DEM, Visibility, and Intervisibility
Elevation data is available in both raster and vector format
Elevation information can be interpolated to calculate different forms of surfaces:
Steepness of slopes
Azimuth or orientation (aspects)
Visibility and intervisibility
The term digital elevation model (DEM) is a generic term used to describe a continuous digital elevation surface (raster data) – could be referred to as DSM or as DTM
A digital surface model (DSM) includes the elevation of all features in the terrain above sea level such as bare grounds, buildings, trees, powerlines, etc.
A digital terrain model (DTM) contains only elevations of the bare ground above sea level, without any buildings, trees, etc.
A DTM has the following features
A variety of representation forms
In digital form, various forms of representations can be easily produced, such as topographic maps, vertical, and cross sections, and 3-D animation
No accuracy loss of data over time
As time goes by, paper maps may become deformed, but the DTM can keep its precision owing to the use of digital medium
Greater feasibility of automation and real-time processing
In digital form, data integration and updating are more flexibly than in analog form
Easier multi-scale representation
DTM can be arranged in different resolutions, corresponding at different scales
Other alternatives to DTM have been used:
Digital elevation models (DEMs), America
Digital height models (DHMs), Germany
Digital ground models (DGMs), UK
Digital terrain elevation models (DTEMs), USGS
Morphological terrain parameters:
Slope is the first derivative of a surface and has both magnitude and direction (aspect)
Slope is a vector consisting of gradient and aspect
Aspect is defined as the direction of the biggest slope vector on the tangent plane projected onto the horizontal plane
Aspect is the bearing (or azimuth) of the slope direction, whose angles range from 0 to 360
In this context, the term slope is still used to refer to the gradient
Slope and aspect can be based on:
The algorithm used
The local topography
The resolution of the original data
Slope calculation is based on the relationship between horizontal distance and the vertical change in elevation (rise/run (%)) – basic Pythagoras theorem.
Method to calculate slope in raster uses a 3x3 moving window (kernel) to create a trend surface
Kernel passes over every pixel of the grid to calculate the maximum rate of change of value (slope) based on its neighbouring cells
Either use 4 closest cells or all 8 cells
Method to estimate slope compared the 4 adjacent cells to determine the rise values of the central cell (2 axes)
Keep the highest run/rise value
Calculate the slope in percentage -> Rise/Run * 100 (%)
Orientation (aspect) of a pixel is derived from the slope
Aspect identifies the down slope direction of the maximum rate of change in value from each cell to its neighbours
Estimation of the aspect by comparing the 4 rise values (4 axes)
Keep the highest rise/run value
Estimate the direction of the slope (north, east, south, or west)
Surface Analysis in GIS using slope and aspect:
Type of Analysis | Surface tool(s) | Application |
|---|---|---|
Creating Contours | Contour, Contour list, Contour with barriers | Contours can be useful for finding areas of the same value. You could be interested in obtaining elevation values for specific locations and examining the overall gradation of the land |
Terrain Relief and Visualization | Hillshade | Analyze how the landscape is illuminated at various times of the day by lowering and raising the sun angle. Provide an attractive and realistic backdrop |
Volumetric Analysis | Cut Fill | You may be leveling a site for building construction and want to demine the volume of material that needs to be excavated and dumped |
We can produce the hillshape by using the slope and aspect
Shows the intensity of lighting on a surface given a light source at a particular location
It can model which parts of a surface would be shadowed by other parts
Conventions:
Sun azimuth, and direction: 315* (NW)
Sun altitude in degrees: 45*
The viewshed is the portion of the land surface that is visible from one or more viewpoints.
Viewshed analysis defines the regions that are visible from a particular point based on the terrain. A viewshed analysis requires 2 input data sets:
A point layer (viewpoints)
An elevation layer – can be in raster (DEM) or vector (TIN)
Line-of-sight (sight-line) operation: the basis for viewshed analysis
Connects the viewpoint to the target point
Interpolates the elevation of the intermediate cells
Determine if the target is visible or not based on elevations of intermediate cell
Expands this operation to every possible cell to determine which one are visible or invisible
Viewshed applications:
Site selection of facilities (e.g. wireless telephone based station)
Housing and resort area development (e.g. visual intrusion of new facilities)
Resource management (e.g. reducing the visual impact of clear-cuts)
Maximizing of billboards & other views
Reducing detection of troops (military)
Maximizing crowd surveillance (event security )
Advertisement, scenic path
Elevation data is available in both raster and vector format
Elevation information can be interpolated to calculate different forms of surfaces:
Steepness of slopes
Azimuth or orientation (aspects)
Visibility and intervisibility
The term digital elevation model (DEM) is a generic term used to describe a continuous digital elevation surface (raster data) – could be referred to as DSM or as DTM
A digital surface model (DSM) includes the elevation of all features in the terrain above sea level such as bare grounds, buildings, trees, powerlines, etc.
A digital terrain model (DTM) contains only elevations of the bare ground above sea level, without any buildings, trees, etc.
A DTM has the following features
A variety of representation forms
In digital form, various forms of representations can be easily produced, such as topographic maps, vertical, and cross sections, and 3-D animation
No accuracy loss of data over time
As time goes by, paper maps may become deformed, but the DTM can keep its precision owing to the use of digital medium
Greater feasibility of automation and real-time processing
In digital form, data integration and updating are more flexibly than in analog form
Easier multi-scale representation
DTM can be arranged in different resolutions, corresponding at different scales
Other alternatives to DTM have been used:
Digital elevation models (DEMs), America
Digital height models (DHMs), Germany
Digital ground models (DGMs), UK
Digital terrain elevation models (DTEMs), USGS
Morphological terrain parameters:
Slope is the first derivative of a surface and has both magnitude and direction (aspect)
Slope is a vector consisting of gradient and aspect
Aspect is defined as the direction of the biggest slope vector on the tangent plane projected onto the horizontal plane
Aspect is the bearing (or azimuth) of the slope direction, whose angles range from 0 to 360
In this context, the term slope is still used to refer to the gradient
Slope and aspect can be based on:
The algorithm used
The local topography
The resolution of the original data
Slope calculation is based on the relationship between horizontal distance and the vertical change in elevation (rise/run (%)) – basic Pythagoras theorem.
Method to calculate slope in raster uses a 3x3 moving window (kernel) to create a trend surface
Kernel passes over every pixel of the grid to calculate the maximum rate of change of value (slope) based on its neighbouring cells
Either use 4 closest cells or all 8 cells
Method to estimate slope compared the 4 adjacent cells to determine the rise values of the central cell (2 axes)
Keep the highest run/rise value
Calculate the slope in percentage -> Rise/Run * 100 (%)
Orientation (aspect) of a pixel is derived from the slope
Aspect identifies the down slope direction of the maximum rate of change in value from each cell to its neighbours
Estimation of the aspect by comparing the 4 rise values (4 axes)
Keep the highest rise/run value
Estimate the direction of the slope (north, east, south, or west)
Surface Analysis in GIS using slope and aspect:
Type of Analysis | Surface tool(s) | Application |
|---|---|---|
Creating Contours | Contour, Contour list, Contour with barriers | Contours can be useful for finding areas of the same value. You could be interested in obtaining elevation values for specific locations and examining the overall gradation of the land |
Terrain Relief and Visualization | Hillshade | Analyze how the landscape is illuminated at various times of the day by lowering and raising the sun angle. Provide an attractive and realistic backdrop |
Volumetric Analysis | Cut Fill | You may be leveling a site for building construction and want to demine the volume of material that needs to be excavated and dumped |
We can produce the hillshape by using the slope and aspect
Shows the intensity of lighting on a surface given a light source at a particular location
It can model which parts of a surface would be shadowed by other parts
Conventions:
Sun azimuth, and direction: 315* (NW)
Sun altitude in degrees: 45*
The viewshed is the portion of the land surface that is visible from one or more viewpoints.
Viewshed analysis defines the regions that are visible from a particular point based on the terrain. A viewshed analysis requires 2 input data sets:
A point layer (viewpoints)
An elevation layer – can be in raster (DEM) or vector (TIN)
Line-of-sight (sight-line) operation: the basis for viewshed analysis
Connects the viewpoint to the target point
Interpolates the elevation of the intermediate cells
Determine if the target is visible or not based on elevations of intermediate cell
Expands this operation to every possible cell to determine which one are visible or invisible
Viewshed applications:
Site selection of facilities (e.g. wireless telephone based station)
Housing and resort area development (e.g. visual intrusion of new facilities)
Resource management (e.g. reducing the visual impact of clear-cuts)
Maximizing of billboards & other views
Reducing detection of troops (military)
Maximizing crowd surveillance (event security )
Advertisement, scenic path
