Re: how to calculate the area of polygon?

gmtspatial -Q will either compute areas for closed polygons or line length for unclosed lines.  Your polygon is not closed so it is assumed to be a line and you get the length of the line and its mid point.
We may want to add a modifier to "force closure of lines to make polygons" but it is not there yet.
If I close your polygon then I get

gmtspatial poly.data -Q
15	-55	100

assuming cartesian data and

gmtspatial poly.data -Q -fg
15	-55.1025678246	707841.648502

if I tell it the data are geographic.

On Dec 5, 2013, at 2:43 PM, Namba Takaya <[email protected]> wrote:

> Dir Sir
> 
> According to your suggestion, 
> 
> I tried to do the script below to calculate the area of the polygon with gmtspatial, but the result is strange as unexpected.
> 
> Could you tell me how to solve the problem?
> 
> gmtspatial poly.data -Q
> 
> poly.data is like
>  20:00  -50:00
>  20:00  -60:00
>  10:00  -60:00
>  10:00  -50:00
> 
> regards,
> 
> takaya
> 
> Date: Thu, 28 Nov 2013 15:48:05 +0100
> From: [email protected]
> Subject: Re: [GMT-HELP] how to calculate the area of polygon?
> To: [email protected]
> 
> In GMT5 you can do this using gmtspatial, 
> 
> If you are stuck with GMT4 it should be possible to do so using grdvolume if you first define a grid file in which all the points inside your polygon has a constant value differing from the points outside, then apply grdcontour to the countour value equal to the value you have chosen inside your polygon.  
> 
> regP 
> 
> 
> On 11/28/2013 03:35 PM, Namba Takaya wrote:
> Dear sir
> 
> 
> Could you tell me how to calculate the area of polygon(hour points) like
> 
> 1.   5:00E  55:00S 
> 2.   6:00E  55:00S
> 3.   6:00E  56:00S
> 4.   5:00E  56:00S
> 
> regards,
> 
> takaya
> 
> -- 
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>   
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