Topic 14: Viewshed and Water analysis Flashcards
Concepts of viewshed analysis
- line of sight operations and viewsheds
- viewsheds are maps that communicate the property of intervisibility: the ability to obtain clear line-of-sight between points
- Viewsheds are the aggregate of sightlines from the viewpoint to ALL locations on the map
Line of sight operations are the basis for viewshed analysis
- sightlines can be plotted on a map and used to generate elevation profiles
Intervisibility vs viewsheds
- intervisibility applies to discrete features and can be performed from one point to any other location
- viewsheds Involves line-of-sight operations and are performed across the DEM itself, identifying the are of visibility from a focal point or points
What affects intervisibility/ viewshed?
- elevation of target: entered directly with coordinate or interpolated from terrain model
- height of observer: typically not useful at ground level
- constraints on view angle: eg., imposed by viewing through a window or telescope
- maximum view distance: limits the radius of possible visibility
- Earth’s curvature: may be necessary to consider for very large viewsheds
Accuracy of viewsheds
- Like other forms of terrain analysis, line-of-sight operations and viewshed analysis are sensitive to DEM resolution/ precision
- low precision data overestimates visibility due to generalizing ground features
Irridance mapping
related to intervisibility where the goal is to compute the amount of solar energy falling on a surface
Applications:
- determine energy inputs for vegetation growth models
- siting solar energy farms
- estimating solar impacts on rates of rock weathering
- determine new site for ski resort that maximizes sunny exposures
- predict behaviour of forest fires through knowledge of moistness/ dryness of litter
What is watershed (hydrological) analysis?
watershed: an upslope area constrained by topographic divides that drain water to a common outlet - often called a drainage basin
watershed analysis: automated terrain analysis that estimates any of the following hydrological features:
- watershed boundaries (basins)
- surface flow direction and accumulation
- location of streams or stream networks
Four major types of hydrological analysis products:
- surface flow direction and sinks
- DEMs and neighbourhood analysis can be used to model flow direction and accumulation (eg., look at the lowest pixel to determine vector going towards that one)
Errors that may be present in flow direction?
- sinks: spurious depressions in the DEM that trap flow
- peaks: spurious high elevation cells that will divert flow
- Need to correct these errors to accurately estimate flow direction and accumulation - Fill operation
Four major types of hydrological analysis products:
- Flow accumulation and stream channels
Flow accumulation is the basis for identifying streams
- each cell in the flow accumulation raster represents the number of cells that contribute to the water flowing through that cell
- can convert to an absolute area by multiplying flow accumulation value by the cell
- can estimate surface flow for an impervious surface by multiplying the area by amount of incoming precipitation
Stream channels are defined as cells exceeding a threshold amount of accumulation (or incoming precip)
- referred to as the channel initiation threshold
Four major types of hydrological analysis products:
- Stream network identification
- A flow accumulation raster is the basis for identifying a stream network
- two types of features must be identified: stream junction points (point of confluence) and stream segments (links)(reaches between junctions)
2 types of stream network identification?
Strahler
- one of the most common ones
- order of streams starting with #1
Shreve
- adding the order of stream flows
Four major types of hydrological analysis products:
- watershed delineation
- watershed delineation is always dependent upon the choice of outlet or pour point along the stream network
- once pour points are designated, a watershed boundary is a pretty straightforward analysis:
trace backwards on your flow direction raster until you reach all the cells that have a flow accumulation =1, code all the cells by a unique value
Limitations to watershed anaylsis
- watershed analysis outputs are often not very accurate when comparing results to ground-based data, so treat it as a model or coarse estimate
- sensitive to errors when DEM is of low resolution or accuracy
