[ncl-talk] understanding function gc_pnt2gc

[Joe Grim]

Alan,

That is a great workaround for this problem!  I'll try your approach unless
I can find a way to get gc_pnt2gc to work the way I want it to.

I looked up the source code, which is located at
ncl/ni/src/lib/nfpfort/sg_tools.f, under FORTRAN function XDPGCDP, and was
written by David Kennison.  It would take me a while to figure it all out,
so hopefully your workaround is fast enough for my purposes.

Thanks again!

Joe

On Thu, Aug 10, 2017 at 9:55 AM, Alan Brammer <abrammer at albany.edu> wrote:

> As another person who uses gc_latlon extensively but didn't know gc_pnt2gc
> existed.
>
> You could use gc_latlon to recreate gc_pnt2gc().  Definitely seems like
> that function is doing some weird stuff.
> Though this wouldn't be directional but that could be worked out
> relatively easily.
>
>
>
> 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)
>
> line         = gc_latlon( lat(0), lon(0), lat(1), lon(1), 1000, 2)
> ;; create the GC path
> distances =  gc_latlon(p_lat, p_lon, line at gclat, line at gclon, 0, 2)  ;;
> find distances to GC path
> shortest   = minind(distances)
>
> print ("The gc_pnt2gc distance is: " + dist)
> print ("The gc_latlon distance is: " + distances(shortest) )
>
> ; The gc_pnt2gc distance is: -0.105952
> ; The gc_latlon distance is: 53.5385
>
> On Thu, Aug 10, 2017 at 10:46 AM, Joe Grim <grim at ucar.edu> wrote:
>
>> Thank you, Dennis.  Yes, I have used the gc_latlon function many times;
>> it is very useful!
>>
>> But, I am hoping to use a function that finds the distance between a
>> point, and where it is closest to a line.  If I can't, I'll just have to
>> write my own function in FORTRAN instead to do that.
>>
>> Hopefully someone else is familiar with the *gc_pnt2gc* function.
>>
>> Joe
>>
>> On Thu, Aug 10, 2017 at 8:12 AM, Dennis Shea <shea at ucar.edu> wrote:
>>
>>> 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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>>>
>>
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