Meaning of remote sensing
Remote sensing is the Collection of data by a recording device that is not in direct contact with the area.
Examples: Satellites, aircraft, drones, and aerial photographs
types of remote sensing
passive remote sensing – measures the energy that is radiated from the earth e.g temperature
ADVANTAGES OF REMOTE SENSING:
- Allows coverage of very large areas.
- Can access inaccessible areas.
- Easy collection of data over a variety of scales and resolutions.
- There is no limitation on the extent of information that can be gathered from a single remotely sensed image.
- Data can easily be processed and analysed fast using a computer.
- Does not disturb the object or the area of interest.
- Cheap and fast method of collecting data of large areas.
- It is easier to locate floods or forest fire that has spread over a large region which makes it easier to plan a rescue mission easily and fast.
DISADVANTAGES OF REMOTE SENSING:
- Remote sensing is a fairly expensive method of analysis especially when measuring or analysing smaller areas.
- Requires a special kind of training to analyse the images.
- Human errors may occur during the analysing process.
- Sometimes different phenomena being analysed may look the same during measurement which may lead to classification error.
- Sometimes large-scale engineering maps cannot be prepared from satellite data which makes remote sensing data collection incomplete.
The 7 elements of a satellite remote sensing system:
A source of energy:
- To be able to see things we need light + to produce light we need energy.
- Sun = primary source of energy.
- Sun = radiates electromagnetic energy that travels through the atmosphere in the form of electromagnetic waves.
- The total range (spectrum) of waves is known as the electromagnetic spectrum.
- We can distinguish between different waves on the basis of their wavelength.
- Long low energy waves – radio waves (measured in km).
- Very short high energy waves – x-ray (measured in um)
- Only a very tiny portion of the waves in the electromagnetic spectrum can be seen with the naked eye.
- That part is known as the visible wavelengths or visible spectrum and consists of the colours of the rainbow
Transmission through the atmosphere:
- Not all electromagnetic waves from the sun reach Earth’s surface.
- Dust particles + gases in the atmosphere cause scattering + absorption of some of the light + radiation.
- The regions of the electromagnetic spectrum which are not seriously affected by scattering + absorption and thus reach the Earth are called atmospheric windows.
- Satellite sensors are designed to be sensitive to the wavelengths that are able to pass through these windows to the Earth’s surface.
Interaction at the Earth’s surface:
- At the Earth’s surface the energy can be absorbed, transmitted or reflected.
- The amounts of energy that will be reflected, absorbed or transmitted or reflected is unique for different Earth features.
- In remote sensing terminology we say that unique features have unique spectral signatures.
- This is very important since it allows us to distinguish between Earth features by measuring the nature of their interaction with electromagnetic energy.
- As our signatures and fingerprints are unique, different phenomena respond in a unique manner to different waves within the electromagnetic spectrum.
- Think about a well-groomed soccer field.
- The grass = reacts in a unique manner to the waves in the visible spectrum.
- Most of the light associated with the green waveband is reflected to our eye while the light associated with other colours is absorbed by the grass and does not reach our eyes.
- In our brain the incoming reflection is compared with thousands of images already stored which we use as references (signatures) to recognise features.
- Within a fraction of a second our brain responds back that the new image best matches the image of “grass”.
- Most satellite sensors are designed to measure the amount of Earth reflection in those regions in the spectrum which we refer to as atmospheric windows.
- They’re active sensors – they emit their own energy + then measures the radiation that’s reflected or scattered back to the sensor.
Recording of reflected energy by a sensor system:
- A conventional digital camera records all wavelengths within a visible spectrum as
a single image.
- Satellite sensors = more sophisticated.
- They can measure + record the reflection within the individual wavelengths of the
visible spectrum as separated numeric images.
- They can even record wavelengths which we cannot see with the naked eye.
- Example = detection of infrared radiation or the x-ray image.
Transmission, reception and processing:
- The image gets processed into a format in which it can be used for analysis by the South African user community of researchers + local + international government departments.
Interpretation + analysis:
- We can attempt the impossible by trying to make sense of the individual numbers written into the cells or pixels of the image.
- Since a numerical image such as that of LANDSAT TM consists of more than 38 million pixels for each of the spectral bands, this is not really an option.
- Computers + image processing software are therefor used to create an image that resembles a picture that we can view + interpret manually or to automatically” interpret the image data according to our instructions.
An explanation of why a satellite image is grid-like:
Earth is divided into latitudes and longitudes (degrees, minutes and seconds). Due to the lines it forms a grid-like effect on the satellite image.
There’s degrees, minutes and seconds which we talk about and because of that the image must work with that to get the exact position on the Earth’s surface.
Remote sensing is used by the satellite to give the exact position and what it’s capturing.