Tag Archives: Developers

At ArcGIS 10, creating spatial references programmatically has become much easier.

Just to recap, a spatial reference is comprised of several parts:

• Coordinate system
• XY, Z, M resolution values
• XY, Z, M tolerance values
• XY, Z, M spatial domains (extents)

The Z and M coordinates are optional and needed only if a feature contains Z values (usually for elevation) or M values (for measurements along a line).

So what we’ve done is rolled the methods for setting domains, resolution and tolerance values into a few key methods, making the whole process more streamlined.

The Old
Prior to ArcGIS 10, if you created a spatial reference coordinate system through ISpatialReference::CreateSpatialReference, ISpatialReferenceFactory::CreateProjectedCoordinateSystem, or ISpatialReferenceFactory::CreateGeographicCoordinateSystem, the resolution, tolerance, and spatial domain values were not initialized. To create a complete spatial reference, you needed to call ConstructFromHorizon, SetDefaultXYTolerance and SetDefaultXYResolution methods.

At 9.3.1 and earlier the suggested steps for creating a spatial reference were as follows:

        public static ISpatialReference3 CreateSpatialReference(int srID)
        {
            // create spatial reference factory
 	Type factoryType = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
 	ISpatialReferenceFactory3 spatialReferenceFactory = (ISpatialReference3)Activator.CreateInstance(factoryType);

            // create the spatial reference
            ISpatialReference3 spatialReference = (ISpatialReference3)spatialReferenceFactory.CreateSpatialReference(srID);

            // From the ISpatialReferenceResolution intereface set the
            // default domain and the default, XY, Z, and M resolution
            ISpatialReferenceResolution spatialReferenceResolution = (ISpatialReferenceResolution)spatialReference;
            spatialReferenceResolution.ConstructFromHorizon();
            spatialReferenceResolution.SetDefaultXYResolution();
            spatialReferenceResolution.SetDefaultZResolution();
            spatialReferenceResolution.SetDefaultMResolution();

            // From the ISpatialReferenceTolerance interface set the
            // default XY, Z, and M tolerance
            ISpatialReferenceTolerance spatialReferenceTolerance = (ISpatialReferenceTolerance)spatialReference;
            spatialReferenceTolerance.SetDefaultXYTolerance();
            spatialReferenceTolerance.SetDefaultZTolerance();
            spatialReferenceTolerance.SetDefaultMTolerance();
            return spatialReference;
        }

The New
At ArcGIS 10, those methods for setting the domain, resolution and tolerance are included in the following methods:

• CreateSpatialReference
• CreateProjectedCoordinateSystem
• CreateGeographicCoordinateSystem

So, at 10 it is merely a case of creating the spatial reference factory singleton and then using the CreateSpatialReference method like this:

        public static ISpatialReference3 CreateSpatialReference(int srID)
        {
            // create spatial reference factory
            Type factoryType = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
            ISpatialReferenceFactory3 spatialReferenceFactory = (ISpatialReference3)Activator.CreateInstance(factoryType);

            // create the spatial reference
            ISpatialReference3 spatialReference = (ISpatialReference3)spatialReferenceFactory.CreateSpatialReference(srID);
            return spatialReference;
        }

And on a one-third-the-length-of-the-blog-post side note, you may notice that we are using the ProgID and the Activator class to get a reference to the spatial reference factory interface.  This is because the spatial reference environment object is a singleton, and may or may not have already been created by another class or method.  If a SpatialReferenceEnvironmentClass singleton instance already exists, then the traditional creation of the ISpatialReferenceFactory interface by creating a new SpatialReferenceEnvironmentClass

ISpatialReferenceFactory3 spatialReferenceFact = new SpatialReferenceEnvironmentClass();

To get around this we use the ProgID and the Activator class which will either create a new SpatialReferenceEnvironmentClass instance if the singleton has not yet been created or it will return you an instance of the already created singleton object without the risk of an invalid cast exception.

This post was contributed by geometry-guru David Raleigh from the geometry team.