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Mapping Mount Vernon

By Special Contributor posted 01-16-2015 15:58

  
By Eric Benson

 George Washington's home with a glimpse of the spectacular view across the Potomac River. | Photo: Rob Shenk
George Washington's home with a glimpse of the spectacular view across the Potomac River. | Photo: Rob Shenk
George Washington, speaking about his home at Mount Vernon, once wrote that, "no estate is more pleasantly situated than this." The view that Washington enjoyed is as impressive today, and is still a focal point for the many visitors to Mount Vernon. Located in Virginia, just 13 miles south of Washington, D.C., on the banks of the Potomac River, Mount Vernon is now owned by The Mount Vernon Ladies' Association, the oldest preservation organization in the country. It was founded in 1853 by Ann Pamela Cunningham with the express purpose of holding in trust for the people of the United States the home of George Washington. As part of this mission, Mount Vernon  strives to stay on the forefront of preservation technology. One such technology is the introduction of a permanent Geographic Information System (GIS) program as part of its historic preservation work. This GIS initiative is transforming Mount Vernon's ongoing viewshed protection efforts.

Since George Washington’s time, visitors to Mount Vernon have marveled at the view across the Potomac River. In the 1950s though, the pressures of suburban development threatened to change the bucolic nature of the opposite shore. The association wisely realized that the exceptional view could no longer be taken for granted, and land conservation was folded into its historic preservation initiatives. Over the next several years, Mount Vernon and other partners across the river lobbied for the creation of a National Park for the specific purpose of protecting the view from the home of George Washington. The vision was finally realized with the dedication of Piscataway National Park in 1968.

Unfortunately, the need to protect the land within Mount Vernon's viewshed did not end with the creation of the park. While Piscataway National Park protects much of the most vulnerable land, beyond its boundaries are numerous properties for which an unfortunate combination of building height and tree removal could negatively affect the view. Today's battles do not involve saving large swaths of land, instead they involve analyzing individual properties to discover their contributions or possible disruptions to the view. GIS analysis provides unparalleled guidance for Mount Vernon’s land conservation efforts.

 The ironically named "Preserve at Piscataway" is one of the few existing areas of development obviously visible from Mount Vernon. Built during a time when little viewshed analysis was incorporated into permitting, this subdivision is clearly visible despite being more than six miles away. | Credit: George Washington’s Mount Vernon
 The ironically named "Preserve at Piscataway" is one of the few existing areas of development obviously visible from Mount Vernon. Built during a time when little viewshed analysis was incorporated into permitting, this subdivision is clearly visible despite being more than six miles away. | Credit: George Washington’s Mount Vernon

 

The Mount Vernon viewshed is a rural portion of Prince George's and Charles counties in Maryland. This land, which is heavily forested, also contains a few farm fields and residential subdivisions mixed among older large-lot homes. A low flood plain along the river quickly rises to a plateau edged with deep, wooded ravines. Despite the varied terrain, the elevation only rises 200 feet from the Potomac River to the highest point. The forest canopies commonly reach above 100 feet in elevation, and the trees play just as integral a role in screening buildings from the visitor's eyes as the topography of the land does.



Explore the viewshed area of concern in this interactive map.


Mount Vernon’s GIS analysis seeks to answer two questions. First, if a building were to be constructed at a given height, how much of it would be visible above the existing tree line? Secondly, are there trees to screen the building?

The GIS Solution

An initial analysis was performed by the Prince George’s County Planning Department of the Maryland National Capital Park and Planning Commission (MNCPPC). However, the analysis only covered the viewshed area within Prince George’s County. Mount Vernon worked with the Chesapeake Conservancy's Conservation Innovation group to develop and refine the analysis tools to cover the entire viewshed within both counties. Using the viewshed tools included in ArcGIS Desktop by Esri, a custom set of tools was developed to perform analysis based upon the elevation of the ground and the elevation of the tree canopy as derived from aerial LiDAR data.

This Digital Terrain Model derived from LiDAR shows the elevation of the ground as well as structure and trees. Lower elevations are darker while higher elevations are lighter. | Credit: George Washington’s Mount Vernon

This Digital Terrain Model derived from LiDAR shows the elevation of the ground as well as structures and trees. Lower elevations are darker while higher elevations are lighter. | Credit: George Washington’s Mount Vernon
 This image shows the same geographical area as the previous image, but the vegetation has been isolated and overlaid on a map to help illustrate the tree cover. The darker the green, the taller the trees. | Credit: George Washington’s Mount Vernon
This image shows the same geographical area as the previous image, but the vegetation has been isolated and overlaid on a map to help illustrate the tree cover. The darker the green, the taller the trees. | Credit: George Washington’s Mount Vernon

 

Using these data, one can select any building footprint, enter in the height of proposed development, and return two different results. The first shows if the proposed development will be visible and if so, how much of it will extend above the tree line. The second result shows all the trees that intersect the viewer’s line of sight to the top of the structure. This analysis can be done quickly and repeatedly with ArcGIS Desktop by Esri or within an interactive browser-based webmap. This webmap was designed to make the analysis simple and available to anyone. With Esri's new ArcGIS Pro software, the results can be modeled in 3D, providing a powerful visualization tool.

Explore for yourself the viewshed analysis tool in this interactive map. Simply enter a building height and draw a shape within the black outline. The tool will automatically return the results. The larger your shape, the longer the analysis will take.

An illustration of a 3D visualization of a subdivision and the surrounding tree cover in ArcGIS Pro. | Credit: George Washington’s Mount Vernon  
An illustration of a 3D visualization of a subdivision and the surrounding tree cover in ArcGIS Pro. | Credit: George Washington’s Mount Vernon

 

 This screenshot of the webmap tool shows where on a specific parcel an 84-foot structure would be visible. The color gradient indicated how much of the structure would be visible. The large pink area indicates trees that provide screening. | Credit: George Washington’s Mount Vernon
This screenshot of the webmap tool shows where on a specific parcel an 84-foot structure would be visible. The color gradient indicated how much of the structure would be visible. The large pink area indicates trees that provide screening. | Credit: George Washington’s Mount Vernon

 

This level of analysis allows the association to be proactive rather than reactive to development threats. Properties can be fully investigated for their potential impact to the view before a development project is proposed. The analysis also allows for the proper oversight without creating unnecessary restrictions. By providing these tools directly to the county planners, the viewshed analysis can be incorporated directly into the planning and permitting process, ensuring that protections are implemented at the start of planning, saving developers and the county time and money.

Real World Result

Last year, a 64-acre parcel that had been targeted for conservation came on the real estate market. Situated on a slope leading up to an existing subdivision, this parcel held the only trees screening this development. Should these trees be removed, the existing subdivision and any new construction would become immediately visible. Mount Vernon purchased the property and in conjunction with the Maryland Environmental Trust and the Conservancy for Charles County, and placed an easement on it restricting the removal of trees and allowing for additional tree plantings by Mount Vernon in the future. The property is now back on the market with these restrictions in place, illustrating the valuable role of GIS analysis in helping Mount Vernon protect George Washington’s view.

 This 3D visualization shows the trees that shield an existing subdivision from view. Because the subdivision is sited on the top of a hill, only a small group of trees on the slope below prevent it from being seen. | Credit: George Washington’s Mount Vernon
This 3D visualization shows the trees that shield an existing subdivision from view. Because the subdivision is sited on the top of a hill, only a small group of trees on the slope below prevent it from being seen. | Credit: George Washington’s Mount Vernon

 

 The property that Mount Vernon purchased and placed an easement on in order to protect trees shielding a subdivision from view. | Credit: George Washington’s Mount Vernon
The property that Mount Vernon purchased and placed an easement on in order to protect trees shielding a subdivision from view. | Credit: George Washington’s Mount Vernon

 

Eric Benson manages the Geographic Information System (GIS) program at Mount Vernon. He oversees conservation efforts to protect land visible from the mansion, known as the viewshed. He has earned a Master’s in GIS from The University of Maryland College Park and has a background in surveying, cartography, information technology, and local government.



#PreservationTools #HistoricSites #mapping

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