Layers such as digitized contours, water bodies, and railroads from USGS topographic maps are categorized as:

The categorization of layers such as digitized contours, water bodies, and railroads from USGS topographic maps as Digital Line Graphs (DLGs) is rooted in the nature of how these features are represented. Digital Line Graphs are a specific type of vector data that encode geographic information using points, lines, and polygons, which makes them particularly suited for representing discrete features on the Earth's surface.

Digitized contours, for instance, represent elevation changes and are depicted as lines connecting points of equal elevation. Water bodies and railroads, similarly, are represented as line features that can be easily analyzed and manipulated within a GIS environment. Because DLGs are derived from the original topographic maps and provide geographic information in a digital format, they are invaluable for cartographic and analytical purposes.

This classification underscores a key aspect of GIS handling of geographic information. While raster layers represent continuous data (such as satellite imagery), and vector data is a broader term encompassing both DLGs and other vector formats, the specific term "Digital Line Graphs" refers to that structured representation of map elements traditionally found in vector format from topographical datasets.