Here's an aerial photo taken on November 7, 1959. It is a view looking from south to north across Simi Valley. The structures at the bottom half of the photos, roughly in the center are the small house(bottom most) and the main house (more center) of the Marr Ranch, which was located north of Alamo and East of Tapo Street. At the top of the photo, slightly to the right of the center was the Rocketdyne test site.
Nearly all traces of the LHOP sets used in the filming of the tv series are gone, destroyed by fire in 2003. The town of Walnut Grove set was dynamited in the series finale in 1983. Still, this hallowed ground where millions got their first inkling about the existence of Pa, Ma, Mary, Laura & Grace, is a sort of touchstone. Seen above is the hill from the opening credits as it appears in a recent photo. Located at Big Sky Ranch near Simi Valley, CA.