CREANDO UNA CAPA VECTORIAL CON CÓDIGO PYTON

En QGIS  abrimos la consola de Pyton y escribimos el siguiente código:

#create memory layer
#type, CRS, fields in a uri
mem_layer = QgsVectorLayer("Polygon?crs=epsg:4326&field=id:integer""&field=area:double&index=yes",
                            "Polygon",
                            "memory")
                           
#add Map Layer to Registry
QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

#Prepare mem_layer for editing
mem_layer.startEditing()

#points to add (first and last point must coincide)
points = [[QgsPoint(-150,61),QgsPoint(-151,61), QgsPoint(-151,62),QgsPoint(-150,61)]]

#Set feature
feature = QgsFeature()

#Set geometry
feature.setGeometry(QgsGeometry.fromPolygon(points))

#Area determination (remember: projection is not in meters)
geom = feature.geometry()

area= geom.area()

#set attributes values
feature.setAttributes([1, area])

mem_layer.addFeature(feature, True)

#stop editing and save changes
mem_layer.commitChanges()

Ejecutamos y creamos la capa con un triangulo.