Re: Lambert conformal conic projection question

Hi Yang,

My understanding is that the central latitude, along with central
meridian, merely defines the origin for the rectangular projection
space, but otherwise does not materially affect the shape of
projection. Thus I can see it would make a big difference in what gets
returned from a calculation like wrf_ll_to_ij(), but I wouldn't expect
it to affect the shape and positioning of a contour plot (?)

Is that what you are seeing -- some differences in the alignment or
shape of the contour plot?

Rick

On Nov 19, 2009, at 10:39 PM, Yang Gao wrote:

> To whom it may concern:
>
> I am curious about the contour plot for Lambert conformal conic
> projection. The following is the necessary information to supply to
> the ncl script. However, there is
>
> another parameter:central latitude. As you know, when we do
> projection, we need to supply:
> Parallel1,
> Parallel2,
> central meridian,
> central latitude.
>
> I just wonder there may be some error if we don't supply centeral
> latitude. I am curious how the ncl program know where is the central
> latitude. I guess it might use
>
> the average of Parallel1 and Parallel2. But this is not always true.
>
>
>
>
> res@mpProjection = "LambertConformal"
> res@mpLambertParallel1F = file1->Latin
> res@mpLambertParallel2F = file1->Latin
> res@mpLambertMeridianF = file1->Lov
>
>
> Today, I am trying to use wrf_ll_to_ij to find the location of the
> observational site. But it seemed there is about one grid shift
> compared the way I use before. I assume the method I use before is
> correct. I am not sure whether the difference may result from the
> central latitude issue.
>
>
> opt@MAP_PROJ = 1
> opt@TRUELAT1 = 15.0
> opt@TRUELAT2 = 40.0
> opt@STAND_LON = 114.0
> opt@REF_LAT = LAT(0,0,60,60)
> opt@REF_LON = LON(0,0,60,60)
> opt@KNOWNJ = 61
> opt@KNOWNI = 61 ; grid number, not array number, so
> it is 1 not 0
> opt@DX = 3000.
> opt@DY = 3000.
>
> opt@retrunInt = True
> loc1 = wrf_ll_to_ij( 120.978, 31.0935, opt )
>
>
> Thanks
>
> Yang
>
>
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Received on Fri Nov 20 09:28:09 2009