<div dir="ltr"><div class="gmail_default" style="font-size:small">I've been looking into this too, and I've come to the tentative conclusion that this function is doing the correct thing.</div><div class="gmail_default" style="font-size:small"><br></div><div class="gmail_default" style="font-size:small">Joe, I think you're right that gc_pnt2gc uses the entire great circle around the globe, and not just the shortest section between the points. If you imagine slicing the globe at the great circle between the two lat,lon pairs that you gave, then the distance will be the length of the line drawn from your base point and perpendicular to the slice you just made.</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 12:09 PM, Rick Brownrigg <span dir="ltr"><<a href="mailto:brownrig@ucar.edu" target="_blank">brownrig@ucar.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">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 (?)<div><br></div><div><br></div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 10:41 AM, Joe Grim <span dir="ltr"><<a href="mailto:grim@ucar.edu" target="_blank">grim@ucar.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">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.<br></div><div class="m_8707625187699278684HOEnZb"><div class="m_8707625187699278684h5"><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 10:39 AM, Joe Grim <span dir="ltr"><<a href="mailto:grim@ucar.edu" target="_blank">grim@ucar.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div><div>Alan,<br><br></div>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.<br><br></div>I looked up the source code, which is located at ncl/ni/src/lib/nfpfort/sg_tool<wbr>s.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.<br><br></div>Thanks again!<span class="m_8707625187699278684m_-147886874246614735HOEnZb"><font color="#888888"><br><br></font></span></div><span class="m_8707625187699278684m_-147886874246614735HOEnZb"><font color="#888888">Joe<br></font></span></div><div class="m_8707625187699278684m_-147886874246614735HOEnZb"><div class="m_8707625187699278684m_-147886874246614735h5"><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 9:55 AM, Alan Brammer <span dir="ltr"><<a href="mailto:abrammer@albany.edu" target="_blank">abrammer@albany.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_default" style="font-family:verdana,sans-serif">As another person who uses gc_latlon extensively but didn't know gc_pnt2gc existed. <br></div><div class="gmail_default" style="font-family:verdana,sans-serif"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif">You could use gc_latlon to recreate gc_pnt2gc(). Definitely seems like that function is doing some weird stuff. <br></div><div class="gmail_default" style="font-family:verdana,sans-serif">Though this wouldn't be directional but that could be worked out relatively easily. </div><div class="gmail_default" style="font-family:verdana,sans-serif"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif"><div class="gmail_default"><br></div><div class="gmail_default">p_lat = 41.6</div><div class="gmail_default">p_lon = -76.84 </div><div class="gmail_default"><br></div><div class="gmail_default">lat = (/52.9921,55.3543/)</div><div class="gmail_default">lon = (/-168.693,-160.346/)</div><div class="gmail_default">dist = gc_pnt2gc(p_lat,p_lon,lat,lon)</div><div class="gmail_default"><br></div><div class="gmail_default">line = gc_latlon( lat(0), lon(0), lat(1), lon(1), 1000, 2) ;; create the GC path</div><div class="gmail_default">distances = gc_latlon(p_lat, p_lon, line@gclat, line@gclon, 0, 2) ;; find distances to GC path</div><div class="gmail_default">shortest = minind(distances)</div><div class="gmail_default"><br></div><div class="gmail_default">print ("The gc_pnt2gc distance is: " + dist)</div><div class="gmail_default">print ("The gc_latlon distance is: " + distances(shortest) )</div><div class="gmail_default"><br></div><div class="gmail_default"><span style="white-space:pre-wrap">; </span>The gc_pnt2gc distance is: -0.105952<br></div><div class="gmail_default">; The gc_latlon distance is: 53.5385<br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 10:46 AM, Joe Grim <span dir="ltr"><<a href="mailto:grim@ucar.edu" target="_blank">grim@ucar.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div>Thank you, Dennis. Yes, I have used the gc_latlon function many times; it is very useful!<br><br>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.<br><br></div>Hopefully someone else is familiar with the <b>gc_pnt2gc</b> function.<span class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472HOEnZb"><font color="#888888"><br><br></font></span></div><span class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472HOEnZb"><font color="#888888">Joe<b><br></b></font></span></div><div class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472HOEnZb"><div class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472h5"><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Aug 10, 2017 at 8:12 AM, Dennis Shea <span dir="ltr"><<a href="mailto:shea@ucar.edu" target="_blank">shea@ucar.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div><div>Hi Joe,<br></div><div><br></div>I have never used the '<b>gc_pnt2gc</b>' function. Hopefully, somebody will address that function.<br><br></div><div>There us an alternative:<br> <a href="https://www.ncl.ucar.edu/Document/Functions/Built-in/gc_latlon.shtml" target="_blank">https://www.ncl.ucar.edu/Docum<wbr>ent/Functions/Built-in/gc_latl<wbr>on.shtml</a><br><br></div>Try the following interactively<br><br></div>%> ncl <return><br></div><div>Then enter the following<br></div><div><div><span><br>p_lat = 41.6<br>p_lon = -76.84<br>lat = (/52.9921,55.3543/)<br>lon = (/-168.693,-160.346/)<br></span>dist = gc_latlon(p_lat,p_lon,lat,lon,<wbr>10,2)<br></div><div>print(dist)<br></div><div><br>Variable: dist<br>Type: float<br>Total Size: 8 bytes<br> 2 values<br>Number of Dimensions: 1<br>Dimensions and sizes: [2]<br>Coordinates: <br>Number Of Attributes: 4<br> units : degrees<br> gclon : <ARRAY of 20 elements><br> gclat : <ARRAY of 20 elements><br> spacing : ( 6.551194, 5.948726 )<br><br>(0) 58.96074<br>(1) 53.53853<br><br>===<br></div><div>Cheers<br></div><div>D<br></div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472m_5731772318593252345h5">On Wed, Aug 9, 2017 at 1:21 PM, Joe Grim <span dir="ltr"><<a href="mailto:grim@ucar.edu" target="_blank">grim@ucar.edu</a>></span> wrote:<br></div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472m_5731772318593252345h5"><div dir="ltr"><div><div><div><div><div><div>Hi,<br><br></div>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:<br><br> p_lat = 41.6<br> p_lon = -76.84<br> lat = (/52.9921,55.3543/)<br> lon = (/-168.693,-160.346/)<br> dist = gc_pnt2gc(p_lat,p_lon,lat,lon)<br> print ("The distance is: " + dist)<br><br></div>Here is the output:<br>(0) The distance is: 0.105952<br><br></div>I would expect the distance to be something around 53 degrees.<br><br></div>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.)<br><br></div>Thank you!<span class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472m_5731772318593252345m_-6134214703216481583HOEnZb"><font color="#888888"><br><br></font></span></div><span class="m_8707625187699278684m_-147886874246614735m_-7453196106184047091m_-6280377129885226472m_5731772318593252345m_-6134214703216481583HOEnZb"><font color="#888888">Joe Grim<br></font></span></div>
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