Exam 14: Viewshed and Watershed Analysis

Correct Answer:

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Viewshed analysis is a geographical analysis that determines the visibility of a particular area from a specific point or along a line, such as a scenic highway. When running a viewshed analysis along a linear feature, the process involves several steps to determine the areas visible from the feature and the extent of the visibility. Here's how it is typically done:

1. **Data Collection**: The first step is to collect the necessary spatial data. This includes a high-resolution Digital Elevation Model (DEM) of the area, which represents the terrain's surface as a grid of elevation values. Additionally, you need the spatial data for the linear feature (e.g., the scenic highway), usually in the form of a polyline vector layer.

2. **GIS Software**: Use Geographic Information System (GIS) software that supports viewshed analysis, such as ArcGIS, QGIS, or other specialized tools.

3. **Defining Observer Points**: Along the linear feature, define a series of observer points. These points are locations from which the viewshed will be calculated. The spacing of these points can vary depending on the level of detail required and the computational resources available. For a highway, you might place observer points at regular intervals or at locations of particular interest (e.g., viewpoints, rest stops).

4. **Setting Parameters**: For each observer point, set the parameters for the viewshed analysis. This includes the observer's height (which could be the average height of a person or the height of a vehicle), the target height (if looking for visibility of specific objects), and the maximum distance of visibility (which could be limited by natural conditions like fog or by the maximum distance at which details can be discerned).

5. **Running the Analysis**: Perform the viewshed analysis for each observer point. The GIS software will use the DEM to calculate which areas are visible and which are not, taking into account the curvature of the Earth, refraction, and terrain features like hills and valleys.

6. **Combining Results**: Once the viewshed is calculated for each observer point, the results are combined to create a composite viewshed map. This map shows the areas visible from any point along the linear feature. Overlapping viewsheds from adjacent points can be merged to show continuous visibility along the highway.

7. **Analyzing and Refining**: Analyze the composite viewshed map to identify the areas with the best views and any significant blind spots. You may need to refine the analysis by adjusting the parameters or adding more observer points in areas with complex terrain.

8. **Visualization and Interpretation**: Finally, visualize the results in a map or 3D model to interpret and communicate the findings. This can help in decision-making processes, such as planning where to place scenic overlooks, assessing the visual impact of new construction, or enhancing the experience for travelers along the scenic highway.

By following these steps, planners and analysts can effectively run a viewshed analysis along a linear feature like a scenic highway to understand and manage the visual resources of a landscape.