In Roslyn the land folds down toward the duck pond, and the old stone viaduct carries the road above a valley cut long before anyone thought to pave it. Advanced Landscape Design, Inc. works that grade the way it asks to be worked: beds shaped to the slope, turf mowed and edged along walks that never stay straight for long. A plan comes first here, drawn for the particular yard, and then the ordinary care that keeps the drawing true. In winter the same drives and paths are plowed, shoveled and salted before the walk down to the train. When the forsythia in Gerry Park goes off like a flare, the mowers come back out.