What is an Overlay?
An Overlay is a dynamic map service and not cached or tiled. The map performance is slower with overlays because the data must draw every time the map is panned or zoomed. In the NHD Viewer, all overlays are off by default. However, overlays provide flexibility in that the user can determine which data themes (transportation, hydrography, structures, etc.) they want to view and which feature layers to turned on within a theme. This gives the user control over which layers of information they are viewing.