Viewshed Analysis

Viewshed Analysis

• A viewshed is the portion of land visible from one or more viewpoints
• Viewshed analysis requires a point layer of viewpoints and a DEM or TIN
• The line-of-sight operation connects a viewpoint and a target
• If the line of sight is blocked, the target is invisible
• Viewshed analysis expands line-of-sight to every possible cell or TIN facet
• Raster-based viewshed analysis follows steps to determine visibility from a viewpoint to each cell
• TIN-based viewshed analysis has rules to define a triangle as visible or invisible
• A cumulative viewshed map can use counting or boolean operations to show visibility from multiple viewpoints
• Accuracy depends on surface data accuracy, data model, and visibility rules
• Visibility can vary within a cell, so its classification can depend on the cell's target location

Parameters of Viewshed Analysis

• Viewpoint location affects the viewshed
• Scenarios include fixed or selected viewpoints
• Tools like contouring and hill shading help select viewpoints at high elevations
• Elevation of the viewpoint is increased by observer’s height or the height of a structure
• Viewing azimuth sets horizontal angle limits
• Viewing radius sets the search distance for visible areas

Applications of Viewshed Analysis

• Viewshed analysis is used for site selection for facilities like forest lookout stations and cell towers
• It can also be applied when planning housing and resort area developments
• It is closely related to landscape analysis of visual quality and visual impact
• It can be integrated with least-cost path calculations to provide scenic paths
• Finally, viewshed analysis can be a tool for preparing 3-D visualization

Watershed Analysis

• A watershed is the area that drains surface water to a common outlet
• The Watershed Boundary Dataset (WBD) is used to delineate a specific class watershed
• Watershed analysis uses DEMs and raster operations to delineate watersheds and derive topographic features
• Automated watershed delineation uses a series of steps, starting with a filled DEM

Filled DEM

• A filled DEM is free of depressions or sinks and represents an area of internal drainage
• Depressions must be removed from an elevation raster
• A common method involves increasing the cell value to the lowest overflow point

Flow Direction

• A flow direction raster indicates the direction water flows out of each cell in a filled DEM
• D8 method assigns flow direction to the surrounding cell with the steepest distance-weighted gradient
• The D8 method doesn't allow flow to be distributed to multiple cells
• D8 method produces good results in zones of convergent flows in well defined valleys
• D∞ method partitions flow from a cell into two adjacent cells

Flow Accumulation

• A flow accumulation raster tabulates, for each cell, the number of cells that will flow there
• This tabulation is based on the flow direction raster
• Cells with high accumulation values are stream channels; those with zero are ridge lines
• An accumulation value multiplied by cell size equals the drainage area

Stream Network

• A stream network is derived from a threshold accumulation value
• A higher threshold yields a less dense stream network
• The threshold represents the amount of discharge needed to maintain a channel head
• The resulting stream network should match a network obtained from traditional methods

Stream Links

• After deriving a stream network, each section of the stream raster line is assigned a unique value and flow direction
• A stream link raster resembles a topology-based stream layer
• Intersections/junctions are like nodes, and stream sections between junctions are like arcs/reaches

Areawide Watersheds

• The final step is to delineate a watershed for each stream section
• This operation uses the flow direction raster and the stream link raster as inputs
• A denser stream network will result in more, but smaller, watersheds

Point-Based Watersheds

• Instead of each stream section, some projects require delineating watersheds based on points of interest
• These points of interest are called pour points or outlets
• Delineation of watersheds based on a pour points follows the same procedure as areawide, but substitutes a point raster for the stream link raster
• The point raster, representing a pour point, must be located over a cell within the stream link

Factors Influencing Watershed Analysis

• DEMs play a crucial role in watershed analysis
• DEMs can vary in both resolution and quality
• Stream networks differ when derived from DEM using the different resolutions
• A higher-resolution DEM tends to generate a smaller watershed area
• An algorithm used to derive flow directions is also important
• D8 is primarily used in GIS packages because it is simple and efficient
• However, it does not do well on highly variable flood plains and wetlands

Applications of Watershed Analysis

• An important application of watershed analysis is the area of watershed management
• The Clean Water Act is aimed at restoring and protecting waters
• It also involves a multiagency to create the Watershed Boundary Dataset (WBD)
• This involves providing necessary for data for hydrologic modeling