I can't believe it has been over a month since I've posted last time, but I guess life is catching up with me. I recently became a dad and my seven month old is pretty demanding. He is awesome of course, but nonetheless, I find it hard to find time for this blog or aviation in general.
I wanted to take some time to go over two major map types and why they are important. Here are a few questions you may expect on the exam.
1. VFR terminal area chart (VTA) is
a. an example of a Lambert Conformal Conic Projection
b. an example of a Transverse Mercator Projection
c. only accurate concerning airspace delineations
d. a smaller version of a WAC
2 Straight line drawn on a Mercator Projection chart most closely represents
a. a rhumb line
b. an arc of a great circle
c. a track made good
d. a track line
any my fave:
3. On a ____, 1 minute of latitude is equal to ____.
a. Transverse Mercator Projection chart, 60 nautical miles
b. Transverse Mercator Projection chart, 1 nautical mile
c. Lambert Conformal Projection chart, 60 nautical miles
d. Lambert Conformal Projection chart,6 nautical miles
Let's get into what you need to know to answer these:
What is a Rhumb Line? and Great Circle? According to Wikipedia:
In navigation, a rhumb line (or loxodrome) is a line crossing all meridians at the same angle, i.e. a path of constant bearing. Following a rhumb line covers more distance than following a geodesic, but it is easier to navigate.
and a great circle of a sphere is a circle that runs along the surface of that sphere so as to cut it into two equal halves. The great circle therefore has both the same circumference and the same center as the sphere. It is the largest circle that can be drawn on a given sphere.
Below is probably the best graphical representation of the difference between the great circle and the rhumb line.
In plain language:
to find the shortest distance between two points, use the great circle, but if you wish to get there by following the same course, use the rhumb line.
There are two type of charts primarily used for aviation navigation:
Lambert Conformal Conic Projection - think of putting a cone on top of the globe and envision a section of that cone to represent a specific area. Have a look at this graphic.
Key principles of this chart are:
- Distance distortion is limited. As the cone 'sticks' closely to the sphere, distance is pretty uniform on the entire chart
- Straight lines most closely represent an arc of a great circle.
- Lines of latitude (horizontal), curves concave towards the poles
- Lines of longitude (vertical), converge towards an imaginary point over the poles.
The second method is a Mercator Projection used to produce a chart that involves putting a cylinder around the earth and then using a section of that cylinder to represent a specific area. You can align the cylinder either parallel to the longitude line (meridian) or latitude line (equator). When you align it parallel to the equator, it is called a Transverse Mercator Projection. A Transverse Mercator Projection is one of the most accurate methods of covering a small area.
Key principles of this chart are:
- Straight lines most closely represent a rhumb line.
- Greater distortion is produced compared to a Lambert Projection, distance becomes exaggerated closer to the poles.
- Lines of latitude are straight parallel lines.
- Lines of longitude are straight parallel lines.
- 1 minute of latitude is equal to 1 nautical mile
I found an interesting way to remember charts and what projection method they are associated with:
WAC - Lambert Conformal. (World Aeronautical Chart)
VNC - Lambert Conformal. (VFR Navigation Chart)
VTA - Transverse Mercator.(VFR Terminal Area Chart)
Now that you know all this, what are the answers?
Here they are:
Thanks and good luck.