[GRASS-dev] Re: [GRASS GIS] #785: wx location wizard: doesn't show all datum transform opts

[GRASS GIS]

#785: wx location wizard: doesn't show all datum transform opts
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  Reporter:  hamish    |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect    |      Status:  new                      
  Priority:  critical  |   Milestone:  6.4.0                    
 Component:  wxGUI     |     Version:  svn-develbranch6         
Resolution:            |    Keywords:  location wizard          
  Platform:  Linux     |         Cpu:  x86-64                   
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Comment (by hamish):

 > This was a kind of quick fix just to see if it is doing the right stuff.

 yup

 > > * trying to make a UTM location:
 > > {{ Traceback (most recent call last):

 fixed in devbr6

 > > Suggest adding '+' to the start of the proj4 term.
 >
 > I don't understand. Could you explain.

 currently line 2 in the transform descr looks like "towgs84=...." or
 "nadgrids=..."
 more familiar would be "+towgs84=..." or "+nadgrids=..."


 > > there is still some work to be done I'm afraid.. a g.setproj
 > > session should give some guidance. for other projections it
 > > is not asking all it needs to. e.g. LCC, TMerc never asks for
 > > lat_0, lon_0, etc.
 >
 > Could you explain?

 e.g. for transverse Mercator projection you need to set up the central
 meridian (lon_0) for the projection and false eastings (x_0) and false
 northings (y_0). Currently the wizard doesn't ask you these essential
 terms and leaves them unset.

 what needs to be asked, their types, and default values can be found
 in the $GISBASE/etc/proj-parms.table file(s).


 > I'll take a look. I want to try and fix the prior issues first.

 ok, one thing at a time..

 > Question, is there any harm in including +dx=0 +dy=0 terms when other
 > information about transforms is also included?

 probably, even if we get away with it today, in the future it could cause
 problems.  The 3-term +towgs84=dx,dy,dz  so having +dx,+dy,+dz as well
 just makes it a matter of programmer's chance as to which is used.
 also for others in datum.table there is dx=99999, which will probably mean
 something to g.proj.

 > The reason is that the proj4 string is being built by accretion of
 > information out of the datums.table and ellipse.table, and then any
 > datum transforms are being included using the datatrans= argument
 > for g.proj.
 >
 > So does +dx=0, etc. need to be filtered out?

 yes, I think it does.


 > If I enter g.proj proj4='+proj=utm +zone=12 +datum=nad27' datatrans=-1
 > I get a list of potential datum transforms to pick from.
 > If I enter g.proj proj4='+proj=utm +zone=12 +ellps=clark66' datatrans=-1
 (or ellipse parameters instead of clark66)
 > I do not get a list of transforms. Is this because...
 >
 > 1) transforms are not needed if an ellipse is specified

 yes. these are datum transform opts. no datum, no transform.

 a datum (in grass) consists of an ellipsoid (shape of the Earth) and
 some notion about where there center of that spheroid sits in space
 (ie where is the center of the planet, what to call 0,0,0). The
 +towgs84 params or grid file gives the translational (and for 7-term,
 rotational) differences relative to WGS84's axes-anchor position.


 I don't think this affects GRASS (except for m.proj), but see also
 http://trac.osgeo.org/proj/wiki/FAQ#WhydoIgetdifferentresultswith4.5.0and4.6.0


 see also "proj -le" and "proj -ld"


 Hamish

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Ticket URL: <http://trac.osgeo.org/grass/ticket/785#comment:18>
GRASS GIS <http://grass.osgeo.org>