# [ncl-talk] calculate area of polygon box

Dennis Shea shea at ucar.edu
Mon Feb 5 12:13:44 MST 2018

```https://www.ncl.ucar.edu/Document/Functions/Built-in/area_poly_sphere.shtml
Arguments *lat*
*lon*

Latitudes and longitudes, in degrees, of the polygon (vertex) locations.

*These must be in clockwise order. This is the user's responsibility.*
*=====================*

ncl 0>  rsph = 6371
ncl 1> latv =
(/31.00,29.70,29.15,28.82,28.43,28.01,27.98,27.47,27.38,28.64,28.99,29.40,29.56,29.99,30.05,30.01,30.61,30.64,31.00/)
ncl 2>  lonv =
(/77.98,77.96,79.01,80.02,80.98,81.57,81.98,83.01,84.00,83.99,83.01,83.01,80.98,81.00,80.01,79.64,79.63,79.01,77.98/)

ncl 3> qarea_poly = area_poly_sphere(latv, lonv, rsph)
ncl 4> print ("qarea_poly="+qarea_poly)

ncl 5> latv = latv(::-1)       ; make clockwise
ncl 6> lonv = lonv(::-1)
ncl 7> qarea_poly = area_poly_sphere(latv, lonv, rsph)
ncl 8> print ("qarea_poly="+qarea_poly)

(0)     qarea_poly=94602.8

On Mon, Feb 5, 2018 at 10:50 AM, Kunal Bali <kunal.bali9 at gmail.com> wrote:

> okay, thanks for this suggestion, Adam sir.
>
> I tried
>
> ncl 9> rsph = 6371 *--> to get area in km^2*
>
>
> ncl 10>  latv = (/31.00,29.70,29.15,28.82,28.
> 43,28.01,27.98,27.47,27.38,28.64,28.99,29.40,29.56,29.99,30.
> 05,30.01,30.61,30.64,31.00/)
> ncl 11> lonv = (/77.98,77.96,79.01,80.02,80.98,81.57,81.98,83.01,84.00,83.
> 99,83.01,83.01,80.98,81.00,80.01,79.64,79.63,79.01,77.98/)
> ncl 12> qarea_poly = area_poly_sphere(latv, lonv, rsph)
>
> ncl 13> print ("qarea_poly="+qarea_poly)
>
> (0) *qarea_poly=5.0997e+08   ---> the calculated area is very large. it
> seems like wrong.*
>
> *I mean from figure (previous mail), the longitude ranges from 78 to 84 =
> 6 degree = ~660 km *
> *
> latitude rages almost 1 degree = 110 km*
> *                                                                  so it
> could be 660 x 110 = ~72600 km^2 and from the above function it's *
> *5.0997e+08*
>
>
>
>
> ---
> Kunal Bali
>
>
>
>
>
> On Mon, Feb 5, 2018 at 10:16 PM, Adam Phillips <asphilli at ucar.edu> wrote:
>
>> Hi Kunal,
>> The examples shown on the area_poly_sphere documentation page also
>> illustrate how to use gc_clkwise and gc_qarea, the latter having the
>> requirement of input lat/lon arrays having a size of 4. This was done to
>> show that the results are the same between gc_qarea and area_poly_sphere
>> (when input lat/lon arrays are of size 4). Try just calling
>> area_poly_sphere and all should be well.
>>
>> On Mon, Feb 5, 2018 at 9:32 AM, Kunal Bali <kunal.bali9 at gmail.com> wrote:
>>
>>> Dear Dennis Sir,
>>> Thanks for this suggestion.
>>>
>>> I tried that one, but I think these examples are applicable to
>>> rectangular box only with the 4 lat lon grid points.
>>>
>>> I followed the examples on that page. e.g.
>>>
>>> ncl 0> rsph = 1.0
>>> ncl 1> lonv = (/77.98,77.96,79.01,80.02,80.9
>>> 8,81.57,81.98,83.01,84.00,83.99,83.01,83.01,80.98,81.00,80.0
>>> 1,79.64,79.63,79.01,77.98/)
>>> ncl 2>  latv = (/31.00,29.70,29.15,28.82,28.4
>>> 3,28.01,27.98,27.47,27.38,28.64,28.99,29.40,29.56,29.99,30.0
>>> 5,30.01,30.61,30.64,31.00/)
>>> ncl 3> qorder     = gc_clkwise(latv, lonv)
>>> ncl 4> qarea_gc   = gc_qarea(latv, lonv)
>>> fatal:gc_qarea: the last (rightmost) dimension of arrays must be 4
>>> fatal:["Execute.c":8576]:Execute: Error occurred at or near line 4
>>>
>>>
>>>
>>>
>>> ---
>>> Kunal Bali
>>>
>>>
>>>
>>>
>>>
>>> On Mon, Feb 5, 2018 at 9:09 PM, Dennis Shea <shea at ucar.edu> wrote:
>>>
>>>> Did you look at any of the functions?
>>>>
>>>> Click "Functions" ; Click "Alphabetical Listing"; Search 'area'
>>>>
>>>> Keep going till you find something: Like .......
>>>>
>>>> http://www.ncl.ucar.edu/Document/Functions/Built-in/area_pol
>>>> y_sphere.shtml
>>>>
>>>> On Mon, Feb 5, 2018 at 5:32 AM, Kunal Bali <kunal.bali9 at gmail.com>
>>>> wrote:
>>>>
>>>>> Dear NCL users,
>>>>>
>>>>> I have coordinates such as
>>>>> lonv = (/77.98,77.96,79.01,80.02,80.98,81.57,81.98,83.01,84.00,83.9
>>>>> 9,83.01,83.01,80.98,81.00,80.01,79.64,79.63,79.01,77.98/)
>>>>>
>>>>>   latv = (/31.00,29.70,29.15,28.82,28.43,28.01,27.98,27.47,27.38,28.6
>>>>> 4,28.99,29.40,29.56,29.99,30.05,30.01,30.61,30.64,31.00/)
>>>>>
>>>>>
>>>>> from that, I am able to plot the polygon line e.g.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> So, could anyone please let me know how to calculate the area (in km)
>>>>> of this polygon box?
>>>>>
>>>>>
>>>>> thank you
>>>>> ---
>>>>> Kunal Bali
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> ncl-talk mailing list
>>>>> ncl-talk at ucar.edu
>>>>> List instructions, subscriber options, unsubscribe:
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>>>>>
>>>>>
>>>>
>>>
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>>
>>
>> --
>> Associate Scientist,  Climate and Global Dynamics Laboratory, NCAR
>> www.cgd.ucar.edu/staff/asphilli/   303-497-1726 <(303)%20497-1726>
>>
>> <http://www.cgd.ucar.edu/staff/asphilli>
>>
>
>
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