In GIS, what usually constitutes a 'data layer'?

A 'data layer' in Geographic Information Systems (GIS) primarily refers to a single dataset that contains geographic information. This dataset could include various types of data, such as vector data (points, lines, polygons) or raster data (grids of pixels). Each layer represents a specific theme or type of information, such as land use, transportation networks, or elevation, which can be analyzed independently or in conjunction with other layers.

This concept is foundational in GIS as it allows users to overlay different types of information to conduct spatial analysis and create more insightful visualizations. For instance, you might have a data layer representing roads that can be visualized alongside another layer showing population density, enabling analyses like assessing accessibility or infrastructure needs.

Other options do not accurately define a 'data layer.' A graphical representation of data relates more to how layers are rendered visually rather than what constitutes the layer itself. The structure for organizing related datasets refers to metadata or database architecture rather than the individual layers of spatial data. Lastly, while visual layers can change the aesthetics of a map, they do not inherently represent the data content that a data layer contains.