Feature Data

In this section, we revisit the data/point.geojson dataset.

ArchGDAL.registerdrivers() do
    ArchGDAL.read(filepath) do dataset
        print(dataset)
    end
end

Feature Layers

ArchGDAL.registerdrivers() do
    ArchGDAL.read(filepath) do dataset
        layer = ArchGDAL.getlayer(dataset, 0)
        print(layer)
    end
end

The display provides

• the name of the feature layer (point)

• the geometries in the dataset, and their brief summary.

• the fields in the dataset, and their brief summary.

You can also programmatically retrieve them using

• ArchGDAL.getname(layer): the name of the feature layer

• ArchGDAL.nfeature(layer): the number of features in the layer

• featuredefn = ArchGDAL.getlayerdefn(layer): the schema of the layer features

• ArchGDAL.nfield(featuredefn): the number of fields

• ArchGDAL.ngeomfield(featuredefn): the number of geometries

• fielddefn = ArchGDAL.getfielddefn(featuredefn, i): the definition for field i

• geomfielddefn = ArchGDAL.getgeomfielddefn(featuredefn, i): the definition for geometry i

Field Definitions

Each fielddefn defines an attribute of a feature, and supports the following:

• ArchGDAL.getname(fielddefn): the name of the field (FID or pointname)

• ArchGDAL.gettype(fielddefn): the type of the field (OFTReal or OFTString)

Each geomfielddefn defines an attribute of a geometry, and supports the following:

• ArchGDAL.getgeomname(geomfielddefn): the name of the geometry (empty in this case)

• ArchGDAL.gettype(geomfielddefn): the type of the geometry (wkbPoint)

Individual Features

We can examine an individual feature

ArchGDAL.registerdrivers() do
    ArchGDAL.read(filepath) do dataset
        layer = ArchGDAL.getlayer(dataset, 0)
        ArchGDAL.getfeature(layer, 2) do feature
            print(feature)
        end
    end
end

You can programmatically retrieve the information using

• ArchGDAL.nfield(feature): the number of fields (2)

• ArchGDAL.ngeomfield(feature): the number of geometries (1)

• ArchGDAL.getfield(feature, i): the i-th field (0.0 and "a")

• ArchGDAL.getgeomfield(feature, i): the i-th geometry (the WKT display POINT)

More information on geometries can be found in Geometric Operations.