Mercator Projections Flashcards
What is another name for a Direct Mercator chart?
Standard Mercator chart.
What is scale expansion proportional to on Mercator charts?
Distance from the great circle of tangency.
What does the conversion angle equal?
Half the convergency.
What projection is used by Mercator charts?
Cylindrical.
How do rhumb line tracks and great circle tracks appear on Direct Mercator projection?
Rhumb lines are straight lines.
Great circles are concave to the equator.
How do rhumb line tracks and great circle tracks appear on Transverse Mercator projection?
Rhumb lines are concave to the nearer pole.
Great circles are concave to the Central Meridian.
Why are straight lines (rhumb line tracks) on Direct Mercator charts not suitable for long distance navigation?
Because a rhumb line track is not the shortest distance.
Is the Direct Mercator projection conformal?
Yes, since it satisfies the chart conformity requirements.
On a Direct Mercator chart, what is the simplest way to calculate scale at the equator from scale at some latitude?
Divide the denominator of the scale at that latitude by the cosine of that latitude.
Define: False Equator.
The great circle of tangency used in Oblique Mercator projections.
How do rhumb line tracks and great circle tracks appear on an Oblique Mercator projection?
Rhumb lines are concave to the nearer pole.
Great circles are concave to the False Equator.
What are common uses for Oblique Mercator projection?
» producing charts of regularly used routes
» producing charts of countries that are long and thin
How does scale vary on a Direct Mercator projection in terms of:
(a) size
(b) fraction
(c) denominator
?
As we depart the equator:
(a) scale expands
(b) scale fraction gets larger
(c) scale denominator gets smaller
Above which latitudes are Direct Mercator charts considered unusable?
Above 70°.
What is the acceptable limit of chart scale error?
1 %.
How do:
(a) parallels of latitude
(b) meridians of longitude
appear on a Direct Mercator chart?
(a) unequally spaced, parallel, straight, horizontal lines
(b) equally spaced, parallel, straight, vertical lines
What effect does hemisphere have on scale calculations?
It has no effect.
Define: Transverse Mercator chart.
Cylindrical projection with the parallel of origin (great circle of tangency) along a meridian.
Define: Central Meridian.
The great circle of tangency used in Transverse Mercator projections.
Define: convergency.
The change of longitude between two meridians (relative to TN).
If a straight line is drawn on a Direct Mercator chart, what kind of a route (track) is that?
Rhumb line track, since the line intersects all meridians at a constant angle.
On a Direct Mercator chart, when do rhumb line tracks and great circle tracks coincide?
Along the equator and the meridians.
Describe: convergency on Direct Mercator charts.
There is no convergency on Direct Mercator projections.
Define: Oblique Mercator chart.
Cylindrical projection with the parallel of origin along any great circle of tangency other than a meridian or the equator.
Where are Mercator projections the most accurate?
Within 2.5° (± 500 nm) of the great circle of tangency.
This area would produce about 0.1 % chart error.
On a Direct Mercator chart, what is the simplest way to calculate scale at some latitude from the equatorial scale?
Multiply the denominator of the scale at the equator by the cosine of that latitude.
Define: Direct Mercator chart.
Cylindrical projection with the parallel of origin (great circle of tangency) along the equator.
Define: conversion angle.
The difference between the great circle tracks and the rhumb line track.
How to convert between rhumb line tracks and great circle tracks?
Calculate the conversion angle and apply it in the correct sense.
On a Direct Mercator chart, how does departure vary with the change of latitude?
It reduces with increasing latitude.
What can we use the ABBA formula for?
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
Define the ABBA formula for its two uses.
ScaleФA · CosineФB = ScaleФB · CosineФA
DepartureФA · CosineФB = DepartureФB · CosineФA
On a Direct Mercator chart, how does:
(a) departure
(b) chart distance
change with increasing latitude?
(a) reduces
(b) remains constant
Define: transverse.
Perpendicular.
List the main types of Mercator charts.
(1) Direct
(2) Transverse
(3) Oblique
What was the original purpose of Mercator charts?
Equatorial navigation.
Why are Mercator charts unsuitable for long-range navigation?
Because of the scale expansion.
