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

Neil Robinson halfhaggis at gmail.com
Wed Aug 13 11:22:32 EDT 2008

Gerald I. Evenden wrote:
> 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 
> orientations.
Don't ask me -- I just live here! I would have been a lot less 
frustrated if the powers that be could've just kept north at the top.
> 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' 
> option.
> 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.
Indeed. As it stands, qgis, which allows configuration of a custom 
projection using proj4 parameters, doesn't appear to support the -m flag 
or have an equivalent flag or setting I could use.
Since I started this whole investigation to get qgis to work properly, 
my fight is still not over. At least I can measure distances now, which 
is my most important requirement.

I'll see what the qgis people have to say. Thanks for your time

> Any comments from the maptools group?
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