ILWIS objects

Tables

A table is a data object storing columns. A table usually contains tabular information which refers existing geographic information stored in maps. The table then contains extra information on the mapping units, polygons, segments or points in your maps, for instance cadastral information, soil information, etc. Such a table is called an attribute table. Raster, polygon, segment and point maps with a class or ID domain can have an attribute table. The domain of an attribute table should always be the same as the domain of the map to which the table relates.

Further, tables may store columns that contain the answers of an operation on a map, for instance an output cross table or output tables of a statistical operation. The domain of a table generated by an operation may be created by the operation itself or the table may use domain None. Tables that use domain None cannot be linked to a map.

Tables can be displayed and edited in a table window. A table window also offers you the possibility to display graphs. You can calculate with the columns in a table by typing a Table Calculation formula on the command line of a table window. For more complicated calculations, you can use the Columns menu in the table window.

Contents and structure of a table:

A table consists of records and columns. The smallest unit in a table is called a field.

Record names in a table are defined by the domain of the table.

In a table window which displays a table, record names appear on (gray) record buttons that are arranged along the left side of the table.
Attribute tables use the same class or ID domain as the map to which they refer. When, for instance, you have an attribute table for a soil map (both using class domain Soils), then the classes of that domain appear as units in the map and as records in the attribute table. Or, when you have an attribute table for a map with cadastral plots (both using ID domain City), then the IDs of that domain appear as units in the map and as records in the table.
Attributes on the classes or identifiers in a map are stored in attribute columns which have their own domain (class, ID, or value), see below.
Tables cannot have a value domain because conceptually spoken you cannot have extra information on value maps; extra tabular information on maps that represent interpolated height values, measured rainfall figures, or the calculated total number of people per building block, etc. does not exist.
A table that uses domain None cannot be linked to a map. Records in a table that uses domain None are simply identified by record numbers.

Columns in a table have a name, a description, and each column uses a domain.

In a table which displays a table, column names appear on (gray) column buttons that are arranged along the top of the table, just below the command line and the menu bar.
The domain of each column depends on the kind of data you want to store in that column: select or create for example a value domain if you want store pH values for your soil classes or the String domain if you want to store a text-description for each soil class.
You can add as many columns to a table as you like. Also by performing calculations, you can obtain new columns.
When a column uses a value domain, the column also has a value range and a precision.
Optionally, the domain of a column may have a representation, which is handy in case you wish to display a map that has an attribute table by one of the attributes, i.e. using the Attribute check box in the Display Options dialog box of that map.
If a table has a certain domain that also exists in another table, or if a column of a table has a certain domain that also exists in another table, then you can join columns of the other table into the current table.

For more information on columns, see ILWIS objects : columns.

Names of tables:

In ILWIS 3, object names comply with Windows long file names. Also Universal Naming Convention (UNC) paths are supported. For more information, see How to use long object names.

To create a table:

The easiest way to create an attribute table is to open the Properties dialog box of a map, and then select the Attribute check box in the dialog box. Subsequently, create the table by clicking the little create button that appears next to the Attribute drop-down list box. The Create Table dialog box appears in which you can specify a name and description for the table. The domain of the table is already filled out and is the same domain that is used by the map. For other methods, see How to create a table.

When you create a table with domain None, the table cannot be linked to a map.

To display and edit a table:

The easiest way to open and edit existing tables is to double-click a table in the Catalog; the table is shown in a table window. For other methods of opening tables, see How to display maps and tables.

In a table window, a table can be displayed and edited in the table viewer and in record view: the table viewer gives you an overview of the whole table, and record view displays the contents of a certain record. For more information, see How to edit a table. It is not possible to edit the fields of dependent and read-only columns or the fields of table-owned columns in dependent tables. Table information can also be shown as graphs.

When an attribute table is linked to a map or to its domain, and the map is displayed in a map window, you can also double-click in the map to edit the attributes in the attribute table. For more information on the double-click action, see Map window : advanced functionality.

Operations on tables:

Calculations within tables are usually performed on columns. You can calculate with columns by typing a Table Calculation formula on the command line of a table window. When the definition symbol = is used in a calculation, the output column is dependent. For more complicated column operations such as aggregations, calculating cumulative values, joins, and least square fit calculations, you can use the Columns menu. For more information on columns, see ILWIS objects : columns.

When a table is the result of an operation such as the Cross operation or the statistical operations, the table itself is dependent and the columns created by the operation are table-owned. For more information on dependent objects, see Basic concepts : dependent data objects.

Operations that can be performed on a table as a whole are listed on the Operations, Table Operations menu in the Main window:

The Transpose Table operation can be used to switch all records to columns and all columns to records.
The Table to Point map operation can be used to create a point map from a table. The table should contain at least columns with X- and Y-coordinates for the points.
The Change domain of table operation can be used to create a new table which uses another domain than the current table.

Technical information:

A table consists of an ASCII object definition file (.TBT) and a binary data file (.TB#). The object definition file of a table stores the table name, table description, whether the table is dependent or not, the number of columns in the table, column names, column descriptions, and for dependent columns the column definitions, etc. Further, the object definition file contains references to the table's domain, the table's data file, and the domains that are used by the individual columns. The .TB# file stores the data of the columns.

By viewing the properties of a table, you can see which domain the table is using and whether the table is dependent or not. For dependent tables, you can manage dependencies: break dependencies, make dependent tables up-to-date, delete the dependent table's data file, etc. For more information, see Basic concepts : properties of objects.

Limitations:

In principle, the maximum number of columns in a table is 32000 and the maximum number of records in a table is 2 billion. In practice however, limitations will occur by available memory. Furthermore, ILWIS is not designed to work with very large data sets: to speed up processing you may need to split tables into smaller logical parts.