features.md

Feature Data

using ArchGDAL

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

dataset = ArchGDAL.read("data/point.geojson")

Feature Layers

Retrieve a layer from a dataset using ArchGDAL.getlayer

layer = ArchGDAL.getlayer(dataset, 0)

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.layerdefn(layer): the schema of the layer features
• ArchGDAL.nfield(featuredefn): the number of fields
• ArchGDAL.ngeom(featuredefn): the number of geometries
• ArchGDAL.getfielddefn(featuredefn, i): the definition for the i-th field
• ArchGDAL.getgeomdefn(featuredefn, i): the definition for the i-th geometry

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

• ArchGDAL.getname(geomdefn): the name of the geometry ("" in this case)
• ArchGDAL.gettype(geomdefn): the type of the geometry (wkbPoint)

Individual Features

We can examine an individual feature

ArchGDAL.getfeature(layer, 2) do feature
    print(feature)
end

You can programmatically retrieve the information using

• ArchGDAL.nfield(feature): the number of fields (2)
• ArchGDAL.ngeom(feature): the number of geometries (1)
• ArchGDAL.getfield(feature, i): the i-th field (0.0 and "a")
• ArchGDAL.getgeom(feature, i): the i-th geometry (the WKT display POINT)

More information on geometries can be found in Geometric Operations.