[ncl-talk] understanding function gc_pnt2gc

[Rick Brownrigg]

I was looking at the source as well -- not easy to understand. The function
gives the correct answer for some simple test cases (eg, GC along prime
meridian and a test point equator 90-degrees away).  But clearly a wrong
answer for your case, and seems wrong if the routine is using the entire
great circle (?)



On Thu, Aug 10, 2017 at 10:41 AM, Joe Grim <grim at ucar.edu> wrote:

> I wonder if gc_pnt2gc uses the entire great circle around the globe, and
> not just the shortest section between the two points defining the great
> circle, in calculating the distance.
>
> On Thu, Aug 10, 2017 at 10:39 AM, Joe Grim <grim at ucar.edu> wrote:
>
>> 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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>>>
>>
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