Re: RotatedMercator map projection

On Sep 16, 2008, at 12:48 PM, Seth McGinnis wrote:

> I got it figured out earlier today, so I'm all good. You got me
> pointed in the right direction, though, so thanks!
>
> Suppose your data is in Lambert Conformal projection, with
> appropriate @lat2d & @lon2d arrays. To plot the data in its
> native projection, so that you have it properly placed relative to
> geographic features on the globe but the grid still ends up nice
> and square, you pass these resource values to gsn_csm_contour_map()
> (where "proj" is a variable in the NetCDF file that contains
> information about the map projection):
>
> res_at_mpProjection = "LambertConformal"
> res_at_mpLambertMeridianF = proj_at_longitude_of_central_meridian
> res_at_mpLambertParallel1F = proj_at_standard_parallel(0)
> res_at_mpLambertParallel2F = proj_at_standard_parallel(1)
> res_at_tfDoNDCOverlay = True
>
> The equivalent for data in a rotated pole or rotated lat-lon
> projection (which I now know is *not* the same as rotated Mercator)
> is this:
>
> res_at_mpProjection = "Mercator"
> res_at_mpCenterLonF = proj_at_grid_north_pole_longitude - 180
> res_at_mpCenterLatF = 90 - proj_at_grid_north_pole_latitude
>

Yes, although are you sure the correct projection is mercator rather
than cylindrical equidistant?
I'm not saying it's the case in your situation, but usually 'rotated
lat-lon' refers to a rotated CE projection.
  -dave

> Cheers,
>
> --Seth
>
>
>> Hi Seth,
>>
>> I am not sure I can visualize what you are trying to do. Can you
>> send the script and the data
>> or at least an image of what you have now?
>> -dave
>>
>> On Sep 16, 2008, at 12:56 AM, Seth McGinnis wrote:
>>
>>> Ah! I get it now. I didn't realize they were different. Thank
>>> you!
>>>
>>> So, it turns out my data is actually rotated pole. I have lat2d &
>>> lon2d arrays, so the data is showing up in the right place on the
>>> map, but I want to use a map projection that will make it square.
>>> Any thoughts? In Ezmap terms, I believe it has non-zero PLAT &
>>> PLON. Is that what mpCenterLatF & mpCenterLonF specify for the
>>> normal Mercator projection?
>>>
>>> --Seth
>>>
>>>
>>>> Hi Seth,
>>>> The RotatedMercator projection is not actually the same as a
>>>> rotated pole projection. It ignores the 'mpCenterLatF' value,
>>>> and the 'mpCenterRotF' value behaves in a different manner from
>>>> the normal Mercator projection.
>>>> You will see the difference if you give 'mpCenterRotF' a non-zero
>>>> value.
>>>>
>>>> For an explanation of the difference between these projections
>>>> from the perspective of the underlying software, see:
>>>>
>>>> http://www.ncarg.ucar.edu/supplements/ezmap/#PROJECTIONS
>>>>
>>>> Scroll down to 'Cylindrical Projections'
>>>>
>>>> If the grid is plotted in its native projection then you must
>>>> either have the tfDoNDCOverlay flag set True, or you are not
>>>> communicating
>>>> the 2d coordinates correctly to the plotting routine.
>>>>
>>>> -dave
>>>>
>>>>
>>>>
>>>> On Sep 15, 2008, at 4:17 PM, Seth McGinnis wrote:
>>>>
>>>>> Does anyone have any examples of how to plot data that's in a
>>>>> rotated Mercator (aka rotated pole or rotated lat-lon) coordinate
>>>>> system?
>>>>>
>>>>> I've been making plots using the following map projection
>>>>> resources:
>>>>>
>>>>> res_at_mpProjection = "RotatedMercator"
>>>>> res_at_mpCenterLonF = 83
>>>>> res_at_mpCenterLatF = 42.5
>>>>>
>>>>> and as far as I can tell, it doesn't look any different than
>>>>> leaving the projection unspecified. (The data has lat2d and lon2d
>>>>> arrays. Using the same attributes with the corresponding values
>>>>> for data in a polar stereographic or transverse mercator
>>>>> projection
>>>>> results in the native grid being plotted square on the page.)
>>>>> Maybe I have the wrong values there, but I tried it with
>>>>> mpCenterLatF = 0, 45, and 90, and it made no difference to the
>>>>> resulting plot. Should I be specifying the projection parameters
>>>>> in a different way?
>>>>>
>>>>> The only examples I can find of the RotatedMercator projection on
>>>>> the NCL website are in the "Map Only" application examples 12 &
>>>>> 13,
>>>>> and it looks to me like the RotatedMercator example is
>>>>> identical to
>>>>> the Mercator example.
>>>>>
>>>>> Thanks,
>>>>>
>>>>> --Seth
>>>>>
>>>>> ----
>>>>> Seth McGinnis
>>>>> Associate Scientist
>>>>> ISSE / NCAR
>>>>> ----
>>>>> _______________________________________________
>>>>> ncl-talk mailing list
>>>>> ncl-talk_at_ucar.edu
>>>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>>
>>>
>>
>

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Received on Tue Sep 16 2008 - 12:58:33 MDT