To start defining existing elevations make sure the Existing tab is selected and then in the Ground panel on the right-hand side click on the + symbol and add a Feature Surface or a Triangle Surface.
A feature surface is used if you want to define elevations using a collection of points, contours and break lines enclosed in outlines. You would typically select this option if working with PDF files or CAD files containing points or lines. If you already have a triangulated surface created in another program select a triangle surface to import the data directly from CAD, LandXML or a Kubla Project file.
