ILWIS objects: Data objects

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Data objects

Data objects; are spatial data in form of vector and raster and tabular data. In new ILWIS, all types of vector objects (point, segment, polygon) are considered “Feature Coverage” and raster objects are referred to as “Raster Coverage”. Raster objects include coverages derived from rasterizing vector maps, acquired from air/space-borne platforms, obtained as results of ILWIS operations. Tabular objects are in form of table consisting of columns and records. Data objects are visualized in a Catalog via icons such as:

Raster Coverage
An example of a raster coverage opened in an ILWIS map panel

A raster coverage consists of pixels (picture elements) of a certain size, e.g. 30m x 30m. Pixels are either codified by IDs, class names, values, or colors depends on the coverage domain. Beside a domain, a raster coverage has to have a georeference. In ILWIS, most spatial operations are performed on raster coverages. Raster coverages are obtained from:

  • rasterizing existing feature coverages
  • creating new raster maps using ILWIS create operation
  • scanning maps or photographs
  • using existing raster coverages obtained by air/space borne sensors
Feature coverage
An example of a feature coverage opened in an ILWIS map panel

A feature coverage in ILWIS refer to maps containing closed areas (polygons), lines (segments) and points. A feature coverage has to have a domain, a coordinate system and a representation. Features in a feature coverage are either codified by IDs, class names or values; depending on the domain of the coverage. A feature coverage can be obtained via digitizing an analog map or vectorizing a raster coverage. Closed areas (polygons) in a feature coverage, represent e.g., uniquely codified areas such as cadastral plots, or mapping units such as land use classes, geological formations or soil types. Lines (segments) in a feature coverage represents e.g., boundaries of polygons, roads, rivers, railways, or isolines, while points represent water wells, meteorological stations, location of field measurements, etc.


A table consists of records and columns. It is called “attribute table” when it is connected to a coverage. The domain of the attribute table has to be the same as the domain of the map to which it relates. An attribute table, contains additional tabular data which linked to mapping units, points, segments, or polygons of the coverage. Such an attribute information can be viewed at mouse clicked pixels when the map is displayed (see Coverage panel). Raster and feature coverages which use a domain type thematic or IDs can have attribute tables while maps with numeric domain cannot have attribute. A table can be edited when is displayed in a table panel.


A table consists of column(s). Each column has a domain. A column with a value domain contains values, a column with a thematic domain contains class names, a column with domain IDs contains IDs, etc. Columns can also have domain “String” which can be used to type for instance descriptions. Columns with a class domains, or user-defined value domains can also have representations. The column representation is used to display the attributes data in a coverage panel as defined by users.

An example of histogram of a raster coverage calculated in ILWIS

A histogram lists frequency information on values, classes or IDs of a coverage in form of a table. In a table panel, histogram of a coverage is presented as a table and as a graph. A histogram is automatically calculated just before a coverage is displayed. A raster histogram lists the number of pixels, the percentages and the areas per value, class or IDs in the raster coverage. If the input raster map uses a value domain, also cumulative number of pixels and cumulative percentages are calculated. Moreover, summary statistics (mean, standard deviation, percentage intervals, etc.) is calculated for each column and presented in the panel. A histogram of a polygon coverage lists number of polygons and the total perimeter and total area of polygons per class, IDs, or value. A histogram of segment coverage lists the number of lines and their total length per class, IDs or value. A histogram of a point coverage lists the number of points per class, IDs or value.