What is a Basemap?
A basemap provides a user with context for a map. You can add information to a basemap by overlaying other information on top of it. Basemaps contain reference information that may provide different geospatial information based on what the cartographer is trying to communicate. Forexample, the NHD basemap has Watershed Boundaries, country and state boundaries,city names, and generalized rivers. Someone interested in viewing hydrography maynot be interested in major highways or National Park boundaries so these layers are excluded from the basemap.
The National Map (TNM) basemaps are tiled or cached services. This means that they
are made up of snapshots of data. These tiles improve map performance because rather
than the viewer having to call data from the database and wait for it to draw, instead,
it can simply display a picture of the data. Users wanting to view more information
may turn specific overlay layers on in the Table of Contents.