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Most GIS software organizes spatial data in a thematic approach that categorizes data in vertical layers. The definition of layers is fully dependent on the organization's requirements. Typical layers used in natural resource management agencies or companies include forest cover, soil classification, elevation, road network (access), ecological areas, hydrology, etc.

Spatial data layers are commonly input one at a time, e.g. forest cover. Accordingly, attribute data is entered one layer at a time. Depending on the attribute data model used by the data storage subsystem data must be organized in a format that will facilitate the manipulation and analysis tasks that will be required. Most often, the spatial and attribute data may be entered at different times and linked together later. However, this is fully dependent on the source of data.

The clear identification of the requirements for any GIS project is necessary before any data input procedures, and/or layer definitions, should occur.

It is mandatory that GIS users fully understand their needs before undertaking a GIS project.

Experience has shown that a less than complete understanding of the needs and processing tasks required for a specific project, greatly increases the time required to complete the project, and ultimately affects the quality and reliability of the derived GIS product(s).