[gdal-dev] S57 dataset boundary issue

[s duclos]

Frank & Nikhil, 


Frank asked:

>are, I'm not sure what I or others can do about
>the performance cost of checking.  

               
If I understand the problem, then it cost nothing!

You have to check the winding while loading a OGRGeometryH
like so:
                double area = 0;
                for (unsigned int i=0; i<vert_count-1; i++) {
                    double x1 = OGR_G_GetX(hRing, i);
                    double y1 = OGR_G_GetY(hRing, i);
                    double x2 = OGR_G_GetX(hRing, i+1);
                    double y2 = OGR_G_GetY(hRing, i+1);
                    area += (x1*y2) - (x2*y1);
                }


If the area < 0 then it is CW (clockwise) and > 0 is CCW (Counter CW).

Note that in S57 the exterior ring must be CW and
interior ring are CCW.

But in S57 there is no order between rings. So interior ring
might come first. I think WKT fix this. In my code I assume
that OGR place the exterior ring first.


We have been trough this before. At the time the question 

was if S57 allow for many exterior ring for the same poly.

I guess it all depend on HO and how they encode the data.

I'm using mostly Canadian ENC and found no problem.

But other HO might encode ENC differently ..




rdgs,


Sylvain Duclos.







>________________________________
> From: Frank Warmerdam <warmerdam at pobox.com>
>To: Nikhil Sai Parupalli <nikhil.parupalli at iictechnologies.com> 
>Cc: "gdal-dev at lists.osgeo.org" <gdal-dev at lists.osgeo.org> 
>Sent: Tuesday, October 9, 2012 12:25:29 PM
>Subject: Re: [gdal-dev] S57 dataset boundary issue
> 
>
>
>
>
>On Mon, Oct 8, 2012 at 10:28 PM, Nikhil Sai Parupalli <nikhil.parupalli at iictechnologies.com> wrote:
>
>HI All, 
>>,
>>   
>>Thanks for the reply in below data
>>
>>  NAME_RCNM (IntegerList) = (7:130,130,130,130,130,130,130)
>>   NAME_RCID (IntegerList) = (7:691,391,690,52,1336,1503,1512)
>>   ORNT (IntegerList) = (7:2,2,2,2,1,1,1)
>>   USAG (IntegerList) = (7:1,1,1,1,1,1,1)
>>   MASK (IntegerList) = (7:255,255,255,255,255,255,255)
>>
>> RCID 691 ORNT has  2 and USAG is 1 based on this data we could identify that RCID 691 is of exterior boundary with reverse direction.
>>
>>So if I have 1000 RCID in  NAME_RCID which is vise versa for the above condition I need to display that RCIDS list as errors.
>>So it makes 1000 conditional checks hence reducing performance.
>>
>>Please let us know further.
>
>
>Nikhil, 
>
>
>I'm afraid I don't follow your point.  Are you trying to
>do some sort of ring orientation validation and identify
>ones that don't match your expectations?  And you
>are concerned about the performance cost?  
>
>
>I don't know why you are doing this, and if you
>are, I'm not sure what I or others can do about
>the performance cost of checking.  
>
>
>Best regards,-- 
>---------------------------------------+--------------------------------------
>I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
>light and sound - activate the windows | http://pobox.com/~warmerdam
>and watch the world go round - Rush    | Geospatial Software Developer
>
>
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