NCL Home > Documentation > Functions > Lat/Lon functions

gc_inout

Determines if a list of lat/lon specified points are inside or outside of spherical lat/lon polygon(s).

Prototype

	function gc_inout (
		plat  : numeric,  
		plon  : numeric,  
		lat   : numeric,  
		lon   : numeric   
	)

	return_val  :  logical

Arguments

plat
plon

Latitude and longitude, in degrees, of points on the globe that you want to check whether they fall inside or outside of polygon(s) defined by lat and lon.

The arrays must agree in number of dimensions and dimension sizes.

lat
lon

Latitudes and longitudes, in degrees, of the vertices of one or more spherical polygons.

If these are multi-dimensional arrays, then the rightmost dimension must contain the polygonal data, and the rest of the dimensions must be the same as the dimensions of plat and plon.

It is assumed that the polygons do not cross themselves and that the vertices can be contained in a hemisphere. If the polygons are not closed (that is, if the first listed vertex doesn't equal the last), then this function will close the polygon for you.

The arrays must agree in number of dimensions and dimension sizes.

Return value

The return value will be a logical array with the same dimensions as plat and plon.

True is returned if the specified point is inside the polygon and False otherwise.

Description

This function determines if specified points are inside or outside of a list of spherical polygons. The polygons must be closed polygons; if the first listed vertex does not equal the last then the code will add an additional vertex to close the polygon. A point is considered inside if it is strictly inside or on a boundary arc. Given the discrete nature of floating point arithmetic, a point is considered to be on a boundary arc if it is within 1.e-10 degrees of it.

Missing values are not honored as such.

See Also

gc_onarc

Examples

Example 1

The following:

begin
;
;  Point at center of diamond.
;
  inout = gc_inout(0.0, -2.0, (/ 0.0,  1.0,  0.0, -1.0,  0.0/), \
                              (/-3.0, -2.0, -1.0, -2.0, -3.0/) )
  print(inout)
end

produces:

Variable: inout
Type: logical
Total Size: 4 bytes
            1 values
Number of Dimensions: 1
Dimensions and sizes:   [1]
Coordinates: 
(0)     True
Example 2

The following:

begin
;
;  Point on a boundary arc.
;
  plat = 20.
  plon =  0.
  qlat = (/0.0, 45.0, 45.0, 0.0/)
  qlon = (/0.0, 45.0,  0.0, 0.0/)
  inout = gc_inout(plat, plon, qlat, qlon)
  print(inout)
end
produces:

Variable: inout
Type: logical
Total Size: 4 bytes
            1 values
Number of Dimensions: 1
Dimensions and sizes:   [1]
Coordinates: 
(0)     True
Example 3

The following:

begin
  lat = (/                                                       \
            (/ (/  0.0,   0.0,  2.0/), (/ 0.0,  0.0,  1.0/) /),  \
            (/ (/  0.0,   0.0,  2.0/), (/ 0.0,  0.0,  1.0/) /),  \
            (/ (/ 89.0,  89.0, 89.0/), (/ 0.0,  0.0, 80.0/) /)   \
         /)
  lon = (/                                                          \
            (/ (/ 0.0,    2.0,   1.0/), (/ -1.0,   1.0, -1.0/) /),  \
            (/ (/ 0.0,    2.0,   1.0/), (/ -1.0,   1.0, -1.0/) /),  \
            (/ (/ 0.0,  120.0, 240.0/), (/  0.0,  90.0, 45.0/) /)   \
         /)

  p0_lat = (/ (/ 1.0,  0.0/),  \
              (/ 1.0, -0.1/),  \
              (/90.0, 45.0/)   \
            /)
  p0_lon = (/ (/ 1.0,  0.0/),  \
              (/ 0.0,  0.0/),  \
              (/ 0.0, 45.0/)   \
            /)

  inout = gc_inout(p0_lat, p0_lon, lat, lon)
  print(inout)
end
produces:

Variable: inout
Type: logical
Total Size: 24 bytes
            6 values
Number of Dimensions: 2
Dimensions and sizes:   [3] x [2]
Coordinates: 
(0,0)   True
(0,1)   True
(1,0)   False
(1,1)   False
(2,0)   True
(2,1)   True