[GRASSLIST:10323] Re: robinson (winkel) projection

[William Kyngesburye]

One important thing I found - use a sphere, not an ellipsoid.  If you  
use an ellipsoid, there will be an approx. 20000m N-S shift at around  
45 deg N and S (for the Robinson, at least), which is quite noticable  
at any scale.  I agonized over this for a while, using WGS84  
ellipsoid out of habit, and even thought it was a bug, until I  
figured out the cause.  -> use a sphere for global projections.


On Feb 15, 2006, at 12:29 PM, Ian MacMillan wrote:

> Otto, I have made some robinson locations for my work, and the  
> process is pretty straight forward.  After selecting 'create new  
> location' when first opening grass, select that you would like to  
> use 'Other Projection' when asked for your coord. system.  When  
> asked to specify your projection name, just use 'robin'.  Select  
> your datum as usual.  You will then have to input your central  
> parallel, central meridian, and preferred units.  For most robinson  
> projections that you see in atlases, the central parallel is the  
> equator (0), and the central meridian is the Greenwich meridian  
> (also 0).  I usually use meters as my preferred units.
>
> Degrees would be really nice as a standard unit for these kinds of  
> projections, but since their absolute length varies depending on  
> your location on the earth, they can't be used in grass.  My  
> workaround to this has been to make a vector grid in a latlong  
> location that is spaced about ten or fifteen degrees apart for the  
> entire earth.  I then project (with v.proj) that vector file into  
> my robinson location as a first step so that I can see where I am  
> in the world.
>
> Once you are done with all of that, use r.proj and v.proj at your  
> leisure in your new location.
>
> Hope this helps,
> Ian
>
> On Feb 15, 2006, at 4:25 AM, Otto Dassau wrote:
>
>> Hi,
>>
>> I would like to convert a lat/lon map into a robinson (winkel)  
>> projection.
>>
>> does anybody has experience with the creation of a robinson (winkel)
>> projektion in GRASS? It seems to be supported (robin, winkel,...)  
>> but I
>> haven't found out, how to create a robin location in GRASS and which
>> parameters a normaly used.
>>
>> thanks
>>   Otto

-----
William Kyngesburye <kyngchaos at kyngchaos.com>
http://www.kyngchaos.com/

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