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Re: cgmap_gshhs.pro minarea issue [message #86614 is a reply to message #86594]

Fri, 22 November 2013 09:34

pvelissariou
Messages: 4
Registered: November 2013

Junior Member

On Thursday, November 21, 2013 7:09:28 PM UTC-5, pvelis...@fsu.edu wrote:
> Apparently, in the recent versions (>= 2.2) of gshhs database
>
> the units of the header.area changed from 1/10 km^2 to 1/10 m^2.
>
> For cgmap_gshhs to work properly the line:
>
> polygonArea = header.area * 0.1 (ok for gshhs < 2.2)
>
> should be changed to:
>
> polygonArea = header.area * 1.0e-7 (for gshhs >= 2.2)

David,

From gshhs.c file we have:
line = (h.area) ? 0 : 1; /* Either Polygon (0) or Line (1) (if no area) */
if (version >= 9) { /* Variable magnitude for area scale */
m = h.flag >> 26;
scale = pow (10.0, (double)m); /* Area scale */
}
area = h.area / scale; /* Now im km^2 */
f_area = h.area_full / scale; /* Now im km^2 */

so, in the cgmap_gshhs.pro
we should have something like:
magnitude = 10.0 ^ ( - ISHFT(f, -26) ) for version >= 9
and
magnitude = 0.1 for version < 9

and
polygonArea = header.area * magnitude

in order to produce the correct results for the polygon area

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[Message index]

		cgmap_gshhs.pro minarea issue By: pvelissariou on Thu, 21 November 2013 16:09
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Thu, 21 November 2013 16:33
		Re: cgmap_gshhs.pro minarea issue By: pvelissariou on Thu, 21 November 2013 18:05
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Thu, 21 November 2013 20:10
		Re: cgmap_gshhs.pro minarea issue By: Takis.Velissariou on Fri, 22 November 2013 07:57
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Fri, 22 November 2013 08:22
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Fri, 22 November 2013 08:39
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Fri, 22 November 2013 08:46
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Fri, 22 November 2013 08:50
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Fri, 22 November 2013 09:37
		Re: cgmap_gshhs.pro minarea issue By: pvelissariou on Fri, 22 November 2013 09:34
		Re: cgmap_gshhs.pro minarea issue By: pvelissariou on Sat, 23 November 2013 20:03
		Re: cgmap_gshhs.pro minarea issue By: David Fanning on Sat, 23 November 2013 21:15

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