Visual Unit Analysis and Scenic Distinction

This article reveals the important role of visible attribute assessment in the landscape planning processes.  The authors' approach describes scenic distinction and the development of supplemental graphic displays.  To read the entire article, visit http://www.fs.fed.us/psw/publications/documents/psw_gtr035/psw_gtr035_03_tetlow.pdf

Authors:

R. J. Tetlow

S. R. Sheppard

The authors begin by stating that “This approach to describing scenic distinction, supplemented by graphic displays, permits comparison of visual attributes for the landscape units in a study area, supporting planning and design” (pg. 117).  They explain that the intent of visual analysis is to ensure that visual qualities are given consideration during the process of environmental design and landscape management.  However, various visual analysis methods have been developed to meet specific needs.  “ the authors discuss how some methods are developed based on issues that relate to the general landscape or landscape in an indirect way (such as visibility assessment from static views), while others modify previous results or research.  The authors argue that more flexible methods are needed to address more comprehensive information and to relate to specific landscapes and their inherent qualities.  They suggest that the visual unit concept used in work by Litton and Shizowa (1971), and Tetlow (1975) would offer a “logical and useful framework for evaluation of the landscape, proposing its division into units which are coherent for planning or analysis purposes” (pg. 117).  This paper aims to adapt this framework for practical and economic use.

The authors describe the visual unit as being “a portion of the landscape enclosed and limited by topography, bounding an observer’s field of view.  That spatial enclosure enables the viewer to accumulate and form a unified impression of his surroundings (Tetlow and Shepard 1976).  They explain that each unit has a “distinct visual character and a degree of unity”, and that its specific scenic distinction is created through the “combination of the landscape elements within and around it” (pg. 118).  The authors discuss how these units are rarely completely enclosed, and that there are ‘portals’ or openings that function as thresholds into the view.  Portals are significant dips in the skyline that provide access into and through the view, and allow the observer to visually orient themselves.  The drawing below illustrates the boundaries and portals associated with a visual unit.

Not all units have an easily apparent boundary; more commonly the boundary follows a complex high ground around the unit’s valley floor and upper slope.  The authors suggest that even though most units are not enclosed, a “false” enclosure or rim can be used to intervene between the valley floor and the unit’s boundary.  They define the rim as the extent of the view from valley floor to the upper slope before it the view becomes invisible or foreshortened.  The illustration below is an example of the rims, vulnerability sectors, and visibility sectors of a view.

In the first part their scenic distinction rating, the authors explain that “mapped geographical arrangement of visual units and their portals indicates the sequence of differing landscapes to be seen along possible travel routes” (pg. 118).  They also describe how scenic elements or individual features that contribute the scenic merit may be identified within or beyond the visual unit.  Next, the authors discuss the key terms associated with visual unit mapping.  Visibility sectors, minor variations in the landscape character or minor changes to the line of site, are used to subdivide the site into more complex visual units that provide specific information.  Vulnerability sectors are the landscape’s potential “to absorb or be visually disturbed by man’s activities (Litton 1974)” (pg. 122).  

Scenic distinction factors “describe the spatial dimensions and visual character of the unit, its water forms, its distinctive features and accents, its linkage with other units, and any degrading contrasts” (pg. 120).  In the last section, the authors describe the actual scenic distinction rating system.  They argue that units with indistinct enclosure and little visual variety receive a typically low distinction; units with defined topographic enclosure, clear orientation, but with few visual elements receive a moderate distinction; units with strong orientation and contrasts in features (water, skyline, reliefs, and vegetation) receive high distinction, and units with the best examples of vivid scenic elements receive a very high distinction.  The following examples depict twenty scenic distinction factors.

This article revealed important qualities and challenges associated with visual unit analysis.  I think that the scenic distinction rating system is very applicable to my thesis because it helps prioritize views from lowest distinction to the very highest.  I feel that the terminology used in this article will also be very helpful in describing scenic elements within a vista and determining the view boundaries.