Mercator Projections

Question 1

What is another name for a Direct Mercator chart?

Answer 1

Standard Mercator chart.

Question 2

What is scale expansion proportional to on Mercator charts?

Answer 2

Distance from the great circle of tangency.

Question 3

What does the conversion angle equal?

Answer 3

Half the convergency.

Question 4

What projection is used by Mercator charts?

Answer 4

Cylindrical.

Question 5

How do rhumb line tracks and great circle tracks appear on Direct Mercator projection?

Answer 5

Rhumb lines are straight lines.

Great circles are concave to the equator.

Question 6

How do rhumb line tracks and great circle tracks appear on Transverse Mercator projection?

Answer 6

Rhumb lines are concave to the nearer pole.

Great circles are concave to the Central Meridian.

Question 7

Why are straight lines (rhumb line tracks) on Direct Mercator charts not suitable for long distance navigation?

Answer 7

Because a rhumb line track is not the shortest distance.

Question 8

Is the Direct Mercator projection conformal?

Answer 8

Yes, since it satisfies the chart conformity requirements.

Question 9

On a Direct Mercator chart, what is the simplest way to calculate scale at the equator from scale at some latitude?

Answer 9

Divide the denominator of the scale at that latitude by the cosine of that latitude.

Question 10

Define: False Equator.

Answer 10

The great circle of tangency used in Oblique Mercator projections.

Question 11

How do rhumb line tracks and great circle tracks appear on an Oblique Mercator projection?

Answer 11

Rhumb lines are concave to the nearer pole.

Great circles are concave to the False Equator.

Question 12

What are common uses for Oblique Mercator projection?

Answer 12

» producing charts of regularly used routes

» producing charts of countries that are long and thin

Question 13

How does scale vary on a Direct Mercator projection in terms of:

(a) size

(b) fraction

(c) denominator

?

Answer 13

As we depart the equator:

(a) scale expands
(b) scale fraction gets larger
(c) scale denominator gets smaller

Question 14

Above which latitudes are Direct Mercator charts considered unusable?

Answer 14

Above 70°.

Question 15

What is the acceptable limit of chart scale error?

Answer 15

1 %.

Question 16

How do:

(a) parallels of latitude

(b) meridians of longitude

appear on a Direct Mercator chart?

Answer 16

(a) unequally spaced, parallel, straight, horizontal lines
(b) equally spaced, parallel, straight, vertical lines

Question 17

What effect does hemisphere have on scale calculations?

Answer 17

It has no effect.

Question 18

Define: Transverse Mercator chart.

Answer 18

Cylindrical projection with the parallel of origin (great circle of tangency) along a meridian.

Question 19

Define: Central Meridian.

Answer 19

The great circle of tangency used in Transverse Mercator projections.

Question 20

Define: convergency.

Answer 20

The change of longitude between two meridians (relative to TN).

Question 21

If a straight line is drawn on a Direct Mercator chart, what kind of a route (track) is that?

Answer 21

Rhumb line track, since the line intersects all meridians at a constant angle.

Question 22

On a Direct Mercator chart, when do rhumb line tracks and great circle tracks coincide?

Answer 22

Along the equator and the meridians.

Question 23

Describe: convergency on Direct Mercator charts.

Answer 23

There is no convergency on Direct Mercator projections.

Question 24

Define: Oblique Mercator chart.

Answer 24

Cylindrical projection with the parallel of origin along any great circle of tangency other than a meridian or the equator.

Question 25

Where are Mercator projections the most accurate?

Answer 25

Within 2.5° (± 500 nm) of the great circle of tangency.

This area would produce about 0.1 % chart error.

Question 26

On a Direct Mercator chart, what is the simplest way to calculate scale at some latitude from the equatorial scale?

Answer 26

Multiply the denominator of the scale at the equator by the cosine of that latitude.

Question 27

Define: Direct Mercator chart.

Answer 27

Cylindrical projection with the parallel of origin (great circle of tangency) along the equator.

Question 28

Define: conversion angle.

Answer 28

The difference between the great circle tracks and the rhumb line track.

Question 29

How to convert between rhumb line tracks and great circle tracks?

Answer 29

Calculate the conversion angle and apply it in the correct sense.

Question 30

On a Direct Mercator chart, how does departure vary with the change of latitude?

Answer 30

It reduces with increasing latitude.

Question 31

What can we use the ABBA formula for?

Answer 31

We can use it to calculate:

» scale at one latitude from scale at another latitude without calculating the equatorial scale

» changing departure at various latitudes

Question 32

Define the ABBA formula for its two uses.

Answer 32

Scale_ФA · Cosine_ФB = Scale_ФB · Cosine_ФA

Departure_ФA · Cosine_ФB = Departure_ФB · Cosine_ФA

Question 33

On a Direct Mercator chart, how does:

(a) departure

(b) chart distance

change with increasing latitude?

Answer 33

(a) reduces
(b) remains constant

Question 34

Define: transverse.

Answer 34

Perpendicular.

Question 35

List the main types of Mercator charts.

Answer 35

(1) Direct
(2) Transverse
(3) Oblique

Question 36

What was the original purpose of Mercator charts?

Answer 36

Equatorial navigation.

Question 37

Why are Mercator charts unsuitable for long-range navigation?

Answer 37

Because of the scale expansion.