[ncl-talk] understanding function gc_pnt2gc

[Joe Grim]

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
>>
>> _______________________________________________
>> ncl-talk mailing list
>> ncl-talk at ucar.edu
>> List instructions, subscriber options, unsubscribe:
>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.ucar.edu/pipermail/ncl-talk/attachments/20170810/ca08e165/attachment.html