SURVEYING METHODS Flashcards
- EDM - GNSS - Remote sensing - GIS
What are the benefits of EDM
- easy
- quick
- accurate
- economy
- reliable
- independent of terrain conditions
What changes has EDM brought forward?
- Traversing on a grandiose scale
- Inclusion of more distances into triangulation - scale error control
- setting out and photogrammetric control
offshore position fixing
deformation monitoring to sub-millimeter accuracies
What are the methods of EDM measurement?
- Pulse method
- Phase difference method
- Interferometry
Explain the Pulse method
A short, intensive pulse of radiation is transmitted to a
reflector target, which immediately transmits it back, along
a parallel path, to the receiver.
The measured distance is computed from the velocity of the signal multiplied by the time it took to complete its journey.
Explain the Phase difference method
It is a method of measuring phase differences of
continuous waves for distance measurement.
The instrument measures the amount (wavelength) by which the reflected signal is out of phase with the emitted signal.
Explain the Interferometry method
Used for high precision distance measurements over short distances.
They are based of the principle of optical interference. Interferometers have a range of about 80m and are mainly used indoors.
Carrier waves
To overcome these unsuitable waves for transmission through the atmosphere, they are superimposed electronically on other waves known as carrier waves.
Carrier waves are of a much higher frequency and act as a medium for carrying distance information. This process of electronic superimposition is known as modulation.
What are the types of modulation?
- Amplitude modulation (light waves)
- Frequency modulation (microwaves)
Sources of errors in EDM measurement
- Errors in the determination of light velocity
- Modulation frequency errors
- Refractive index errors
- Errors in phase difference determination
- Zero errors (calibration errors)
What does GNSS stand for?
(Global Navigation Satellite Systems)
What are the 3 major components/segments of GNSS
- Space segment
GNSS satellites orbiting above earth - Control segment
Ground-based network of master control stations, data uploading stations, and monitor
stations.
Master station - adjust satellites’ orbit parameters/ on-board high-precision clocks
Monitor station - monitor status and signal of satellites
Uploading station - upload change in status - User segment
Contains antennas and receivers to determine info like position, velocity, time
GNSS steps to determine time and position
- satellites
- propagation
- reception
- computation
- application
Satellite constellation geometry
Dilution of Precision (DOP) depends on the distribution of the satellites in space.
Clear visibility, many satellites, low DOP, good geometry (and vice versa)
Dilution of Precision (PDOP) used to measure geometric strength as an expression of the quality of the satellites geometry, which is essential for ambiguity resolution.
Desirable PDOP value: 1<x>6</x>
What are the GNSS Error sources
- Satellite clocks
- Orbit errors
- Ionospheric delays
- Receiver noise
5, Multipath
GNSS applications
- Transportation
- Timing
- Machine Control
- Marine
- Surveying
- Defence
- Port Automation
Types of Remote Sensing/Earth observation
- Visible & Reflective IR remote sensing
Radiation source: The sun
Object: Reflectance - Thermal remote sensing
Radiation source: Object
Object: Thermal radiation (emissivity, temperature) - Microwave remote sensing
Radiation source: Object/ Radar
Object: Microwave radiation/ Backscatter coefficient
Remote sensing process
- source of energy
- propagation through the atmosphere
- interaction with targets
- re-transmission through the atmosphere
- imaging platform
- image data
- interpretation and analysis
- information products
- users
Why mapping? (importance)
- creates a model of the world.
- required to describe resources and
environment. - an indispensable instrument for planning
sustainable development.
What is an image?
Data that is organized in a grid of columns and rows that usually represents a geographical area
How are images made?
- Passive remote sensing
result of light from the sun reflecting off
objects on the earth’s surface - Active remote sensing
result of backscattering of energy from objects on the earth’s surface
Pixels (concept)
Each pixel stores a digital number
(DN) measured by the sensor.
Represents individual areas
scanned by the sensor.
The smaller the pixel or the
higher the spatial resolution of
the sensor, the easier it is to see
detail.
Swath
It is a strip of the earth’s surface
Sensors collect 2D images of the surface in a swath below the sensor
Light classification
- Gamma
- x-rays
- uv
- visible
- infrared
- microwave
- tv/radio
(wavelength increases downwards)
Types of images
- Remotely sensed images
Continuous data - quantitative, remote sensing
Types:
a. Panchromatic - 1 band/layer
b. Multispectral - 2+ bands - Thematic images
Discrete data - qualitative, GIS
Characteristics of Remote sensing Data
- Spatial resolution
determines the smallest object or level of detail that can be perceived in an image - Spectral resolution
Defines the number and size of the bands a sensor is able to record - Radiometric resolution
sensitivity of a sensor to incoming radiance/ how much change in radiance there must be on the sensor before a change in recorded brightness value takes place - Temporal resolution
duration it takes a sensor to return to a previously imaged location
What is GIS
Geographic Information System (GIS) is defined as a special type of information system that is used to input, store, retrieve, process, analyze and visualize geospatial data and information in
order to support decision making.
basically a computer-based information system
for handling spatially referenced data and information.
Name the 2 types of geographic data that GIS handles
- Spatial data
describe the location and shape of
geographic features and their spatial relationships - Attribute Data
describe the characteristics of spatial data.
Geometric data models in GIS
- Vector model
uses discrete points, lines and/or areas corresponding to discrete objects with name or code number of attributes. - Raster model
employs regularly spaced grid cells set out in a specific sequence.
Basic GIS functions
- Data input and processing
Map digitizing, map/photo scanning, editing,
topology building, format conversion etc. - Database management
Data archiving, data retrieval, data updating
etc. - Spatial analysis
Query, measurements, reclassification,
coverage rebuilding, overlay operations,
connectivity analysis etc. - Graphic output and visualization
Map projection, graphic representation, map
production, DEM generation, bird’s eye
views etc.
GIS data capture and editing
- data identification
- data collection and capture (spatial/non-spatial data)
- data processing and verification
- data ( correct?)
- GIS database
What is spatial analysis
It is the means of adding value to geographic data that allows the conversion of data into useful information and knowledge.
It gives more insight into a place and helps in
focusing and prioritizing actions or decisions.
