[GRASSLIST:8434] Re: problems in gnomonic projection cont'd

[Milan Salek]

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Milan Salek


Milan Salek napsal(a):
> Hello all,
> 
> several weeks I am struggling with reprojection from/to gnomonic projection and
> I am more and more convinced that something is wrong in r.proj/v.proj dealing
> with gnomonic projection.
> 
> I was trying to give you the simplest example of the problem that still persists.
> 
> Enclosed you will find two locations, rad_m and fi_lambda, both roughly for the
> same region, the former in gnomonic projection and the latter in lat/lon.
> 
> In the gnomonic projection there is an artificial array with some distinct
> values creating a cross at a given location (it is the reference point of the
> projection but it could be any arbitrary point)
> 
> d.proj -l -d works nicely, the point/cross gives accurate values:
> 
>           EAST:             NORTH:               LON:               LAT:
>           -62.5              237.5        14.44612648        50.01013321
> 
> 
> When using proj for the LON/LAT:
> proj  +proj=gnom +lat_0=50.008 +lon_0=14.447 +a=6379000 +x_0=301500 +y_0=-217500
> +es=0. +no_defs -f %6.0f <<EOF
> 14.44612648        50.01013321
> 
> it gives
> 
> 301438  -217262 which, in respect to above mentioned results of d.proj -l -d,
> and the false easting/northing given by proj parameters, gives nice match.
> 
> Here it works OK.
> 
> But, when I am in location fi_lambda and want to reproject 00test map into it,
> it imports well but everything is by 0.2 degree to north.
> 
> Reprojection:
> r.proj input=00test location=rad_m mapset=salek output=00test method=nearest
> 
> Then, when pointing to the little point/cross:
> 
> d.where -l -d
>               LON:               LAT:
> 
>          14.447125        50.20371875
>                           ------------
> 
> The latitude should be near 50.01. The little difference in LON does not bother
> me, but the LAT does.
> 
> Similar results are in Spearfish area, even the difference is the same.
> 
> If you want to import the data yourself, try in rad_m:
> 
> r.in.bin input=<PATH>/input_gnom.int output=00test bytes=2 north=217500
> south=-310500 east=426500 west=-301500 rows=528 cols=728
> 
> Has someone an idea why the latitude is wrong?
> 
> Best regards
> 
> Milan
>