Preserving Viewsheds Efficiently Through GIS Mapping

Usually, when those of us in the land preservation community think of preserving land, we think of placing that particular piece of land under a conservation easement (or perhaps preserving it in some other way). But what if the property is so unique that placing even nearby properties under a conservation easement becomes useful or even necessary?

Such is the case for Mount Vernon, George Washington’s country estate in Fairfax County, Virginia. One of Mount Vernon’s most memorable features is that the property overlooks the Potomac River, providing visitors with a view across the river into Maryland for several miles. This breathtaking view has astounded visitors for centuries, but the quality of the viewshed became threatened in the mid-20th century as suburban development around Washington, D.C. began encroaching on the area.

In a recent blog post bearing the disarmingly simple title of “Mapping Mount Vernon,” Eric Benson, the GIS Manager at Mount Vernon, explains how Geographic Information System (GIS) mapping is now being used to protect the stunning view from Mount Vernon. Benson begins by going over the history of efforts to preserve Mount Vernon’s viewshed. These earlier efforts had already resulted in the creation of Piscataway Park directly across the Potomac River. Piscataway Park, which is part of the National Park System, was created in the 1960s and early 1970s and consists of nearly 5,000 acres of land. For many scenic properties, protecting that much of the viewshed is only a dream, and it probably seemed at one time that Mount Vernon’s viewshed had been adequately protected.

Nevertheless, even though the creation of Piscataway Park protected the majority of Mount Vernon’s viewshed, the particular topography of the area means that even distant development can mar the view. For example, Benson shows how a subdivision named The Preserve at Piscataway, which is located over six miles away near Accokeek, Maryland, can be clearly seen from Mount Vernon. The following map that I created to illustrate the issue shows that this is because most of those six miles—closer to 6.2 miles, actually—are over water, specifically the Potomac River and Piscataway Creek:

Notice, in particular, how the line of sight across the water skirts right past those nearly 5,000 acres of land preserved in Piscataway Park.

While this sort of issue might have been more difficult to predict in the past, the advent of modern GIS technology allows for much more sophisticated analysis of viewscapes so that such issues can be preemptively spotted and proactively remedied. For example, Benson explores how maintaining sufficient tree cover in small, specific places can effectively shield much larger subdivisions from view. Thus, Benson’s team is able to take steps to preserve these comparatively small, but key, pieces of land without unnecessarily expending funds to preserve land that will have no meaningful effect on the viewshed. This is a powerful approach. Instead of either blindly attempting to preserve everything or simply capitulating to the encroaching development, this more targeted approach makes an efficient use of resources by allowing Mount Vernon’s preservation goals to be met while also allowing reasonable development to take place.

Benson explains in his blog post in much greater detail how his team has been using GIS analysis to locate potential threats to Mount Vernon’s viewshed and, once located, to determine how best to protect the viewshed from those threats. (If you missed the link to Benson’s post earlier, you can read the post by clicking here.) I encourage you to read his blog post for yourself; it comes highly recommended.