[GRASSLIST:2377] Re: v.proj help

[Christian G. Tveen]

Again thank you! very helpfull information!

Best regards Christian

Glynn Clements wrote:

>Christian G. Tveen wrote:
>
>  
>
>>I had't really realized that units had to be meters (I'm very much used 
>>to only use lat/lng even in projected maps.)
>>
>>I have two further questions: From where did you obtain the precise 
>>values for the boundaries ? (I saw my rounded values written by v.proj)
>>    
>>
>
>1	$ grass5 /opt/grass-data/global5/glynn
>2	GRASS:~ > g.region n=57:54:28N s=54:24:11N w=7:38:57E e=15:33:24E
>3	GRASS:~ > r.mapcalc one=1
>4	GRASS:~ > exit
>5	$ grass5 /opt/grass-data/merctest/glynn
>6	GRASS:~ > g.region s=7000000 n=8000000 w=-500000 e=1000000
>7	GRASS:~ > r.proj in=one location=global5
>8	GRASS:~ > g.region rast=one
>9	GRASS:~ > g.region -p
>
>Line 6 took a bit of trial and error.
>
>Note: I'm not suggesting this as a standard approach.. I was also
>trying to figure out your projection parameters (I initially entered
>lat_ts=57, which put your region somewhere around Spitzbergen), so I
>was repeatedly projecting maps back and forth between locations.
>
>  
>
>>A more general question is how to obtain the initial WIND/DEFAULT_WIND 
>>file. When starting out, I only have the LL boundaries. Do I manually 
>>calculate the Mercator boundaries for those files, or is there a clever 
>>way ? (g.region initially requires a DEFAULT_WIND file, so somehow I 
>>need a boot-strap)
>>    
>>
>
>Use m.proj2 to project some sample coordinates. For projections which
>transform rectangles to rectangles (e.g. lat/lon -> Mercator),
>projecting the opposing corners will suffice.
>
>For projections which will distort the grid, you need to use knowledge
>of the projection to select appropriate points; think about what the
>projection will do to a rectangle.
>
>E.g. when projecting lat/lon to a typical (northern hemisphere)
>conical projection, a rectangle will be converted to a ring segment
>(i.e. a "rectangle" in polar coordinates). The minimum projected Y
>coordinate would be obtained by projecting the intersection of the
>central meridian and the southern parallel; the maximum Y would come
>from either the NE or NW corner, and the min/max X from the SE and SW
>corners.
>
>One mechanism to automate this is to create a sites list which samples
>the source region boundary, convert those to the destination
>projection with s.proj, then set the destination region with
>"g.region sites=...". This will work for any likely projection (it
>won't work in some extreme cases which "fold" the map, but those
>aren't particularly common).
>
>Note: there isn't a command to change the default region; you just
>have to overwrite the DEFAULT_WIND file with a suitable WIND file.
>FWIW, the default region doesn't have any significance beyond
>"g.region -d".
>
>  
>

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