[ncl-talk] understanding function gc_pnt2gc

[Dennis Shea]

Hi Joe,

I have never used the '*gc_pnt2gc*' function. Hopefully, somebody will
address that function.

There us an alternative:
  https://www.ncl.ucar.edu/Document/Functions/Built-in/gc_latlon.shtml

Try the following interactively

%> ncl <return>
Then enter the following

p_lat = 41.6
p_lon = -76.84
lat = (/52.9921,55.3543/)
lon = (/-168.693,-160.346/)
dist = gc_latlon(p_lat,p_lon,lat,lon,10,2)
print(dist)

Variable: dist
Type: float
Total Size: 8 bytes
            2 values
Number of Dimensions: 1
Dimensions and sizes:   [2]
Coordinates:
Number Of Attributes: 4
  units :       degrees
  gclon :       <ARRAY of 20 elements>
  gclat :       <ARRAY of 20 elements>
  spacing :     ( 6.551194, 5.948726 )

(0)     58.96074
(1)     53.53853

===
Cheers
D


On Wed, Aug 9, 2017 at 1:21 PM, Joe Grim <grim at ucar.edu> wrote:

> Hi,
>
> I have used the function gc_pnt2gc, but it doesn't appear to be doing what
> I think it should be doing.  I assume this is because I am misunderstanding
> it.  What I think it does is give the distance in degrees between a lat/lon
> point (p_lat, p_lon), and its closest approach to the great circle line
> between two other lat/lon pairs (lat[2], lon[2]: and I assume that this
> great circle line is the shortest greatest circle line between these
> lat/lon pairs.)  Here is an example snippet of what I am trying to do:
>
>  p_lat = 41.6
>  p_lon = -76.84
>  lat = (/52.9921,55.3543/)
>  lon = (/-168.693,-160.346/)
>  dist = gc_pnt2gc(p_lat,p_lon,lat,lon)
>  print ("The distance is: " + dist)
>
> Here is the output:
> (0)     The distance is: 0.105952
>
> I would expect the distance to be something around 53 degrees.
>
> Could someone please explain to me what I am misunderstanding about this
> function?  And, do you know of another function that does what I am trying
> to do?  (That is, find the shortest distance between a point and a line on
> the Earth.)
>
> Thank you!
>
> Joe Grim
>
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