A last-minute revision squeeze of this chapter: the definition of remote sensing, its eight stages, the electromagnetic spectrum, sun-synchronous versus geostationary satellites, whiskbroom versus pushbroom scanners, the four types of resolution, data products, and the seven elements of visual interpretation, all in one place. The full detailed notes are available separately for a complete read.
1 The one definition asked every single time
Remote Sensing is the total process used to acquire and measure information about some property of objects or phenomena on the earth’s surface by a recording device (sensor) that is not in physical contact with the object or phenomenon under study; the term was first used in the early 1960s.
Box 6.1’s Sun-Synchronous versus Geostationary comparison and Table 6.1’s satellite resolution figures are asked directly almost every year.
2 The eight stages of remote sensing (a-h)
- (a) Source of Energy: sun, or self-emission/artificial (flashgun, radar)
- (b) Transmission of energy from source to the earth’s surface
- (c) Interaction of energy with the earth’s surface
- (d) Propagation of reflected/emitted energy through the atmosphere
- (e) Detection of the energy by the sensor
- (f) Conversion of energy received into photographic/digital data
- (g) Extraction of thematic information from the data products
- (h) Conversion of information into Map/Tabular forms
3 Electromagnetic spectrum
| Order (long to short wavelength) | Used in remote sensing? |
|---|---|
| Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma | Only Visible, Infrared and Microwave are used |
Visible spectrum: about 400-700 nanometres (violet to red). Speed of light: 300,000 km/second.
4 Sun-Synchronous versus Geostationary satellites (Box 6.1)
| Characteristic | Sun-Synchronous | Geostationary |
|---|---|---|
| Altitude | 700-900 km | ~36,000 km |
| Coverage | 81°N to 81°S | 1/3rd of the globe |
| Orbital period | ~14 orbits/day | 24 hours |
| Resolution | Fine (182 m to 1 m) | Coarse (1 km x 1 km) |
| Use / example | Earth Resources (IRS) | Telecom & Weather (INSAT) |
India’s earth receiving station: Shadnagar, near Hyderabad.
5 Whiskbroom versus Pushbroom scanners
| Whiskbroom | Pushbroom |
|---|---|
| Single rotating mirror + single detector | Linear array of many fixed detectors |
| Mirror oscillates 90°-120°; bit-by-bit, line-by-line | No moving mirror; entire line captured at once |
| TFOV = total swept field; IFOV = instant view | Detector count = swath ÷ spatial resolution |
SPOT HRV-I: swath 60 km (60,000 m) ÷ resolution 20 m = 3,000 detectors.
6 Four types of resolution
| Type | What it measures |
|---|---|
| Spatial | Distinguishing two closely spaced objects as separate objects |
| Spectral | Recording power of the sensor in different EMR bands |
| Radiometric | Discriminating fine radiance differences between two targets |
| Temporal | Revisit time interval for reimaging the same place |
7 Table 6.1: key sensor figures (memorise the extremes)
| Sensor | Spatial resolution | Bands | Radiometric range |
|---|---|---|---|
| Landsat MSS (coarsest) | 80.0 x 80.0 m | 4 | 0-64 |
| Landsat TM | 30.00 x 30.00 m | 4 | 0-255 |
| SPOT HRV-I | 20.00 x 20.00 m | 3 | 0-255 |
| IRS PAN (finest) | 5.80 x 5.80 m | 1 | 0-127 |
8 Data products: image versus photograph
- “All photographs are images, but all images are not photographs.”
- Photographic images: optical region 0.3-0.9 µm; 4 film types (B/W, colour, B/W infrared, colour infrared)
- Digital images: made of pixels; each pixel’s value = Digital Number (DN)
9 Seven elements of visual interpretation
| Element | Book’s own example |
|---|---|
| Tone/Colour | Healthy vegetation = red/magenta in FCC; clear water = dark/black |
| Texture | Rice/wheat = fine; sugarcane/millet = coarse |
| Size | Distinguishes industrial complexes from residential dwellings |
| Shape | Railway line’s long, gently-curving linearity vs a road |
| Shadow | Qutub Minar identifiable only by its shadow |
| Pattern | Planned residential layouts, orchards with uniform spacing |
| Association | Educational institution with a playground nearby |
Two categories: Image characteristics (tone, shape, size, pattern, texture, shadow) and Terrain characteristics (location, association).
- Remote sensing = measuring/recording without contact, via a sensor; term first used in the 1960s
- 8 stages (a-h): source → transmission → interaction → atmosphere → detection → conversion → extraction → presentation
- Only Visible, Infrared, Microwave regions used; X-rays NOT used
- Sun-synchronous (700-900 km, fine resolution, IRS) vs Geostationary (~36,000 km, coarse, INSAT)
- Whiskbroom (rotating mirror + 1 detector) vs Pushbroom (many fixed detectors, no mirror); SPOT HRV-I = 3,000 detectors
- 4 resolutions: spatial, spectral, radiometric, temporal
- 7 interpretation elements: tone/colour, texture, size, shape, shadow, pattern, association
Can I name all eight stages of remote sensing in order? Can I state every row of the Sun-Synchronous vs Geostationary comparison from memory? Can I explain, with the SPOT HRV-I number, how a pushbroom scanner’s detector count is calculated? Can I name all four types of resolution and all seven elements of visual interpretation?