Principles of Remote Sensing Questions and Answers

Answer: C

Remote sensing uses electromagnetic waves.

Remote sensing gathers information about objects without touching them, by recording the electromagnetic radiation they reflect, emit or scatter: visible and infrared light in optical sensors, thermal infrared in thermal sensors, and microwaves in radar and passive microwave instruments.

Sound waves are used in sonar and seismic surveys, not in remote sensing from aircraft and satellites. 'Electric waves' is not a separate kind of wave, and wind waves are waves on water.

Answer: D

The small-scale photo is 1 : 250000.

A map or photo scale is a fraction, so the larger the denominator, the smaller the fraction and the smaller the scale. At 1 : 250000, 1 cm on the photo covers 2.5 km on the ground: a large area with little detail, which is what 'small scale' means.

1 : 10000, 1 : 25000 and 1 : 50000 are larger scales, showing smaller areas in more detail. The common trap is to think a bigger number means a bigger scale.

Answer: D

The point on the ground vertically beneath the lens centre is the Ground Nadir (printed 'Gound Nadir').

In aerial photography the plumb line through the camera's perspective centre meets the photograph at the photo nadir and the ground at the ground nadir. On a truly vertical photograph the nadir, principal point and isocentre coincide, but on a tilted photograph they separate.

Fiducial marks are reference marks on the frame used to locate the principal point, the camera lens centre is the exposure station in the air, and the isocentre lies on the bisector of the tilt angle, not directly beneath the lens.

Answer: D

GPS works on the principle of trilateration.

A receiver measures the travel time of signals from several satellites and converts each into a distance. Each distance places the receiver on a sphere centred on that satellite; the intersection of spheres from at least three satellites fixes the position, and a fourth satellite resolves the error in the receiver's clock. Positioning by distances is trilateration.

Triangulation and trisection (resection) fix a position from measured angles, not distances, which GPS does not measure. 'Reselection' and 'revolving movement' are not positioning principles.

Answer: B

The focal length is 657.8 mm.

The scale of a vertical aerial photograph is the focal length divided by the flying height above the ground:

\[ \begin{aligned} S &= \frac{f}{H - h} \\ H - h &= 3780 - 820 = 2960\ \text{m} \\ f &= \frac{2960}{4500}\ \text{m} = 0.6578\ \text{m} \end{aligned} \]

So f = 657.8 mm. The common error is to use the flying height above sea level: \(3780/4500 = 0.84\) m, which is not offered. The other options do not come from this formula with the given heights.

Answer: D

The Doppler effect is a change in frequency.

When a source of waves moves towards an observer, successive wave crests are emitted closer together, so the observer receives a higher frequency; when it moves away, a lower frequency. The pitch of a passing siren drops for this reason. Doppler radar and Doppler weather radar use this frequency shift to measure the speed of moving targets such as raindrops.

The amplitude of the waves (loudness) does not change because of the motion itself. Velocity and displacement belong to the source; the Doppler effect is the change in frequency that results from the source's velocity.