Lecture 3

Question 1

in situ data

Answer 1

▪ 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

Question 2

currently, the easiest and most common way of assigning spatial coordinates is through a
http

Answer 2

Global Navigation Satellite System

Question 3

Global Navigation Sattelite SySTEMS: 5

Answer 3

1. Global Positioning System (GPS) United States (1978)
2. Indian Regional Navigation Satellite System (NAVIC) India (2013)
3. Global Navigation Satellite System (GLONASS) Russia (1982)
4. Galileo Europe (2011)
5. BeiDou Navigation Satellite System (BDS) China (2000

Question 4

Origins of GPS

Answer 4

▪ 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

Question 5

Selective Availability

Answer 5

▪ 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

Question 6

GPS

Answer 6

▪ 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

Question 7

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

Answer 7

27

20,200

Question 8

How GPS works?

Answer 8

.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

Question 9

Signal Transit Time

Answer 9

trying to figure out when signal was sent and when it was received

Question 10

GPS: On the ground

Answer 10

▪ 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

Question 11

collect the signal from __ satellites, and your position on Earth’s surface can be determined through triangulation

Answer 11

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

Question 12

Differential GPS(DGPS)

Answer 12

▪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

Question 13

Real Time Kinematic sattelite

Answer 13

▪ 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

Question 14

Land Surveying

Answer 14

▪ 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

Question 15

What has the “Total Stations” done for Land Surveying

Answer 15

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”

Question 16

Census

Answer 16

when resources allow, a complete survey of every possible datum can be completed

Question 17

Digitization

Answer 17

▪ 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

Question 18

Heads-up digitizing

Answer 18

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

Question 19

microdensitometers

Answer 19

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)

Question 20

Remote Sensing

Answer 20

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

Question 21

remote sensing systems come in two flavours:

Answer 21

▪ 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

Question 22

Resolution: 4 types

Answer 22

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

2. 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
3. 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
4. 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

Question 23

Panchromatic sensors

Answer 23

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

Question 24

multispectral sensors

Answer 24

▪ 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

Question 25

NDVI (normalized-difference vegetation index) can be used…

Answer 25

to assess the health of plants

Question 26

Hyperspectral sensors

ultraspectral sensors

Answer 26

▪ hyperspectral sensors record energy in 10’s to 100’s of narrow spectral bands, while ultraspectral sensors record energy in 100’s to 1000’s of very narrow spectral bands
▪ very specific bands can be selected to maximize the contrast between the object of interest and its background and improve the probability that the desired information will be extracted from the remotely sensed data

Question 27

from a GIS perspective, _____ and _____ resolution issues may be most important

Answer 27

spatial, temporal

▪ there is typically a trade-off between spatial and temporal resolution, such that High temporal resolution imagery is usually lower spatial resolution and vice versa

• also, high spatial and temporal resolution imagery usually translates into greater volumes of data and files sizes, which can pose problems

Question 28

▪ ______:launched in 1972, is the longest running satellite imagery system

Answer 28

landsat

-type of platform

Question 29

______: features high spatial resolution (1.5 m panchromatic / 6 m multispectral), a pointable sensor, and greater radiometric resolution than Landsat

Answer 29

SPOT
▪ imagery is not free, starting at €0.30 per km2 – a basic SPOT image is 60 x 60 km,
So this ammounts to >1000 euros per image
$364 US per square KM

-type of platform

Question 30

IKONOS & GeoEye

Answer 30

▪ IKONOS & GeoEye-1
▪ IKONOS–>1 m panchromatic and 3.28 m multispectral spatial resolution
▪ GeoEye-1–> 41 cm panchromatic and 1.65 m multispectral spatial resolution

Question 31

______ is the primary source of google earth imagery

Answer 31

GeoEye-1 is the primary source of Google Earth Imagery

Question 32

Hyperspectral platforms: ________

Answer 32

MODIS–>36 bands, 2 day resolution, but low spatial resolution (250 m to 1 km)

Question 33

Platforms: 4 main ones

Answer 33

Landsat

SPOT

IKONOS & GeoEye

MODIS

Question 34

Thermal Imagery

Answer 34

▪ most remote sensing platforms focus on the reflection of short wave radiation, usually in the visible and shortwave infrared bands – thermal infrared is longwave radiation, and the amount of energy release by an object is directly proportional to its temperature
▪ all objects have a typical temperature - and therefore emit thermal radiation – and when that temperature starts to change we get interested

Question 35

LIDAR

Answer 35

▪ light detection and ranging

• laser light (1040-1060 nm for land; 532 nm for water) scans the ground surface and reflects back to the source 200,000 times per second
• the light travels at 2 x 108 m s-1, and when combined with accurate GPS positioning data for the source, can be used to determine a precise distance between the source and the reflector
• this produces a dense collection of elevation data points called masspoints

▪ the laser pulse has a footprint of about 30 cm, and within that footprint all or some of the light can be reflected

• this is important with vegetation cover, and multiple returns will be produced as the light passes through the vegetation reflecting off structures as it passes
• the last return is the ground surface
• each return is distinguishable and contributes to the point cloud
• 4 returns occure, the 4th being the ground
• LiDAR surveys can be conducted at night since this is an active remote sensor

▪ the masspoints can then be interpolated (eg, using inversedistance weighting) to produce a smooth surface

• alternatively, a triangular-irregularnetwork (TIN) may be produced
• when the last return points are used, a digital surface model (DSM) is created which contains the elevation characteristics of all the trees, shrubs, and human-made structures

.DSM’s are useful for evaluating vegetation height and biomass and building height information

▪ masspoints can be further filtered to find the bare-Earth digital terrain model (DTM) to produce an image of ground terrain with all vegetation and buildings removed

Question 36

Radar

Answer 36

▪ radio detection and ranging, like LiDAR, is an active remote sensing system and is not dependent on solar radiation
▪ radar sensors transmit microwave radiation, which can easily passes through clouds, and reflects off the ground surface
▪ like passive systems, radar sensors emit radiation in specific wavelengths or bands
▪ shorter wavelengths are affected more by water, K-band radar is typically used for detecting atmospheric, surface, and soil-water presence

Question 37

▪ radar images are interpreted based on _______

–white objects are strong reflectors while black objects are poor

Answer 37

brightness

-water is good at absorbing microwave radiation, so it looks black