HEC-RTS performs an "on-the-fly" coordinate system transformation and related software applications. This allows HEC-RTS to display map layers that were created in different coordinate systems in a unified view. For example, you can display your watershed with background maps from Internet sources such as Google® Maps and Open Street Map. This feature works correctly only if you define a geo-referenced coordinate system for the watershed and include a coordinate system definition (*.prj file or equivalent) for each map layer included in the watershed. The positions of elements in the watershed's primary layers (time series icons, stream alignment, and study layers) are stored in the watershed coordinate system. You can set the watershed's coordinate system when creating a new watershed. This superimposes a grid on layer features to establish x- and y-coordinates in the World Coordinate System (WCS) for each point on the layer. The x-coordinate is referred to as "easting" and the y-coordinate is referred to as "northing" following customary use in surveying and mapping. The user can select the extent of this grid, the units of measurement of the grid, and even the location of the origin of the grid. This appendix provides you with information about the various coordinate systems that are supported by HEC-RTS.