[gdal-dev] Area of hourglass shaped polygon

Chaitanya kumar CH chaitanya.ch at gmail.com
Fri Mar 26 09:52:36 EDT 2010

```Ramiro,

The C API function OGR_R_IsValid( OGRGeometryH ) may help you.

On Fri, Mar 26, 2010 at 2:02 AM, Ramiro Gonzalez <
ramirogonzalez at suremptec.com.ar> wrote:

> I won't calculate the area. I'll show an 'invalid area' message to the
> user.
>
> Thanks.
>
> 2010/3/19 Peter J Halls <P.Halls at york.ac.uk>
>
> Whilst I agree with Chris that the 'winding' method for calculating polygon
>> areas would need tweaking to work with a shape like this, the 'complementary
>> areas' method or the triangulation method would.  See Stephen Wise, GIS
>> Basics, London: Taylor & Francis (2002) pp61-64 or Joseph O'Rourke,
>> Computational Geometry in C, Cambridge (2nd Edn): Cambridge University Press
>> (1998) pp18-22.
>>
>> Best wishes,
>>
>> Peter
>>
>>
>> Chris 'Xenon' Hanson wrote:
>>
>>> On 3/18/2010 10:20 AM, Ramiro Gonzalez wrote:
>>>
>>>> I use gdal to calculate the area of polygons drawn by a user over a map.
>>>> The points are added to the polygon in same order the user selects them.
>>>> An hourglass shaped polygon can be created adding 4 points like a 'z'.
>>>> When the polygon has the shape of an hourglass, the measure returned by
>>>> get_Area() is smaller than the sum of the areas of the two independent
>>>> triangles.
>>>> I'd like to know if there is a way to calculate the area in this
>>>> situation?
>>>> I tried to intersect the polygon with another that contains it but the
>>>> operation fails when the polygon cross itself(return NULL). I could use
>>>> this to inform that the area calculated is invalid, but I'd like to know
>>>> if there is a better way of knowing this.
>>>>
>>>
>>>  I'm pretty sure this would be considered nonsensical topology. Area
>>> calculations usually
>>> rely on the points in a polygon winding either consistently clockwise or
>>> counter-clockwise, but yours are mixed.
>>>
>>>  Don't do that. It's bad.
>>>
>>>  If you want to measure the area of a shape like that, create it out of
>>> two normal
>>> consistently-wound triangles stacked tip-to-tip.
>>>
>>> Thanks,
>>>> Ramiro
>>>>
>>>
>>>
>> --
>>
>> --------------------------------------------------------------------------------
>> Peter J Halls, GIS Advisor, University of York
>> Telephone: 01904 433806     Fax: 01904 433740
>> Snail mail: Computing Service, University of York, Heslington, York YO10
>> 5DD
>> This message has the status of a private and personal communication
>>
>> --------------------------------------------------------------------------------
>>
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>
>
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--
Best regards,
Chaitanya kumar CH.
/tʃaɪθənjə/ /kʊmɑr/
+91-9848167848
17.2416N 80.1426E
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