public ElevationSurfaceLayer FindElevationSurfaceLayer( 
   string layerURI
)

{
    // retrieve the elevation surface layers in the map 
    var surfaceLayers = map.GetElevationSurfaceLayers();
    // find a specific elevation surface layer by its URI
    var surfaceLayer = surfaceLayers.FirstOrDefault(l => l.Name == "Surface2");
    // or use the FindElevationSurfaceLayer method, passing the layer URI
    surfaceLayer = map.FindElevationSurfaceLayer("layerUri");
}