Lecture 3 Flashcards
in situ data
▪ in situ data are collected on the ground by a human or by special data collection instruments
▪ the are often called contact measurements, since the data collector is acquire data from the environment it is directly in contact with
currently, the easiest and most common way of assigning spatial coordinates is through a
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Global Navigation Satellite System
Global Navigation Sattelite SySTEMS: 5
- Global Positioning System (GPS) United States (1978)
- Indian Regional Navigation Satellite System (NAVIC) India (2013)
- Global Navigation Satellite System (GLONASS) Russia (1982)
- Galileo Europe (2011)
- BeiDou Navigation Satellite System (BDS) China (2000
Origins of GPS
▪ the theory of a GPS begins with testing for Einstein’s theory of general relativity
- specifically to test how speed affects time – put some atomic clocks in space-highly precise
▪ when the Soviets launched Sputnik in 1957, Americans realized they could precisely track it through the distortion of its signal as it orbited Earth
- this was quickly picked up by the US military, and by 1973 the Defence Navigation Satellite System was in the works
Selective Availability
▪ initially, the US used ‘selective availability’ – there were 2 satellite signals transmitted, 1 high quality for military use and 1 low quality for civilian use
▪ Clinton orders selective availability to be abandoned by 2000, and all GPS users get access to the high quality service
▪ instantly, the errors in GPS data are reduced from >100 meters to <20 m
GPS
▪ the GPS consists of a network of Earth orbiting satellites that constantly transmit information toward Earth’s surface; a ground-based receiver accumulates this information to pinpoint its exact location
▪ the constellation consists of 27 satellites orbiting in 6 planes, each 60° apart, at an altitude of 20,200 km above Earth’s surface – each satellite orbits Earth twice each day
▪ from the surface, you are typically in sight of 9 satellites, provided you have a clear view of the sky
-the more satellites you can see, the more accurate your positional data
The GPS constellation consists of __ satellites orbiting in 6 planes, each 60° apart, at an altitude of ______ km above Earth’s surface – each satellite orbits Earth twice each day
27
20,200
How GPS works?
.on board each satellite is a very precise atomic clock
▪ the satellite continually transmits the precise time of the message, parameters to calculate the location of the satellite, and the general health of the system
▪ in space, the civilian and military signals are the same, but a 2nd frequency picked up by military receivers result in extra error correction, resulting in better accuracy for military use
Signal Transit Time
trying to figure out when signal was sent and when it was received
GPS: On the ground
▪ on the ground, the gps recieever intercepts the signal sent by each satellite
▪ by determining the transit time – the time the signal takes to get from the satellite to the receiver – your distance from the satellite can be determined
- collect the signal from 3 satellites, and your position on Earth’s surface can be determined through triangulation
▪ unfortunately, your receiver does not have an atomic clock on board, so errors in the transit time can be big
- a transit time error of 0.0000001 seconds is about 300 metres of positioning error
adding a 4th satellite signal allows for the transit time error to be calculated and factored into the measurements –more satellites improve the error reduction
collect the signal from __ satellites, and your position on Earth’s surface can be determined through triangulation
3
adding a 4th satellite signal allows for the transit time error to be calculated and factored into the measurements –more satellites improve the error reduction
Differential GPS(DGPS)
▪In Differential GPS(Dgps) two receivers are used
- the reference receiver is located over a point with a known location, perhaps a horizontal control monument, while the other is a roaming receiver
- the reference will assess the signal it receives and correct for any errors against its known position– these corrections are then transmitted to the roaming receiver to substantially improve the accuracy
Real Time Kinematic sattelite
▪ real time kinematic satellite navigation uses the GPS, but instead of relying on the transit time, it uses the phase information of the satellite signal carrier wave ▪ in normal GPS, the satellite signal can be distorted as it passes through the atmosphere, but the wave form of the signal is not distorted ▪ by ignoring the transit time information but examining the wave form, an RTK receiver can achieve accuracy close to 1cm
-better error connection
Land Surveying
▪ the basic technique of land survey is triangulation, a system of angle measurements that connect points of known position to unknown points
- used a theodilite
- these techniques are time consuming and often very challenging
- Land surveying has evolved too include GPS equipment
What has the “Total Stations” done for Land Surveying
Measures horizontal and vertical angles like a theodolite(slide 18)
- lazar goes from one point to another to determine
- mm precision like triangulation
the total station has largely replaced all previous land surveying equipment into a single device
.”Smart stations” have added GPS into the “Total Station”
Census
when resources allow, a complete survey of every possible datum can be completed
Digitization
▪ digital data is relatively new but analog versions of data and maps have been produced over centuries to millennia
▪ the process of converting an analog source to digital is digitization, and many methods are available depending on the format of the analog product
▪ one of the most direct methods is transcription, where each piece of data is manually entered into a digital database
Heads-up digitizing
is a common tool including in many GIS
-▪ typically begins with scanning a map to create a digital image, which is then added to the GIS
▪ from here, the user can use points, lines, and polygons to identify selected features and they are added as features directly to the GIS
microdensitometers
are very similar to scanners but are capable of achieving very high resolution imagery (up to about 4000 dpi)
▪ colours on the map are transformed into a numerical matrix, commonly in 8-bit format (256 colours)
Remote Sensing
is the measurement or acquisition of information of some property of an object or phenomenon by a recording device that is not in physical or intimate contact with the object or phenomenon under study
▪ usually refers to some sort of orbital sensing platform, but also includes sub orbital or surface platforms
remote sensing systems come in two flavours:
▪ Passive systems simply receive energy reflected of surfaces and translate that incoming energy to an image
-panchromatic, multispectral, hyperspectral systems
▪ Active systems produce their own energy, send it down to Earth’s surface, and then capture the reflected energy and translate that to an image
- radar, LiDAR, sonar
Resolution: 4 types
1.Spectral Resolution
▪ remote sensing relies on the establishment of a deterministic relationship between the amount of energy reflected or emitted in a specific wavelength “band” and the chemical, biological, and physical characteristics of the phenomenon under investigation
▪ spectral resolution is the number and size of the wavelength bands to which the sensor is sensitive
- Spatial Resolution
▪ a measure of the smallest angular or linear separation between two objects that can be resolved by the remote sensing system
▪ for photography, this is defined by the size of the silver halide crystals on the film
▪in digital systems it is defined by the pixel dimensions - Temporal resolution
▪ a remotely sensed image is a snapshot in time, and the data collected only refers to that time of acquisition
▪ in order to identify changes in surface processes or features, subsequent images must be taken
▪ temporal resolution is how often the sensing platform captures data from the same location on Earth’s surface - Radiometric resolution
▪ the sensitivity of a remote sensing detector to difference in signal strength as it records the radiant flux reflected, emitted, or back-scattered from the terrain
▪ typically has improved through time with technology and platform generation
-8 Bit better then 4 Bit
Panchromatic sensors
panchromatic sensors detect radiation in a wide band covering a range of colours, effectively detecting total reflected radiation
– this is normally recorded as a brightness image with dark areas representing low reflection and light areas representing high reflection
multispectral sensors
▪ multispectral sensors record energy in multiple distinct wavelength bands
▪ specific wavelength bands can be selected based on their specific relationship with the reflector, eg, live plants effectively absorb in the PAR band (400 – 700 nm) but reflect in the NIR range (700 – 900 nm
