point order in IGeometry and WKT

[Lenor]

Hello everyone!
I'm using nts 1.12.1. My code looks like that:

public void DoSomething(Envelope extent)
{
 IGeometry extentGeom = _geomFactory.ToGeometry(extent);
 string wkt = extentGeom.AsText();
 SqlGeography sqlGeom = SqlGeography.STGeomFromText(new System.Data.SqlTypes.SqlChars(wkt), 4326); // (*)
}

and in the line (*) I'm getting an exception, because in wkt-string points are ordered in clockwise order, while SqlGeography, according to OGC standard, accepts counter clockwise order.
If I add a line like this:

extentGeom.Normalize();

it doesn't help either, points are still ordered clockwise.

With the nts v.1.7.3 that code works properly, points are ordered counter clockwise.
If I'm doing something wrong, please tell me how to make it work.

Thanks)

[Felix Obermaier]

Parsing from WKT is expensive. Have you tried the SqlGeographyReader/Writer classes?

 

Anyway, if the coordinate order is expected to be CCW you can do

if (!NetTopologySuite.Algorithms.CGAlgorithms.IsCCW(p.Coordinates))

    p = p.Reverse();

 

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[Lenor]

Thanks for your attention)

 

1. It was just an example, for I wanted to see the wkt representation of my shape, so for now speed doesn't matter.

 

2. What I wanted to know, is: why from version 1.7.3 to 1.12.1 point order has changed.

 

I write the following simple code:

var envelope = new Envelope(1, 2, 1, 2);
var geom = geomFactory.ToGeometry(envelope);
var wkt = geom.AsText();

 

 

If I run it using NTS-1.7.3, I receive:

wkt = "POLYGON((1 1,2 1,2 2,1 2,1 1))"

This is counter clockwise order, and it's OGC-standard correct.

 

 

But if I'm using NTS-1.12.1, the string is different:

wkt = "POLYGON ((1 1, 1 2, 2 2, 2 1, 1 1))"

This is clockwise order, and according so OGC standard, it's incorrect.

 

In the 1.12.1, GeometryFactory.ToGeometry(Envelope envelope) method, points are adding in the clockwise order. Method Polygon.Normalize also is ordering points clockwise.

[FObermaier]

Where does it say in the OGC spec that polygon rings have to be counter-clockwise?
NTS behaves exactly like JTS does in this method.

[Lenor]

I may be wrong, but I've found it here:

http://www.opengeospatial.org/standards/sfa

6.1.11.1

The exterior boundary LinearRing defines the “top” of the surface which is the side of the surface from which the exterior boundary appears to traverse the boundary in a counter clockwise direction. The interior LinearRings will have the opposite orientation, and appear as clockwise when viewed from the “top”

 

And can you please specify which version of nts behaves like jts - 1.7.3 or 1.12.1, since thay behave different?

[FObermaier]

I must have overread that.
NTS v1.12.1 behaves like JTS v1.12 in that matter.

[John Diss]

It is confused slightly by SFS for SQL  1.2.1 (http://www.opengeospatial.org/standards/sfs) stating that orientation has no bearing on validity as long as shells and holes are in opposite directions.

To view this discussion on the web visit https://groups.google.com/d/msg/nettopologysuite/-/GhYXQNGcl8EJ.

[FObermaier]

JTS used to return CCW polygons up to version 1.9. and started to return CW polygons at 1.10.

On Tuesday, October 23, 2012 12:23:10 PM UTC+2, Lenor wrote:

[FObermaier]

And here is the explanation:
http://sourceforge.net/mailarchive/message.php?msg_id=30002801

[Lenor]

I see. So, I'm supposed to reverse polygons myself, when I need it.

Thank you very much for your help.