[GRASSLIST:10329] Re: robinson (winkel) projection

[William Kyngesburye]

GRASS bug #2558

I just noticed that my latest comments to help close that bug didn't  
make it into the tracker.  I'll have to dig those up and resubmit them.

I've been working with the GLOBE DEM, which should be in WGS84, and  
is in a WGS84 latlong location.  I first noticed the problem when  
upgrading PROJ from 4.4.7 to 4.4.8.  I think something was fixed in  
proj, and so broke a glitch I was depending on (unknowingly) in the  
GRASS-PROJ interaction.

If the latlong DEM was really sphere-based but in a WGS84 location,  
other projections should also be having problems.  I haven't seen any  
other projections with this shifting issue.


On Feb 15, 2006, at 2:05 PM, Ian MacMillan wrote:

> William, I have not seen this with my Robinson locations.  Are you  
> sure about this?  Could it be that your original dataset was using  
> a sphere but you imported it into a wgs84 location.  I had this  
> trouble originally with Smith and Sandwell data.  Seems like a  
> major bug if your observation is true.  Has anyone else ever seen  
> this with Robinson projections?
>
> -Ian
>
>
> On Feb 15, 2006, at 11:00 AM, William Kyngesburye wrote:
>
>> 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/
>>
>> [Trillian]  What are you supposed to do WITH a maniacally  
>> depressed robot?
>>
>> [Marvin]  You think you have problems?  What are you supposed to  
>> do if you ARE a maniacally depressed robot?  No, don't try and  
>> answer, I'm 50,000 times more intelligent than you and even I  
>> don't know the answer...
>>
>> - HitchHiker's Guide to the Galaxy
>>
>>
> >
> >
> What happens if a big asteroid hits Earth? Judging from realistic  
> simulations involving a sledge hammer and a common laboratory frog,  
> we can assume it will be pretty bad.
>  - Dave Barry
>
>
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-----
William Kyngesburye <kyngchaos at kyngchaos.com>
http://www.kyngchaos.com/

Theory of the Universe

There is a theory which states that if ever anyone discovers exactly  
what the universe is for and why it is here, it will instantly  
disappear and be replaced by something even more bizarrely  
inexplicable.  There is another theory which states that this has  
already happened.

-Hitchhiker's Guide to the Galaxy 2nd season intro