Domain type Value
Value maps and value columns are supposed to contain measured, calculated or interpolated values in contrast to maps and columns that contain for instance class names or identifiers. You should use a value domain for a map when the map contains for example height information such as in a contour map or a Digital Elevation Model. Ranges of values in a value map will be represented by ranges of colors. It is not possible to link attributes to a map with a value domain.
By using a certain value domain for a map or column, you define the kind of values that should be available in the map or column. This comprises the possible minimum and maximum value that you want to be available (the value range) and the number of decimals you want to work with (the precision). Values smaller than the minimum or larger than the maximum are considered undefined.
A number or system value domains are avaible in ILWIS, such as:
It is advised to create create a user-defined value domain when values in a map or column have a specific meaning, are in a specific range and should be represented by specific colors or ranges of colors.
For example, for a height map of a mountainous area, you can create a user-defined value domain Height with values between 3500 and 5000m. You can then create a representation value Height in which you can define the colors for various ranges of values.
As a value domain is linked to a certain representation value or a representation gradual, the value domain you choose thus also defines the default representation (= coloring) in which a map will appear. Maps using the same value domain will by default appear in the same colors.
For more information on representations for a value domain, see ILWIS objects : representations or Representation Value/Gradual editor.
During calculations with MapCalc or TabCalc or when performing operations on value raster maps or columns, you will usually need to specify a domain for the output value map or value column, as well as a value range and precision for the output value map or value column.
ILWIS will come up with defaults for the value range and precision of an output map or column; these defaults are based on actual minimum and maximum values as occuring in the input map(s) and columns(s), or if these are not known, on the value range and precision of the domain(s) of the input maps and columns, and on the default value range and precision of the domain which you selected for the output map.
You can of course adapt the defaults so that the value range and precision for an output map will better fit the expected values in an output map or column or will better suit your needs.
Examples:
See also:
Before creating ILWIS maps and tables