- The relation between a data object and service objects, or between service objects
among themselves, is specified by the properties of the object.
- The properties are different for vector maps, raster maps or tables. The properties
depend largely on the type of domain which is used.
- The link between output data objects, obtained from other input data objects
through an ILWIS operation is defined by dependency links.
- Maps, tables and columns are originally when they are digitized, imported or created
by typing. These objects are called source objects
- Output maps, tables and columns which are created through a calculation or an
operation are called dependent objects.
- The concept of dependency has a number of important advantages:
Easy update: when you have new information and you edited input maps, tables
or columns accordingly, the dependent output maps, tables or columns can be recalculated as the system can execute their definition again. There is thus no
need to repeat operations and choose input and specify output objects again: you
can simply press the Make Up-to-Date button in the Properties sheet of the
dependent output maps, tables or columns.
Easy change of calculation formulas: MapCalc and TabCalc formulas are stored
as the definition of dependent raster maps and columns and can be modified in
the Properties sheet of such a map or column. This enables you to quickly
change calculation formulas and see results displayed in the same raster map or
column. The need to create new maps and columns for similar calculations is
thus reduced.
Minimum use of disk space: when you perform a calculation or another operation
and the outcome is a dependent map or table, in principle only the object
definition file of the dependent map (.MPR, .MPA, .MPS, .MPP) or table
(.TBT) is stored on disk. Object definition files are small ASCII files and they
store amongst others the expression by which the output object is created and
contain a reference to the output data file(s). You can directly continue to perform
calculations and/or operations on the dependent objects that do not have
calculated data files yet.
When an object is Dependent (D), Calculated (C) and Up-to-Date (U) it means that
none of the source objects have been updated since the dependent object was generated.
- When an object is Dependent (D), Calculated (C) and Not Up-to-Date (N) it means
that (one of) the source objects has been updated since the dependent object was
generated. You can make the dependent object up-to-date again by using the Make
Up-to-Date button in the object’s Properties sheet.
- When an object is dependent (D), Not Calculated (N) and Not Up-to-Date (N) it
means that only the object definition file of the dependent object is stored on disk
and that the actual data file is deleted. You can recalculate the dependent object by
using the Calculate button in the object’s Properties sheet.
- It is also possible to convert an object from being a dependent object to a source
object. You can break a dependency link by using the Break Dependency Link
button in the object’s Properties sheet.