Remote Sensing and GIS (Year 2) - Part 1 GIS Flashcards
Direct observation-> E.g. ground survey
What is type of data collection is this ?
Primary
- Digitising data collected by someone else
- E.g. digitising a paper map
- Digitisation of existing data
o Manual digitising
o Digitising tablets
o Photogrammetry
What is type of data collection is this?
Secondary
What type of data is this (discrete or continuous)?
- Data that have distinct spatial extent
- E.g. roads, buildings, fields
Discrete
What type of data is this (discrete or continuous)?
- Data that occur everywhere (data at every point on the map)
- E.g. atmospheric pressure, height, air temperature
Continuous
What are the two models used by computers to represent the world (types of data)?
Raster and Vector
What coordinate system is used by vector data?
Uses Cartesian coordinates (x, y) to represent spatial entities
What are the 5 types of data within the vector model?
- Point is simplest entity -> single (x, y) pair
- Lines -> built by connecting multiple points (each is an x,y point joined together)
- Polygons – closed lines
- Networks – connected lines that don’t form a polygon e.g: Road network or River network
- Surface – mesh of coordinates -> e.g. elevations
What are the attributes of vector data?
- Spatial entities are linked to their attributes
- The attribute table contains this attribute information
- This is what enables us to do analysis
What are the limitations of vector data sets?
- Polygons along shared boundaries (inefficient)
- No information about links in networks
- No information about objects within other objects
What technique can be used to overcome the issues of polygons not being linked in vector data?
Topology which ensure there is connection between entities (topological editing).
How does the raster model display data? (attributes)
- Space (entities, surfaces) represented by tessellations
- array of grid cells (usually square)
- Attributes (i.e. values) assigned to these cells define entities and surfaces
- Each cell has a single value representing the attribute
How do you make the raster display more accurate?
Increase the resolution (decrease the size of each grid cell)
What is the issue with increasing the resolution (raster data)?
Doubling the resolution increases storage requirement by four times -> increasing resolution = more storage required
What is ‘run length’ an example of?
A computer strategy used to decrease the storage requirement for raster data
What techniques are used with raster data when a cell contains more than one attribute? Explain them
- Greatest area allocation (>1 lines/polygons in one cell)
- Centre point allocation (>1 points in one cell)
What is vector data good for? (type of data)
Features that have discrete boundaries (e.g. a building)
What is raster data good for? (type of data)
- Continuous geographic data (e.g. temperature)
- Spatial analysis and modelling
- Data collected on a regular grid (e.g. satellite data)
What are digital terrain models (DTM)?
Height of the land surface in relation to a datum
What are digital terrain models (DTM) also known as?
Sometimes referred to as a digital elevation model (DEM)
What is an elevation model that includes man-made features known as?
(Digital Surface Model)
What are DTM’s based on?
Based on height samples
Why do we simplify the landscape for making DTMs?
We simplify our landscape as we can’t have a value for every point
What does the number of height values in the model depend on?
- The purpose of the model
* The finite data storage available
What are the two ways in which DTM can be collected?
Primary and Secondary
What are the primary data techniques to collect data to form DTMs?
- Ground survey (e.g. GPS, total station) - Time consuming (expensive) + Can be very accurate (a few mm precision) – surveyors use this
- Photogrammetry - Broad set of techniques regarding the taking of photographs -> e.g. taken from different angles and heights + Can have very high spatial/vertical resolution
- Satellite (radar) altimetry - Tends to have low spatial resolution
- SAR (Synthetic Aperture Radars) Interferometry (InSAR) - Can be of very high resolution (mm)
- LiDAR (light detecting and ranging)– use of light beams and measuring the time it takes to return e.g. through the use of a plane
Globally what resolution of DEM is available for public use?
30 metre
At which regions are there higher resolution of DEM available?
- 2 m for the Arctic (high resolution – lots of data)
- UK wide – 5 m
- UK selected areas – LiDAR derived DEM sub meter resolution (e.g. 50 cm for selected parts of England)
How is secondary data used to form DTMs?
Based on other’s data, e.g. digitising contours on maps
What are the issues of using secondary data to form DTMs?
- Time consuming
- Lack of data in relatively flat areas (wide contour spacing)
- Only as good as the map (accuracy/resolution)
How does the raster approach represent surfaces in GIS? (DTM)
Elevation represented by a regular grid of numbers
How does the vector approach represent surfaces in GIS? (DTM)
Elevation represented by irregularly spaced elevation values (spot heights). A Triangulated Irregular Network (TIN) links all of the points to form a surface.
In order to represent surfaces in GIS, for raster data what is required when data has not been collected directly onto a regular grid? (DTM)
Interpolation -> different methods give different results so therefore selection of appropriate method is critical
What are the disadvantages of the raster approach for representing surfaces (elevation) in GIS? (DTM)
- Calculation time for interpolation proportional to the square of the number of data points
- Does not honour original data points
- Representation of discrete data is challenging -> e.g. trying to display a sharp ridge line on a map
What are the advantages of the raster approach for representing surfaces (elevation) in GIS? (DTM)
- Simple to implement and visualise using a computer
* Suitable for continuous data
What are the advantages of the vector approach for representing surfaces (elevation) in GIS? (DTM)
- Suited for inclusion of discontinuities
- Honours data points – no interpolation is required
- Calculation time for triangulation proportional to number of data points
- Structure reflects data density, so time not wasted on areas of low data density
What are the disadvantages of the vector approach for representing surfaces (elevation) in GIS? (DTM)
Time taken to fit surface (for visualisation) to triangulation is high
How do you chose whether raster or vector approach is more appropriate for representing elevation?
Selection must be based on the nature of the data and the purpose of the terrain model
• Features to represent
• Data density
• Purpose of application
What is hypothetical illumination?
Determining illumination values for each cell in a raster. It does this by setting a position for a hypothetical light source and calculating the illumination values of each cell in relation to neighboring cells.
How can DTM be applied? (what context)
Elevation, aspect, and slope influence: • Climate • Vegetation • Hydrology • Geomorphologic processes • Human activity Can be applied in: • Land use planning • Resource management • Site visibility analysis • Hazard prediction
What is visibility analysis and how is it used with DTMs?
Identification of which areas are visible from a particular location
What is slope and aspect analysis and how is it used with DTMs?
Analysis of slope & aspect – Slope = how steep – Aspect = direction of slope – Can be derived from DTM – Different results from different approaches.
How can DTMs be used for hydrological modelling?
• Catchments in which saturated hillslopes are connected to the channel are very ‘flashy’
– Subject to flooding
• DTM can be used to identify hydrologically connected areas
• Input to hydraulic and hydrological models
• Can be used in flood inundation modelling.
What is geodesy?
discipline that deals with the measurement and representation of the earth, including its gravity field, in a three-dimensional time-varying space
Why is geodesy important?
formation of terrestrial coordinate systems
What shape is the earth described as?
oblate ellipsoid -> earth isn’t a perfect ellipsoid
What are the two main types of ellipsoid models in use?
- Some models designed to fit the whole world -> as well as possible
- Some models designed to fit a single region – fits certain areas very well but others poorly
The Airy 1830 ellipsoid coordinate system is an example of an ellipsoid model that has been designed to fit the whole world as best as possible or deigned to fit a single region?
designed to fit a single region -> Britain
The GRS80 coordinate system is an example of an ellipsoid model that has been designed to fit the whole world as best as possible or deigned to fit a single region?
designed to fit the whole world
To define our position on the surface of the ellipsoid we require what?
- Longitude/latitude
2. Rectangular cartesian coordinates
Define geodetic datum
A set of conventions defining how the coordinates relate to the Earth (e.g. Where is the zero meridian located?), together with the mathematical description of the ellipsoid. It specifies the relation between the real world and the conceptual coordinate system
What are the eight parameters that make up a geodetic datum?
– the 3-D location of the origin (three parameters)
– the 3-D orientation of the axes (three parameters)
– the size of the ellipsoid (one parameter)
– the shape of the ellipsoid (one parameter)
What does a geodetic datum require to be made?
An ellipsoid model -> e.g. the World Geodetic System 1984 is based on the GRS80 ellipsoid
To use a datum what do we need?
• To use it we need an infrastructure of reference points: a Terrestrial Reference Frame (TRF)
– aka datum realisation
– We can then find our position relative to these reference points
E.g. OS Trig points
What do geodetic transformations allow us to do?
Tranform between coordinate systems using mathematical function
What is the ellipsoid height?
Distance between the ellipsoid and ground surface (perpendicular to the ellipsoid surface)
We experience uphill and downhill based on what?
The gravity field of the Earth
A ‘level’ surface is perpendicular to what?
The Earth’s local gravity field
What is the geoid?
The shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone, if other influences such as winds and tides were absent. This surface is extended through the continents. The Geoid is that surface with the same ‘height’ throughout the world
The geoid may differ from the best fitting global ellipsoid by up to what?
200m
To compare heights between different places what do we need?
a height datum
What is a height datum?
An arbitrary point at which height is defined as zero. All heights are measured relative to this point. Usually based on a tide gauge (set this 0 point)
What is needed to specify the pattern of differences between the Geoid and ellipsoid?
A Geoid model
What is British Mapping based on?
The OSGB36 and ODN datums and their corresponding TRFs
Why do we project data via map projections?
- Creates 2D map we can visualise -> Cartographers use map projections to represent the curved surface of the Earth on a flat map
- Allows us to do planar maths rather than spherical maths -> greatly eases the analysis of spatial data
What is the impact of map projections?
Some projections corrupt the size of countries or cause distortions
What are the 4 possible spatial properties subject to distortion when creating a flat map?
– Shape
– Area
– Distance
– Direction
What does a conformal map projection preserve?
preserve local shape
What does an equidistant map projection preserve?
preserve true distances from the centre of the projection, or along a special subset of lines
What does an equal area map projection preserve?
preserves area at all scales
What does a true direction map projection preserve?
preserves correct directions (bearings)
What are the classifications of projections?
– Cylindrical
– Conical
– Planar/azimuthal
What are the characteristics of the Planar/azimuthal classification?
- Earth intersects the plane on a small circle
- Good for mapping circular features (often used in the high latitudes) because all points on circle have no scale distortion
- Not suitable for world maps -> generally restricted to North and South pole
What are the characteristics of the Cylindrical classification?
- Geographic grid is projected on to a cylinder
- Cylinder touches the globe along 1 or 2 standard parallels (areas of minimum distortion)
- Can be used for world maps and low latitudes
- Severe distortion at high latitudes
- E.g. Mercator projection (often used on World maps)
What are the characteristics of the Conical classification?
- Earth intersects globe along 1 or 2 parallels in the mid latitudes
- No distortion along circles
- Not suitable for world maps
- Most often used in the northern mid-latitudes
What are the aspects of map projections?
- Normal
- Transverse
- Oblique
What does the aspect of a map projection refer to?
Orientation of projection with the Earth’s axis
What does the perspective of a map projection refer to?
Location of the light source
What are the perspectives of map projections?
- Gnomic
- Stereographic
- Orthographic
What is the gnomic perspective of map projections?
Light at centre
What is the stereographic perspective of map projections?
Light at far side
What is the orthographic perspective of map projections?
Light at infinity
What are pseudocylindrical and pseudoconic map projections?
Map projections that bear less exact resemblance to perspective projections
What is the difference between tangents and secants with reaction to map projections?
A tangent case has little or no distortion whereas a secant case has distortion
Why would use a secant case projection?
If you are interested in displaying certain latitudes more than others
A curved earth (3D) uses which type of coordinate system?
One based on angles (latitude and longitude)
A flat map (2D) uses which type of coordinate system? Why?
Cartesian (x, y) coordinate systems because they are easier to work with
What is the Universal Transverse Mercator coordinate system?
• A single coordinate system for the entire Earth (between 80°S and 84°N)
• Based on (secant) Transverse Mercator projection
– Cylindrical
• Divides Earth into strips 6° of longitude wide
What is the Normal Mercator projection?
- Distortion increases in the y direction
* Fantastic for navigation
What is the Transverse Mercator projection?
Distortion increases in the x direction
Which projection are the British National Grid Coordinates based on?
Based on (secant) Transverse Mercator projection
