[GRASS5] Re: GRASS Projections (fwd) - UTM problem

Bill Brown brown at gomez.gis.uiuc.edu
Wed Nov 24 12:23:24 EST 1999

>> > Only thing needed is a one-liner in geo_init.c
>> This is a newer file (not in 4.2), so I haven't tracked down all
>> the changes in the projection handling - is this documented somewhere
>> online?  Or was this moved from m.proj to the gislib for some reason?
>I merged the files containing definitions and functions that are used by
>both g.setproj and m.proj and put them into include/geo.h (headers) and
>libes/gis/geo_init.c (init functions). This way both programs stay
>syncronised and use same default settings for the different projections.
>This happened sometimes in grass5.0beta4m.


This is a good idea.  But now that it is part of the gis libe, you should
probably rename the public functions using a G_ convention to prevent
name space collisions and for better program readablility.

>> As far as I know, GRASS programs were suppose to use either the proj lib
>> (e.g., r.proj, v.proj, m.proj) or coorcnv lib (m.*ll*) both of which 
>> include the scale factor of .9996 hard coded.  Somebody needs to look into
>> this more - I wouldn't assume the data has been incorrectly converted.
>Yes, I looked at specifications for utm in proj manual, and the scaling
>factor (k_0) is not an option for that projection. You can still use it,
>that is, pass any +k_0= value to proj, and it will not display an error.  
>But it seems to be ignored because the resulting northing/easting values
>are exactly the same, whatever k_0 value you use. In other words, the
>scaling factor for utm is hardcoded in proj. 

So in other words, the conversion process works fine but if a user wants 
to enter an invalid scale factor for UTM, it is ignored, correct?  And for
the sake of consistancy, we want to allow users to enter invalid scale
factors?  If that's it, it's fine with me.  You might still want to check
with Jerry to see if there might have been another explanation for hardcoding
the UTM scale.  

- Bill

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