Module 11 Flashcards
Terrain Analysis
terrain analysis is the process of using topographic data to derive terrain
attributes
is the analysis and interpretation of topographic features through geographic information systems. Such features include slope, aspect, viewshed, elevation, contour lines, flow, upslope flowlines and dowslope flowlines
DEM’s and Terrain Analysis
▪ digital elevation models emerged in
photogrammetry in the 1950s and since then, digital
point elevation data in the form of DEMs have
gradually replaced contours on topographic maps
and have become the main source of topographic
data
▪ supported by DEMs and other forms of digital
topographic data, digital terrain analysis takes the
place of manual terrain analysis – it is now one of the
most powerful tools in GIS for environmental
modeling and landscape visualization
__________ are the most widely used digital terrain model due to their simplicity and ease of computer implementation
digital elevation matrices
TINs
TINs use the vector model to represent terrain by using contiguous, nonoverlapping triangle facets that consist of planes linking the three adjacent elevation points in the network
Breaklines in TIN
▪ breaklines are line features or boundaries of area features with or without height measurements ▪ hard breaklines represent a discontinuity in the slope of the surface, capturing abrupt changes in the surface (eg road cuts, streams) ▪ soft breaklines do not change the local slope of a terrain surface, but mark an important feature of the terrain (eg, coastlines, study area boundaries)
DEM vs TIN
DEM=RASTER
-▪ DEMs can be readily integrated with remotely sensed environmental data – there
are well-established algorithms for digital image processing that can be used to
process DEMs
▪ however, DEMs can have erroneous sinks or depressions generated during their
production as a result of data input errors, insufficient input data, interpolation
effects, or by rounding and averaging errors
TIN=VECTOR
-▪ vector contours are simply digitized contour lines and are only used in GIS for data
capture and visualization, not for analysis
-Triangular irregular networks (TIN) have been used by the GIS community for many years and are a digital means to represent surface morphology. TINs are a form of vector-based digital geographic data and are constructed by triangulating a set of vertices (points). The vertices are connected with a series of edges to form a network of triangles.
DEM sink
▪ a sink is a cell or a set of spatially connected cells in a DEM that are surrounded by higher elevation values -▪ while there are real sinks in nature (sinkholes, quarries), they tend to be rare -▪ sinks are particular problems for hydrologic research, creating false drainage patterns – a DEM that has its sinks removed is known as a hydrologically correct DEM
Removing Sinks in DEMs(2)
1.▪ sinks can be removed or ‘filled’ by raising the values of cells in depressions to the value of the depression’s spill point, the cell with the lowest value surrounding the
sink
▪ a smoothing filter (eg, moving average) is a common method of removing sinks on a large scale, but it may also remove real sinks, as well as raise the average
elevation of the terrain and create artificial flat areas
2.▪ an alternative approach to sink filling is
using the ‘Topo to Raster’ tool in ArcGIS,
which couples the DEM to a watershed
model, showing where the lakes and rivers
are located
▪ since rivers and lakes are natural sinks, the
algorithm knows to keep them, while DEM
sinks found far from the lakes or rivers are
assumed to be error sinks and are filled
MOST DEMs are not hydrologically correct and therefore contain spurious sinks
the ____ instrument onboard the Terra satellite is capable of generating DEM quality elevation data using stereo pairs of imagery
– the product has a horizontal resolution of 30m and a vertical resolution of 15m
ASTER
InSAR
▪ InSAR acquires multiple radar images of the terrain to extract 3-D information, with
accuracies as good as traditional photogrammetric techniques
LiDAR
LiDAR produces masspoints (an x,y location with an elevation) – the space between
points can then be interpolated to produce a continuous elevation surface
Terrain Data Sources(3)
- ASTER
- INSAR
- LIDAR
____ ____ _____(DEMs) provide a basis for extracting terrain-related attributes or features, and can be divided into primary and secondary attributes (2)
Digital Terrain Models
▪ primary attributes: are mainly used to describe the size, shape, and other surface attributes of hillslopes, catchment boundaries, and stream channels – ie, elevation, slope, aspect, profile and plan curvature, flow accumulation, drainage network, flow length, upslope area…
.Secondary attributes:
▪ secondary attributes are used to characterize the spatial variability of specific environmental processes occurring in the landscape
– they tend to be applied to quantify the influence of topography on the spatial distribution and abundance of
water, soil, fauna, and flora
for both DEMs and TINs, _____
is calculated using the normal
vector approach
slope
Aspect
– an important factor that controls the spatial distribution of solar irradiance (thus affecting evapotranspiration), and flora and fauna distribution and abundance
- ▪ like slope, aspect is calculated for each DEM grid cell or every triangle facet in a TIN based on the normal vector
- ▪ aspect is usually described as a compass direction, measured clockwise in degrees from 0 (due north) to 360 (due north), using the following rule
