GIS week 8 Flashcards
(27 cards)
What is terrain analysis, and why is it important?
Terrain analysis studies the Earth’s surface shape, its influence on environmental processes, and how to represent terrain using contours, spot heights, DEMs (raster), and TINs (vector).
Complexity: oblate spheroid < geoid < land surface (terrain).
Using topographic data to derive terrain features.
the morphology of terrain surface.
the influence of terrain on environmental processes.
What are the main digital terrain data sources in GIS?
Ground surveys.
Photogrammetry.
Radar altimetry.
Interferometric synthetic aperture radar (InSAR).
Airborne laser scanning (LiDAR).
Most of these are in the form of, or can be readily transformed to DEM.
What is a Triangulated Irregular Network (TIN), and what are its benefits?
A TIN models surfaces using contiguous, non-overlapping triangles linking 3 adjacent elevation points, offering flexibility, efficiency in terms of data storage, and varying detail across scales.
What is the difference between hard and soft breaklines in TINs?
Hard breakline =a discontinuity in slope of a surface.
Soft breakline = does not change local slope of terrain surface (captures position)
What are primary terrain attributes in GIS?
Elevation, slope, aspect, plan/profile curvature are primary terrain attributes
Flow direction, flow accumulation, flow length, flow order, drainage network and catchment area are hydrological characteristics of a terrain surface.
In GIS, they are mainly derived from DEM or TIN and represented in raster layers
What are secondary terrain attributes, and what do they quantify?
Secondary attributes are computed from two or more primary attributes.
Quantify the role played by topography in redistributing water in the landscape and in modifying the amount of solar radiation received at the surface
These attributes affect soil characteristics, distribution and abundance of soil water, susceptibility of landscapes to erosion by water, and the distribution and abundance of flora and fauna
What does slope measure in terrain analysis, and how is it expressed?
The steepness of the surface; calculated as the maximum rate of elevation change between a cell and its neighbours, in degrees or percent.
What is shown on a slope map derived from a DEM?
A visualisation of slope steepness across a landscape, highlighting steep and flat areas for analysis.
What does an aspect map show, and how is it measured?
The compass direction a slope faces, measured in degrees from North (0°), important for understanding sunlight exposure and hydrology.
What is a terrain profile in GIS, and what does it illustrate?
A vertical cross-section of terrain along a defined line, showing elevation changes, slope gradients, and landform shapes.
What is surface curvature, and what are its two main types?
Curvature quantifies the amount by which a surface deviates from flatness:
* Profile curvature (along slope) is the rate of change of slope in the direction of the maximum slope.
* Plan curvature (perpendicular to slope) is the curvature perpendicular to the direction of the maximum slope. (convergence/divergence)
What does a profile curvature map illustrate in terrain analysis?
Areas where flow accelerates/decelerates (profile) and where water converges/diverges (planform), helping assess erosion risk and flow patterns.
Slope- influences the overall rate of movement downslope
Aspect- determines the direction of flow
Profile curvature- affects the flow acceleration and erosion/deposition rate
Plan curvature- affects the convergence and divergence of flow
What is watershed analysis, and what questions can it answer?
Models the movement of water over the land surface.
Track pollution sources to find potentially affected streams e.g. agrochemicals, outlet pipes from factories.
Estimate stream flow for ungauged areas.
Predict water quality.
Analyse historic and future climate predictions.
Understand sedimentation processes.
Characterise landforms
What is a hydrologically correct DEM, and why is hydrological conditioning important?
A DEM without artificial sinks (errors); conditioning fixes issues like pits from bridges or narrow channels to accurately model water flow.
Checking the DEM for sinks (hydrological conditioning).
Areas which are pits, surrounded by higher elevation values.
Whilst these can occur naturally, often they result from problems with the input DEM.
Bridges, quarries sinkholes or misrepresentation of narrow sided channels can all result in sinks.
What are common causes of sinks in a DEM, and how are they addressed?
Sinks occur from errors in input data (e.g., misrepresented bridges, quarries); they’re corrected by hydrological conditioning to ensure realistic flow paths.
What is a flow direction raster in GIS hydrology?
A raster showing the direction water flows from each cell, based on the steepest downslope neighbour.
What does a flow accumulation map show in hydrological analysis?
It quantifies the number of cells flowing into each location; high values indicate streams, low values indicate ridgelines.
How is a drainage network extracted from a DEM?
By applying a threshold (e.g., 1000 or 7000 cells) to the flow accumulation raster, defining where streams and rivers likely form.
What is a catchment boundary, and how is it delineated in GIS?
The outer limit of an area contributing water to a drainage point, identified using flow direction and accumulation data.
What is line-of-sight analysis in terrain studies?
It evaluates visibility between two points, considering terrain obstructions, useful for infrastructure planning and viewshed analysis.
What does a typical line-of-sight analysis output show?
Areas visible and not visible from a defined observer point, helping assess visual coverage over terrain.
What is a viewshed analysis in GIS?
A study of visible areas from a location, considering terrain elevation; useful in siting, landscape planning, and communication infrastructure.
What is solar radiation mapping, and what are the key types of solar radiation considered?
Estimate spatial and temporal distribution of solar radiation on the Earth’s surface.
Solar radiation is radiant energy emitted by the Sun:
Direct radiation propagates unimpeded in a direct line from the Sun to the receiving surface.
Diffuse radiation is scattered by clouds and dust.
Reflected radiation is the direct and diffuse radiation reflected by nearby terrain towards the location of interest.
Total or global solar radiation is the sum of the direct, diffuse and reflected radiation.
What is a hemispherical viewshed in solar radiation mapping?
A model showing sky visibility angles for each location, combined with sun/skymaps to calculate solar energy at different times.
Upward–looking hemispherical viewshed model (DTM).
Maximum angle of sky obstruction in all directions.
Used in combination with sunmaps and skymaps.
Calculate solar radiation for each location.
