datatondc

Converts data units into normalized device coordinates (NDCs).

Prototype

	procedure datatondc (
		plot  [1] : graphic,  
		x_in  [*] : float,    
		y_in  [*] : float,    
		x_out [*] : float,    
		y_out [*] : float     
	)

Arguments

A plot object that can be created by using one of the many gsn plotting functions, or by calling the NCL create language construct.

One-dimensional arrays of x and y data coordinates. They must be the same length. For map plots, x_in corresponds to longitude values, and y_in corresponds to latitude values.

(output)
One-dimensional arrays of the same size as x_in and y_in. These will contain the x and y NDC output values.

Description

The datatondc procedure uses the built-in transformation mapping feature of NCL plot objects to map data coordinates into Normalized Device Coordinates (NDCs).

The datatondc procedure maps coordinate pairs from x_in and y_in and places the results in the corresponding elements of x_out and y_out respectively. Both x_out and y_out can be the same variables as x_in and y_in.

When the data coordinates are outside of the data transformation specified in the plot, a missing value is placed in both of the corresponding indexes in the output arrays. The missing value chosen is either the x_in missing value, y_in missing value, or a system default.

See Also

ndctodata, NhlDataToNDC

Examples

Example 1

This example creates and draws an XY plot, and then shows how to use datatondc to convert some data coordinates to NDC coordinates, and use these new coordinates to plot markers.

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"

begin
;
; Create some dummy data.
;
  npts = 500
  x    = fspan(0,npts-1,npts)
  y    = 500.+ 0.9 * x * sin(0.031415926535898*x)

  wks = gsn_open_wks("x11","test")

  res             = True
  res@gsnMaximize = True

  xy = gsn_xy(wks,x,y,res)

  x_in  =  x
  y_in  = fabs(y)
  x_out = new(dimsizes(x_in),float)
  y_out = new(dimsizes(y_in),float)

  datatondc(xy,x_in,y_in,x_out,y_out)

;  print("data: (" + x_in + "," + y_in + ") NDC: (" + x_out + "," + y_out +")")

;
; Just for fun, see how you can now use these NDC coordinates to
; plot markers on the XY curve. Note, this can be done much
; easier by using gsn_polymarker directly; this example is just 
; for illustration.
;
  draw(xy)

  gsres               = True
  gsres@gsMarkerColor = "HotPink"
  gsres@gsMarkerIndex = 14
  gsres@gsMarkerSizeF = 10.0
  gsn_polymarker_ndc(wks,x_out(::5),y_out(::5),gsres)

  gsres@gsMarkerColor = "Yellow"
  gsres@gsMarkerIndex = 11
  gsn_polymarker_ndc(wks,x_out(3::5),y_out(3::5),gsres)

  frame(wks)
end

Example 2

This example creates and draws a map plot, and then shows how to use datatondc to convert some longitude coordinates along a single latitude value to NDC coordinates. It also uses the reverse function, ndctodata, to convert the NDC coordinates back to the original values.

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"

begin
  wks = gsn_open_wks("x11","test")

  res             = True
  res@gsnMaximize = True

  map = gsn_map(wks,"CylindricalEquidistant",res)

  x_in = fspan(-180,180,10)
  y_in = new(dimsizes(x_in),float)
  y_in = 30.

  x_out = new(dimsizes(x_in),float)
  y_out = new(dimsizes(y_in),float)

  print("datatondc")
  datatondc(map,x_in,y_in,x_out,y_out)
  print("data: (" + x_in + "," + y_in + ") NDC: (" + x_out + "," + y_out +")")

;
; Use ndctodata to go back to the original lat/lon coordinates. They
; should be the same as the original values.
;
  x_out2 = new(dimsizes(x_out),float)
  y_out2 = new(dimsizes(y_out),float)

  print("ndctodata")
  ndctodata(map,x_out,y_out,x_out2,y_out2)
  print("data: (" + x_in + "," + y_in + ") original data: (" + x_out2 + "," + y_
out2 +")")
end

The output should be:

(0)     datatondc
(0)     data: (-180,30) NDC: (0.0200001,0.58)
(1)     data: (-140,30) NDC: (0.126667,0.58)
(2)     data: (-100,30) NDC: (0.233333,0.58)
(3)     data: (-60,30) NDC: (0.34,0.58)
(4)     data: (-20,30) NDC: (0.446667,0.58)
(5)     data: (20,30) NDC: (0.553333,0.58)
(6)     data: (60,30) NDC: (0.66,0.58)
(7)     data: (100,30) NDC: (0.766667,0.58)
(8)     data: (140,30) NDC: (0.873333,0.58)
(9)     data: (180,30) NDC: (0.98,0.58)
(0)     ndctodata
(0)     data: (-180,30) original data: (-180,30)
(1)     data: (-140,30) original data: (-140,30)
(2)     data: (-100,30) original data: (-100,30)
(3)     data: (-60,30) original data: (-60,30)
(4)     data: (-20,30) original data: (-20,30)
(5)     data: (20,30) original data: (20,30)
(6)     data: (60,30) original data: (60,30)
(7)     data: (100,30) original data: (100,30)
(8)     data: (140,30) original data: (140,30)
(9)     data: (180,30) original data: (180,30)