <br>
> It looks to me that I simply need to multiply the outputs by -1, but
> I'm loath to do that because I don't understand the underlying
> mathematics that calculates the projection. <br>
> Is it really that simple? I could use the "-m" flag and set it to -1,
> but is that wise?<br>
> It appears to give me the correct output:<br>
> <br>
> ~$ proj +proj=tmerc +ellps=WGS84 +lon_0=27 -m -1 Desktop/proj<br>
> -55660.46    -0.00<br>
> -54820.34    1105896.37<br>
> -52324.06    2212444.34<br>
> -48243.45    3320218.65<br>
> -58652.24    2897715.76 <br>
> <br>
> Can you advise?<br>

You have a most strange grid system where the south pole is oriented to the 
top of the map.  I suppose it makes a difference to someone living in the 
southern hemisphere but it is a bit odd when compared to most grid 

All you are doing is rotating the x-y (easting-northing) axis by 180 degrees 
and this can, as you empirically determined, be done my using the '-m -1' 

As to whether an axis rotation option is libproj4's responsibility or the 
applications program's (proj4, lproj, etc.) responsibility and thus a "-" 
option or "+" option respectively is now open for debate.

Any comments from the maptools group?

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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