Mercator ChartsAir Navigation — DGCA CPL practice questions
Question 1 of 10
A normal Mercator chart is which type of projection? [select the correct combination of: (i) Cylindrical (ii) Perspective (iii) Non-perspective (iv) Conformal (v) Conical (vi) Azimuthal]
All 10 questions — Mercator Charts
Air Navigation · DGCA CPL. The correct option is marked on each.
Q1. A normal Mercator chart is which type of projection? [select the correct combination of: (i) Cylindrical (ii) Perspective (iii) Non-perspective (iv) Conformal (v) Conical (vi) Azimuthal]
- A.(i), (ii) and (iii)
- B.(ii), (iv) and (v)
- C.(i), (iii) and (iv)✓
- D.(iii), (iv) and (vi)
Why: The Mercator is: Cylindrical (cylinder wrapped around the globe), Non-perspective (Mercator mathematically modified the simple cylindrical to make it conformal), and Conformal/Orthomorphic (it satisfies both orthomorphism conditions). It is NOT perspective (the mathematical adjustment broke the direct projection relationship). Answer: c — (i), (iii) and (iv)
Q2. A direct Mercator graticule is:
- A.Rectangular✓
- B.Square
- C.Circular
- D.Convergent
Why: Meridians are equally spaced parallel vertical lines. Parallels are horizontal lines (spacing increases toward poles). The graticule is rectangular (not square — only the Equatorial region is close to square; at higher latitudes the cells stretch vertically). Answer: a — Rectangular
Q3. On a normal Mercator chart, rhumb lines are represented as:
- A.Curves concave to the Equator
- B.Curves convex to the Equator
- C.Complex curves
- D.Straight lines✓
Why: The defining property of the Mercator chart for navigation: a constant compass bearing (rhumb line) crosses every meridian at the same angle. Since all meridians are parallel vertical lines on a Mercator, any straight line crosses them all at the same angle. Therefore rhumb lines are straight lines . Answer: d — Straight lines
Q4. On a direct Mercator, great circles can be represented as:
- A.Straight lines
- B.Curves
- C.Straight lines and curves✓
Why: The Equator and all meridians are both great circles AND straight lines on the Mercator. All other great circles are curves. Therefore great circles can be represented as BOTH straight lines (Equator/meridians) and curves (all others). ⚑ Key Note: This is a tricky question. Options (a) and (b) are each partially correct, but (c) is the most complete and correct answer. Always choose (c) in examinations.
Q5. On a direct Mercator, with the exception of meridians and the Equator, great circles are represented as:
- A.Curves concave to the Nearer Pole
- B.Curves convex to the Equator
- C.Curves concave to the Equator✓
- D.Straight lines
Why: Great circles (except meridians and Equator) curve toward the poles — i.e. their centre of curvature is toward the Equator. They are concave toward the Equator . (Note: 'concave to the Equator' and 'convex to the poles' are the same thing — be careful with the wording.) Answer: c — Curves concave to the Equator
Q6. The angle between a straight line on a Mercator chart and the corresponding great circle is:
- A.Zero
- B.Earth convergency
- C.Conversion angle✓
- D.Chart convergence
Why: A straight line on a Mercator chart is a rhumb line . The angle between the rhumb line and the great circle (between two points) is the Conversion Angle = ½ × ch.long × sin(mean lat). (Earth convergency = ch.long × sin lat; chart convergence on Mercator = 0.) Answer: c — Conversion angle
Q7. The rhumb line track from Turin (45N 008E) to Khartoum (15N 032E) is 145°(T). What is the great circle track measured at Turin?
- A.133°(T)
- B.139°(T)✓
- C.145°(T)
- D.151°(T)
Why: Mean latitude = (45 + 15) / 2 = 30°N. Ch.long = 32 - 8 = 24°. Conversion angle = ½ × 24 × sin(30°) = ½ × 24 × 0.5 = 6° At Turin (northern end of a SE track), the GC is more northerly (more poleward) than the RL. GC at Turin = RL - CA = 145° - 6° = 139°(T) Answer: b
Q8. In Question 7, what is the direction of the great circle track from Khartoum to Turin?
- A.319°(T)
- B.325°(T)
- C.331°(T)✓
- D.337°(T)
Why: RL from Khartoum to Turin = reciprocal of 145° = 325°(T). Conversion angle = 6° (same as above). At Khartoum (southern end, the GC is still more poleward than the RL — poleward from 325° is toward 360°/000°, i.e. bigger number). GC at Khartoum = RL + CA = 325° + 6° = 331°(T) Answer: c
Q9. On a Mercator chart, the rhumb line track from Durban (30S 032E) to Perth (30S 116E) is 090°(T). What is the great circle track from Perth to Durban?
- A.291°(T)
- B.312°(T)
- C.228°(T)
- D.249°(T)✓
Why: Both places are at 30°S. Ch.long = 116 - 32 = 84°. Mean lat = 30°S. Conversion angle = ½ × 84 × sin(30°) = ½ × 84 × 0.5 = 21° RL from Perth to Durban = 270°(T) (due west, same latitude). In the Southern Hemisphere, poleward = south. At Perth (eastern end going west), the GC departs more toward the south (poleward in SH) than the RL. GC at Perth = 270° + 21° = 291°(T) would be if going MORE south. Wait — rechecking: going west in SH, poleward means larger track number toward 360°... Actually for Perth→Durban: GC initial at Perth = RL - CA = 270° - 21° = 249°T (SW, toward pole). Note: The GC…
Q10. At 60°S on a Mercator chart, chart convergence is:
- A.greater than Earth convergency
- B.'correct'
- C.less than Earth convergency✓
- D.equal to ch.long × 0.866
Why: On a Mercator chart, all meridians are parallel . Therefore chart convergence = zero . Earth convergency at 60°S = ch.long × sin(60°) = ch.long × 0.866 > 0. Zero < Earth convergency → chart convergence is less than Earth convergency . (Option d gives Earth convergency, not chart convergence.) Answer: c