[ncl-talk] Conformal projection

[Rick Brownrigg]

Hi Carlos,

The map is *probably* OK, I don't know for certain. Looking at planar maps
near the poles is weird, and I've seen what you describe with E/W
longitudes labeling all sides. But look at greenland on a globe and it kind
of makes sense. I think if you set the resource "mpGridAndLimbOn=True", the
lines of longitude will make a bit more sense*.*

Rick


On Tue, Sep 15, 2020 at 12:53 PM Carlos Peralta via ncl-talk <
ncl-talk at mailman.ucar.edu> wrote:

> Hello,
>
> I am following example 3 here:
>
> https://www.ncl.ucar.edu/Applications/Scripts/lcnative_3.ncl
>
> to plot a grb file where I am not sure of what the appropriate
> parallels are.
>
> I get the figure below, which seems correct, but not sure
> why I see the axes displayed with "W" and "E" in both the vertical
> and horizontal.
>
> thanks for any help.
>
>
> My code is below
>
> ;------------------------------------------------
> ; lcnative_3.ncl
> ;------------------------------------------------
> ;
> ; These files are loaded by default in NCL V6.2.0 and newer
> ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
> ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
>
> begin
>
> ;------------------------------------------------
> ; open file and read in data
> ;------------------------------------------------
>   gfile= "./CW_Tskin.grib"
>   f     = addfile (gfile, "r")
>   printVarSummary(f)
>   ;fili  = "ruc2.bgrb.20020418.i12.f00.grb"
>   ;f     = addfile (fili, "r")
>
>   ;vpt       = f->VPTMP_252_HYBL
>   vpt       = f->TMP_GDS3_HTGL(g3_x_0|:,g3_y_1|:)
>   ;lat2d     = f->gridlat_252
>   ;lon2d     = f->gridlon_252
>    lat2d=f->g3_lat_0
>    lon2d=f->g3_lon_1
>
>
> ;------------------------------------------------
> ; create plot
> ;------------------------------------------------
>   wks = gsn_open_wks("png","lcnative")      ; send graphics to PNG file
>
>   res                   = True              ; plot mods desired
>   res at tiMainString      = "Native Lambert Conformal"
>   res at gsnAddCyclic = False
> ;------------------------------------------------
> ; lambert conformal projections are limited using
> ; the corners method rather than the latlon method
> ; seen for cylindrical equidistant projections
> ;------------------------------------------------
>   res at mpLimitMode       = "Corners"         ; choose range of map
>   res at mpLeftCornerLatF  = lat2d at corners(0)
>   res at mpLeftCornerLonF  = lon2d at corners(0)
>   res at mpRightCornerLatF = lat2d at corners(2)
>   res at mpRightCornerLonF = lon2d at corners(2)
> ;------------------------------------------------
> ; there are many instances when you may not know the
> ; appropriate parallels. Here is a method to find out...
> ; 1) as a first guess, start with the parellels in the
> ;    center of the grid.
> ; 2) look at the lines plotted on the map (see below).
> ;    you know the projection is correct when the red
> ;    and blue lines form a right angle between themselves
> ;    and the boundary.
> ; 3) iterate as necessary to get the closest solution.
> ;------------------------------------------------
>   res at mpProjection          = "LambertConformal"
>
>   res at mpLambertParallel1F = (max(lat2d)+min(lat2d))*0.5
>   res at mpLambertParallel2F = (max(lat2d)+min(lat2d))*0.5
>   res at mpLambertMeridianF  = (max(lon2d)+min(lon2d))*0.5
>
> ;------------------------------------------------
>   res at pmTickMarkDisplayMode = "Always"
>   res at mpFillOn              =  False           ; turn off map fill
>   res at mpOutlineDrawOrder    = "PostDraw"       ; draw continental outline
> last
>   res at mpOutlineBoundarySets = "GeophysicalAndUSStates" ; state boundaries
>
> ;------------------------------------------------
> ; usually, when data is placed onto a map, it is TRANSFORMED to the
> specified
> ; projection. Since this model is already on a native lambert conformal
> grid,
> ; we want to turn OFF the transformation.
> ;------------------------------------------------
>
>   res at tfDoNDCOverlay = True
> ; res at tfDoNDCOverlay = "NDCViewport"          ; NCL V6.5.0 or later
>
>
> ;------------------------------------------------
> ; set additional resources and create plot
> ;------------------------------------------------
>   res at cnFillOn              = True            ; color plot desired
>   res at cnLinesOn             = False           ; turn off contour lines
>   res at cnFillPalette         = "gui_default"   ; set color map
>   res at gsnFrame              = False           ; don't advance frame yet
>
>   ;plot = gsn_csm_contour_map(wks,vpt(9,:,:),res) ; draw plot
>   plot = gsn_csm_contour_map(wks,vpt(:,:),res) ; draw plot
>
> ;------------------------------------------------
> ; add lines that help determine correct projection
> ;------------------------------------------------
>   dims  = dimsizes(vpt)                       ; get dimension of grid
>   ;nlat  = dims(1)
>   ;nlon  = dims(2)
>   nlat  = dims(0)
>   nlon  = dims(1)
>
>   gres                  = True                ; polyline mods desired
>   gres at gsLineThicknessF = 3.0                 ; line thickness
>   gres at gsLineColor      = "Red"               ; line color
>   gsn_polyline (wks, plot, lon2d(nlat/2,:), lat2d(nlat/2,:), gres)
>
>   gres at gsLineColor      = "Blue"
>   gsn_polyline (wks, plot, lon2d(:,nlon/2), lat2d(:,nlon/2), gres)
>
>   gres at gsLineColor      = "Black"
>   gsn_polyline (wks, plot, lon2d(:,0)     , lat2d(:,0), gres)
>   gsn_polyline (wks, plot, lon2d(:,nlon-1), lat2d(:,nlon-1), gres)
>   gsn_polyline (wks, plot, lon2d(0,:)     , lat2d(0,:), gres)
>   gsn_polyline (wks, plot, lon2d(nlat-1,:), lat2d(nlat-1,:), gres)
>
>   frame(wks)
>
> end
>
>
>
>
> [image: image.png]
>
>
>
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