C# Vector and Spatial Reference Interfaces

Basic Architecture

The vector interface is within the OSGeo.OGR namespace and the spatial reference interface is within the OSGeo.OSR namespace.

The main classes are as follows

OGR
|
|- DataSource
|
|- Layer
|
|- Feature
|
|- Geometry

OSR
|
|- SpatialReference
|
|- CoordinateTransform

Accessing Feature Geometries

The basic process is DataSource => Layer => Feature => Geometry

Open a DataSource

A DataSource wraps a OGR source (e.g a filename) and is created as follows:

/* -------------------------------------------------------------------- */
/*      Register format(s).                                             */
/* -------------------------------------------------------------------- */
Ogr.RegisterAll();

/* -------------------------------------------------------------------- */
/*      Open data source.                                               */
/* -------------------------------------------------------------------- */
using (DataSource ds = Ogr.Open( "... add your own valid OGR source", 0 ))
{
    if (ds == null) {
        // create an error
    }
    // Do your processing here
}

Access the Layers

Each DataSource will have one or more layers that can be iterated as follows:

/* -------------------------------------------------------------------- */
/*      Iterating through the layers                                    */
/* -------------------------------------------------------------------- */

for( int iLayer = 0; iLayer < ds.GetLayerCount(); iLayer++ )
{
    Layer layer = ds.GetLayerByIndex(iLayer);

    if( layer == null )
    {
        // create an error
    }
    // Do your processing here
}

Access a Layer's Features

Each Layer can have zero or more Feature s. These should be accessed as follows:

layer.ResetReading();
Feature f = null;
do {
    f = layer.GetNextFeature();
    if (f != null)
        // Do your processing here
} while (f != null);

Access a Features's Geometry

Geometry geom = feature.GetGeometryRef();
wkbGeometryType type = geom.GetGeometryType();

Geometry objects are nested - so for instance:

a Geometry of type wkbGeometryType.wkbTIN has multiple daughter Geometry objects of type wkbGeometryType.wkbTriangle,
each Geometry object of type wkbGeometryType.wkbTriangle has a daughter Geometry object of type wkbGeometryType.LinearRing,
each Geometry of type wkbGeometryType.LinearRing contains a number of points.

When you get to the most basic type - which usually seems to be wkbGeometryType.wkbPoint, wkbGeometryType.wkbLineString or wkbGeometryType.wkbLinearRing or their multi- versions or 25D or ZM versions, you can access the point coordinates as follows:

int count = geom.GetPointCount();
if (count > 0)
    for (int i = 0; i < count; i++) {
        double[] argout = new double[3];
        geom.GetPoint(i, argout);
        // do your processing here
    }

注釈

The size of the double[] depends on the number of dimensions of the Geometry.

Access a Feature's data fields

Each Feature object can have a number of data fields associated. The schema for the data fields is defined in a FieldDefn object. The fields can be fetched a follows:

Dictionary<string, object> ret = new Dictionary<string, object>();
if (feature != null) {
    int fieldCount = feature.GetFieldCount();
    for (int i = 0; i < fieldCount; i++) {
        FieldDefn fd = feature.GetFieldDefnRef(i);
        string key = fd.GetName();
        object value = null;
        FieldType ft = fd.GetFieldType();
        switch (ft) {
            case FieldType.OFTString:
                value = feature.GetFieldAsString(i);
                break;
            case FieldType.OFTReal:
                value = feature.GetFieldAsDouble(i);
                break;
            case FieldType.OFTInteger:
                value = feature.GetFieldAsInteger(i);
                break;
            // Note this is only a sub-set of the possible field types
        }
        ret.Add(key, value);
    }
}

Access a Geometry's CRS

If there is a CRS (aka SRS) defined for the Geometry it can be retrieved as follows:

SpatialReference crs = geom.GetSpatialReference()

The SpatialReference is the main class for representing the CRS / projection. The CRS can be turned into a WKT string, e.g. for display purposes, as follows:

string wkt;
crs.ExportToWkt(out wkt, null);

注釈

Sometimes the CRS defined on the layer does not cascade down to the Feature - you need to refer bak to the Layer

Reproject a Geometry

If the Geometry has a valid SpatialReference defined, then the Geometry can be transformed to a new CRS using this command:

if (geom.TransformTo(newProjection) != 0)
    throw new NotSupportedException("projection failed");

However, often it is better to explicitly define the CoordinateTransform to be used

SpatialReference from_crs = new SpatialReference(null)
    // note - if you are defining from wkt - replace the null with the wkt
from_crs.SetWellKnownGeogCS("EPSG:4326");

SpatialReference to_crs = new SpatialReference(null);
to_crs.ImportFromEPSG(27700);

CoordinateTransform ct = new CoordinateTransform(from_crs, to_crs, new CoordinateTransformationOptions())
    // You can use the CoordinateTransformationOptions to set the operation or area of interet etc

if (geom.Transform(ct) != 0)
    throw new NotSupportedException("projection failed");