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Virtual Attribute Fields

Support for virtual attribute fields, a mature TatukGIS functionality, was significantly extended in the TatukGIS Developer Kernel version 11.33.0. Unlike typical attribute fields, virtual fields (sometimes called built-in fields) exist only in memory with values that update dynamically. Virtual fields offer a convenient way for getting information about individual shapes in vector layers and are commonly used for labelling, section rendering, and creating SQL queries.

Before focusing on the new functionality, let's first review the list of previously supported virtual fields:

• GIS_UID
  provides the shape's UID
• GIS_SELECTED
  True if the shape is selected
• GIS_HIDDEN
  True if the shape is hidden
• GIS_AREA
  area of a polygon shape expressed in square meters
• GIS_LENGTH
  length of a line shape or the perimeter of a polygon shape expressed in meters
• GIS_COORD_Z
  Z coordinate of the first vertex of a 3D shape
• GIS_COORD_M
  M coordinate of the first vertex of a 3D shape
• GIS_NOW
  current date and time

Some virtual fields are available also in the TatukGIS Editor product, as built-in attribute table columns in its Data panel.

For more information, visit the documentation at:

• Labeling
• SQL Query
• Statistics
• TGIS_LayerVector class

The DK 11.33.0 introduced additional virtual fields relating to object geometry:

• GIS_MIN_X, GIS_MIN_Y, GIS_MIN_Z, GIS_MIN_M, GIS_MAX_X, GIS_MAX_Y, GIS_MAX_Z, GIS_MAX_M
  from coordinates of the vertices defining the shape's extent
• GIS_CENTER_X, GIS_CENTER_Y, GIS_CENTER_Z, GIS_CENTER_M
  coordinates of the shape’s center (center of the extent)
• GIS_CENTROID_X, GIS_CENTROID_Y
  coordinates of the shape’s centroid
• GIS_NUM_POINTS
  number of points (vertices) composing the shape
• GIS_NUM_PARTS
  number of parts composing the shape

The image below is a graphical representation of virtual fields relating to the geometry of a polygon shape.
 

Virtual fields relating to polygon shape geometry.