Navigation Basics

AST 241

From Van Sickle’s Modern Airmanship chapter 11 6th ed.

Charts & Mapping

• What is your definition of a map?

• Can any map accurately depict the surface of the earth?

• Why?• Is the earth perfectly round? Why or Why

Not?

Mapping

• What is a great Circle?– A sphere cut by a plane through the center

gives the largest circle that can be drawn on that sphere.

– In global navigation, the shortest distance between two points on the earth’s surface is a great circle route.

– Any other circle is a small circle.

Mapping

• If we begin at the center of a sphere and divide it equally into 360 parts, each part becomes one degree.

• Degrees are then divided into 60 parts:• Called minutes which are divided into 60

parts:• Called seconds. Thereafter they become

decimals or fractions of seconds.

Mapping

• Determining position in the earth’s surface requires reference points:

• On a sphere circles work best- we begin at the equator.

Latitude/Longitude

• Latitude ranges from 0 degrees at the equator to 90 degrees at the poles

• What type of circle would the equator be?

• It is a great circle, all other circles of latitude are small circles parallel to it and are thus “parallels” of latitude.

Lat./long.

• ½ of a great circle passing through the poles is called a “meridian” of longitude or simple “meridian.”

• All meridians intersect at rt.angles to parallels.

• In order for meridians to be useful as a reference point we need to establish a reference meridian which is:

Lat./long.

• The prime meridian located in:• Greenwich, England• In the western world, longitude is measured

east or west of the prime meridian 0 – 180 degrees ending at the international date line in the pacific ocean

• We now have intersections from which we can locate any point on earth.

Lat./long.

• The numbers of these intersecting lines are the “coordinates” of the earth.

• There are many different types of coordinates but in aviation we use divisions of degrees as described earlier.

• For example the coordinates for El Paso TX are 31Degrees 47 minutes North, 106 degrees 27 minutes West (Always give Lat. Followed by Long.)

Time

• We all know that it takes the earth roughly 24 hours to make one 360 degree rotation on its axis.

• Thus if we divide 360 degrees by 24 we get 15 degrees. These divisions are where we get our time zones from- roughly

• When the sun is directly overhead of the 75th meridian is is noon EST.

Time

• Since it is difficult to keep track of time in aviation we use the time over the prime meridian as a reference- UTC (GMT) or “zulu time.”

• Thus we have conversion factors to convert our local times to UTC times ie. From EST to UTC add 5 hours.

Map projections

• Why are maps categorized according to projections?

• The two most common projections are the:– Lambert Conformal Conic- (secantally cut

projection- differs from a tangential conic)– Mercator

Map projections

• Refer to Fig. 11 (in Modern Airmanship)

• As mentioned earlier all flat maps produce distortions- it is the job of the map maker to minimize these distortions with respect to the users most urgent needs.

Measuring distance

• Using grid coordinates- 1 minute of a latitude or longitude line is 1 Nautical Mile or 6080 feet:

• (1.15 N.M.)

Measuring Direction

• Direction is expressed in degrees clockwise from the North Pole (true north).

• The difference between true and magnetic north is:

• Variation

Map Scale

• Map Scale- The ratio of the length of distance on a map to the true distance on the surface of the earth.

For example aeronautical sectional charts the scale is 1:500,000 where one unit of the chart is equal to 500,000 units on the surface of the earth.

Great Circle vs. Rhumb line

• What is a great circle?

• A rhumb line is a straight line drawn on a mercator map and cuts each meridian at the same angle.

• On most aeronautical charts a straight line drawn closely approximates a great circle course

Rhumb line ctn.

• For example the in a true great circle route and a straight course drawn on a sectional chart would be about 10 miles on a NY to San Francisco trip.

• Conversely, the difference in distance between a rhumb line course and a great circle course between NY and Paris is about 145 NM. Which is longer? (see fig. 11-4)

What are the coordinates of KHUF?

How about KSIV