GATE Civil Engineering Geomatics Engineering Flashcards

24 sample cards from the Geomatics Engineering deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Differentiate triangulation from trilateration.

   In triangulation, angles are measured and sides are computed from a baseline. In trilateration, the sides (distances) are measured directly (typically with EDM) and angles are computed. Modern surveys often combine both (triangulateration).

2. What is the 'strength of figure' in triangulation?

   Strength of figure is a measure (related to the distribution of angles) of the precision with which lengths are computed through a chain of triangles; it depends on the geometry and the number of conditions. Well-conditioned triangles (angles between $30^{\circ}$ and $120^{\circ}$) give greatest strength.

3. What is a total station and what does it integrate?

   A total station is an electronic surveying instrument integrating an electronic theodolite (for horizontal and vertical angles), an EDM (for distances), and an onboard microprocessor/data logger to compute and store coordinates, elevations, and reductions.

4. On what physical principle does an EDM in a total station measure distance?

   EDM measures distance by transmitting a modulated electromagnetic (infrared/laser) wave to a reflector and measuring the phase difference (or time of flight) of the returned signal: $D=\dfrac{1}{2}\,c\,t$ or $D=\dfrac{n\lambda+\Delta\lambda}{2}$ for phase methods.

5. Differentiate the resection and the free-station (free setup) method using a total station.

   Resection determines the coordinates of the instrument station by observing two or more known control points. A free-station setup places the instrument at an arbitrary point and uses resection so the operator need not occupy a known point.

6. What is a horizontal curve and what are its two main types?

   A horizontal curve connects two intersecting straights (tangents) in plan to provide a gradual change in direction. Main types: simple circular curve, compound curve, reverse curve, and transition (spiral) curve.

7. For a simple circular curve of radius $R$ and deflection angle $\Delta$, give the tangent length and the length of curve.

   Tangent length $T=R\tan\dfrac{\Delta}{2}$; length of curve $L=\dfrac{\pi R\Delta}{180^{\circ}}=R\Delta_{\text{rad}}$. The long chord $=2R\sin\dfrac{\Delta}{2}$.

8. Give the formulas for the external distance and mid-ordinate (apex distance) of a simple circular curve.

   External distance $E=R\left(\sec\dfrac{\Delta}{2}-1\right)$; mid-ordinate (versine) $M=R\left(1-\cos\dfrac{\Delta}{2}\right)$.

9. Relate the degree of curve $D$ to the radius $R$ (arc definition, $30\,\text{m}$ chord/arc).

   For a $30\,\text{m}$ standard arc, $R=\dfrac{1718.9}{D}$ m (since $D$ is the central angle subtended by a $30\,\text{m}$ arc, $R=\dfrac{30\times180}{\pi D}$). The degree of curve is inversely proportional to the radius.

10. What is a transition curve and why is it provided?

    A transition (easement) curve has a radius that gradually changes from infinity (straight) to $R$ (circular curve). It is provided to introduce centrifugal force gradually, allow gradual application of superelevation, improve comfort and safety; the ideal form is the clothoid (spiral) where $L\,R=$ constant.

11. What is a vertical curve and which geometric form is used?

    A vertical curve connects two different grades (gradients) in elevation to provide a smooth transition. A parabola is used because it gives a constant rate of change of grade and equal horizontal spacing of offsets.

12. Differentiate a summit (crest) curve from a sag (valley) curve.

    A summit/crest curve is convex upward, formed where the algebraic difference of grades makes the centre higher (e.g., $+$ grade meeting $-$ grade); design governed by sight distance. A sag/valley curve is concave upward; design governed by headlight sight distance and comfort/drainage.

13. What is remote sensing?

    Remote sensing is the science of acquiring information about an object or area from a distance, without physical contact, by measuring the electromagnetic radiation reflected or emitted from it, typically using sensors on satellites or aircraft.

14. Differentiate active and passive remote sensing.

    Passive remote sensing uses naturally available energy (e.g., reflected sunlight, emitted thermal radiation); the sensor only records. Active remote sensing supplies its own energy source (e.g., RADAR, LiDAR) and records the reflected return, enabling night/all-weather imaging.

15. What is spatial resolution versus spectral resolution in remote sensing?

    Spatial resolution is the smallest ground area (pixel size) a sensor can distinguish. Spectral resolution is the number and width of wavelength bands the sensor can detect; finer (narrower, more) bands give higher spectral resolution. (Temporal resolution = revisit interval; radiometric = bit depth.)

16. What is a Geographic Information System (GIS)?

    A GIS is a computer-based system to capture, store, manage, analyze, and display spatially referenced (geographic) data, integrating spatial (location) data with attribute (descriptive) data for spatial analysis and decision-making.

17. Differentiate the raster and vector data models in GIS.

    Raster represents space as a grid of cells/pixels, each holding a value—good for continuous data (imagery, elevation). Vector represents features as points, lines, and polygons defined by coordinates—good for discrete features with sharp boundaries; it is more compact and topologically precise.

18. What are the basic components of a GIS?

    Hardware, software, data (spatial + attribute), people (users/experts), and methods/procedures. These integrate to capture, manage, analyze, and present geographic information.

19. In aerial photogrammetry, give the relationship between photo scale, focal length, and flying height.

    For a vertical photograph over flat terrain, scale $=\dfrac{f}{H-h}$, where $f$ is the camera focal length, $H$ is the flying height above datum, and $h$ is the average ground elevation. Equivalently scale $=\dfrac{\text{photo distance}}{\text{ground distance}}$.

20. How do you compute the flying height $H$ above ground for a desired photo scale $S=1:n$ with focal length $f$?

    From scale $=\dfrac{f}{H'}$ where $H'=H-h$ is height above ground, $H'=f\times n=\dfrac{f}{S}$. Thus flying height above the datum $H=f\,n+h$, where $h$ is the average terrain elevation.

21. What is relief displacement on a vertical aerial photograph, and how is it computed?

    Relief displacement is the radial shift of an image point (away from the principal point) caused by the object's height above datum: $d=\dfrac{r\,h}{H}$, where $r$ is the radial distance of the image from the principal point, $h$ the object height, and $H$ the flying height above the base.

22. Define overlap (forward and side lap) in aerial photography and typical values.

    Forward (end) lap is the overlap between successive photos along a flight line, typically about $60\%$, required for stereoscopic (3D) coverage. Side lap is the overlap between adjacent flight strips, typically about $30\%$. These ensure complete stereo coverage.

23. What is parallax in stereo-photogrammetry, and how is object height obtained from it?

    Parallax is the apparent displacement of a point due to change in observation position between two photos. Height $h=\dfrac{H\cdot\Delta p}{p+\Delta p}$, where $H$ is flying height, $p$ the parallax at datum, and $\Delta p$ the parallax difference; greater parallax means a higher/closer object.

24. Define accuracy and precision in measurement, and state how they differ.

    Accuracy is closeness of a measured value to the true value (freedom from systematic error/bias). Precision is closeness of repeated measurements to one another (small random scatter). Measurements can be precise but inaccurate if a systematic error is present.

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