Using SqlGeometry.MakeValid to get a counter-clockwise polygon in SQL Server

Question

The data I've loaded from a shapefile into a geometry column in SQL Server (using NetTopologySuite) is not producing valid GeoJSON when I export it because it doesn't follow the right-hand rule.

I thought this would be easy to fix with the MakeValid() function, but it's not correcting the issue.

I tried with a simplified geometry as below:

declare @geom geometry
select @geom = geometry::STGeomFromText('POLYGON ((
0 0,
0 1,
2 1,
2 0, 
0 0
))', 4326)

select @geom, @geom.IsValidDetailed()

SQL Server says it's valid (24400: Valid), so presumably that's why it's not fixing it in the MakeValid function.

What's the best way to resolve this? I've seen an option to use: UPDATE table SET geom = geom.STUnion(geom.STStartPoint()) which works for simple polygons, but I'm not sure how well it will work for more complex geometries.

Answer 1

I've started working with NetTopologySuite, and have the same problem. I found a fix here. Simply put:

if (!polygon.Shell.IsCCW) polygon = polygon.Reverse();

If you have Geometries you will need to cast, and if you have MultiPolygons you will have to iterate through each of them.

Answer 2

I had thought I would fix this in Sql Server using a spatial function like MakeValid, but as that didn't work, I've found a way to fix it when loading the data from the shape file, using NetTopologySuite.

I was using:

var factory = new GeometryFactory();
using (var shapeFileDataReader = new ShapefileDataReader(filePath, factory))

But simply changing the factory worked, like this:

var factory = new OgcCompliantGeometryFactory();
using (var shapeFileDataReader = new ShapefileDataReader(filePath, factory))