Principles of map reading
The basis of air navigation is the triangle of velocities explained earlier in these notes. The use of the triangle to solve navigation problems in flight requires plotting charts, computers and other navigation instruments that are normally denied to the pilot navigator. His navigation techniques must enable observations of flight progress to be interpreted by other methods.
For the pilot navigator, flying the aeroplane and navigating it are concurrent activities, the predominance of one or the other at any instant of time being dictated by the immediate situation. The problem can be simplified if the navigation aspect is approached logically and careful preparation is made. The navigational factors contributing to success will be considered under the following headings
- The need for Accurate Flying
- Pre-Flight Planning
- Aircraft Performance
- Mental Dead Reckoning
- Chart Analysis and Map Reading
- The Use of Radio Aids
The Need for Accurate Flying
It is necessary that the highest standards of accuracy possible are maintained in respect of heading, airspeed and altitude. Precise limits of each are not quoted here, but it is emphasised that skill in accurate flying can only be achieved by constant practice.
It is absolutely necessary to reduce to a minimum the time spent on navigation in the air. Thorough flight planning materially contributes to the success of any flight in this respect.
Flight Planning Sequence
A logical sequence is as follows:
- Review all information relevant to the flight eg Flight Rules, Navigation Warnings etc
- Study the meteorological situation, obtain wind velocities and temperatures required for planning.
- Select a flight planning chart and, if different, a set of charts for the route.
- Determine the route to be followed; considering the aim of the flight, flight rules, the meteorological situation, the availability of navigation aids and any other factors involved.
- Draw in tracks, measure track angles and distances and record them in the flight log.
- Determine safe altitudes and decide on flight altitude or flight level as applicable.
- From knowledge of aircraft performance determine RAS for each flight stage. Enter RAS in log and in conjunction with altitude and temperature calculated TAS.
- Calculate headings to steer for each flight stage and log them.
- Complete the log by the calculation of ground speeds and fuels.
- Carry out a mental re-appraisal of the whole plan to check for obvious errors.
- Prepare the flight charts
- Note positions of alternate airfields and determine flight planning data, destination to alternates.
Flight planning should be carried out on a basis that will require the pilot to establish his position at the following intervals:
- Immediately after setting heading to provide a definite departure point and to establish a departure time on which to base ETA
- At regular points along track to check the progress of the flight so that corrections for track error or time may be made.
- At a final point close to the destination so that final corrections may be made.
With chart preparation no hard and fast rules can be laid down for preparing charts apart from stating that it is necessary to follow certain basic rules:
Chart Analysis and Map Reading
There are four basic features upon which success of map reading depends:
- Knowledge of direction.
- Knowledge of distance or time flown.
- Identification of features.
- Selection of landmarks.
The first step in map reading is to orientate the chart. By so doing the pilot navigator relates the direction of land features to their representation on the chart, which aids recognition.
When the chart has been properly orientated, it becomes easier to compare distance between landmarks on the ground with their corresponding distances on the chart, thus facilitating the fixing of position.
Anticipation of Landmarks
During the flight planning stage the relationship of easily recognisable features to the intended track should be noted and a time established at which the aircraft will be near them. Thus in flight, the map reader is prepared to make his visual observation at a particular time thereby avoiding undue diversion of attention from other aspects of flying the aircraft.
Identification of Features
The basic principles to be adopted in the selection of check features is the ease with which they can be identified. They must be readily distinguishable from their surroundings. The conspicuousness of check features depends upon:
- The Angle of Observation: At low levels features are more easily recognised from their outline in elevation. As altitude is increased the reverse is the case and the plan outlines become more important.
- Dimensions of the Feature: A feature which is long in one direction, but sharply defined in the other is best; the length makes the feature easier to see despite airframe restrictions to downward vision, and its shorter dimension permits accurate estimation of the aircraft’s relation to the feature, either in tracking along it or in timing the movement of flight directly above it.
- The Uniqueness of the Feature: To avoid ambiguity the ideal feature should be the only one of its particular outline in the vicinity.
- Contrast and Colour: These properties play a large part in the identification of a particular feature. Map reading is often complicated by seasonal variation, such as
- The difference between deciduous woods in summer and winter.
- The landscapes before and after extensive snow fall.
Contrast and colour also play their part in identifying coastlines after a long sea crossing.
Fixing by Map Reading
Map reading techniques are largely dependent upon the weather and different techniques are evolved for:
- Conditions which permit continuous visual observation of the ground beneath.
- Conditions which limit visual observations of the ground to unpredicted intervals.
Map Reading in Continuous Conditions
By means of a time scale on the track, the pilot navigator should be prepared to look for a definite feature at a definite time. As a check on identification, additional ground detail surrounding the feature should be positively identified. Thus, when in continuous contact with the ground, map read from chart to ground.
Map Reading at Unpredictable Intervals
This technique is used when flying above or through broken cloud. The pilot should first estimate a circle of uncertainty for his position, based on a 10% error of the distance flown from his last known position. The pilot then studies the ground features over which he is passing, noting outstanding features and the sequence in which they occur. He then attempts to identify these features on his chart within the circle of DR error. This procedure is continued until some idea of the track flown is obtained. Thus, when seeking to establish position, map read from ground to map.
Use of Radio Aids
When map reading, the position of the aircraft is established relative to identifiable land features and the information is interpreted by means of a map. When using radio observations, the radio station takes the place of the landmark. Various different radio aids are available for air navigation.
Principles of Map Reading
Every pilot must be familiar with the general properties of various charts and with the conventional signs used for depicting the various ground features. The conventional signs are reproduced on the reverse side of most topographical charts and those used commonly on ICAO charts are reproduced as an appendix to the MAP section.
The chart should be folded so that complete track coverage is possible with the minimum number of page turns and without re-folding in flight. They should then be numbered and arranged in order of use. It is also a good idea to have an emergency set of charts in a readily accessible spot to relieve any embarrassing situation that might arise.
Chart scale is the ratio of chart distance to earth distance and may be given in one of three ways. The amount of detail which appears on a topographical chart clearly depends upon the scale; the larger the scale the more the detail and vice versa.
Elevation of the ground over which the aircraft flies is of vital importance it can be a valuable feature in map reading and a dangerous barrier to flight. Indications of ground elevation are indicated on charts in one or more of the following ways:
- Contours: Contours are lines joining points of equal elevation. The intervals at which contours are drawn depends on the scale of the chart, this interval is known as the vertical interval is noted on the chart. The horizontal distance between successive contours is known as the horizontal equivalent. The vertical interval on ICAO charts is normally in feet, but on some charts may be in metres: it is therefore imperative that the units are checked.
- Spot Heights: The highest point in a locality is marked by a dot with the elevation marked alongside. The highest spot height on some charts is given in a box. Spot heights are also given for the elevations of all airfields marked on the chart.
- Layer Tinting: Contours are usually emphasised by colouring the area between adjacent contours. The shades of colour chosen normally become deeper with increase of height; on ICAO charts the colours range from white through darker shades of yellow to brown.
- Hachuring: Hachures are short tapered lines drawn on the chart radiating from peaks and high ground. A spot height usually appears. Hachures are used on topographical charts only for incompletely surveyed areas and also on some plotting charts on which physical detail is not provided.
- Hill Shading: Hill shading is produced by assuming that a bright light is shining across the chart sheet so that shadows are cast by the high ground. Difficulty is caused when the shadow obliterates other detail and this method is not extensively used.
Relative Values of Features
Knowing the amount of detail to be expected on maps of different scales and given a knowledge of the conventional signs by which the detail is indicated, the map reader is in a position to appreciate the relative values of the features seen on the ground. The beginner is sometimes confused by the amount of detail confronting his untrained eye. He must learn to distinguish the more significant features and to remain undistracted by the irrelevant background. The following may help to indicate the type of feature which is of value to the map reader.
- Coastlines: Coastlines are most valuable by day or night. While it may be difficult to recognise a particular stretch of coast in an area merely by its appearance, a satisfactory degree of certainty can often be obtained by taking a bearing of its general direction. Study of any map will show how difficult it is to find half a dozen two mile stretches of coast similar in shape and bearing on the whole sheet.
- Water Features: As with coastlines, water features show up well by day and by night. Large rivers, estuaries, canals, lakes and reservoirs are the main water features in order of importance. In using them the season of the year must be taken into account, as in winter floods may cause considerable alteration in their shape, whilst in some parts of the world rivers dry up altogether during the dry season.
- Mountain and Hills: As an aircraft’s height above the ground increases, the countryside below appears to flatten out. Nevertheless the contours of prominent mountains frequently protrude above low lying cloud and mist and provide landmarks when all other features are obscured. In the case of low level map reading, contours assume great importance and even small hills are very helpful in fixing position.
- Towns and Villages: Built up areas are not usually of a distinctive enough shape to be valuable by themselves, but used in conjunction with other features such as rivers, railways and coastlines that lie through or adjacent to them, they are usually easily identified. Large cities are useful in determining the general area of the aircraft’s position, but accurate pinpointing must be done on other associated features.
- Railways: The identification of a particular stretch of railway is often difficult in well developed countries with many railways, particularly when the area of uncertainty is large. In the case of contact navigation, however, where the progress of the aircraft is being continually followed on the map, railways are very useful for position information. In countries with few railways a railway line is a feature of absolute value. Traffic along railways, by day or night, assists considerably by making them more conspicuous.
- Roads: As with railways, the value of roads depends on the extent to which the area has been developed. In the Sudan, for example, roads are of great value. In Great Britain they are practically useless as landmarks, both because of their multiplicity and the difficulty often encountered in distinguishing between major and minor roads. The modern arterial road generally stands out well.
- Woods: Woods make good landmarks, being clearly marked on maps, usually by green areas representing their shape and size. In heavily wooded or forested country the shape of clearings becomes the most valuable feature. Care must be exercised when using woods to fix position since tree felling may have changed their shape since the area was surveyed.
Symbols and Meaning
With modern high performance aircraft flight planning choice may well be restricted to the need to conform to operational limitations. This aspect of the subject is considered in Flight Planning.
Mental Dead Reckoning
Mental DR is the mental calculation of the aeroplane’s progress so that its position can be assessed, alterations to heading are determined and revisions of ETA are calculated as necessary
Track Error Lines
Lines drawn at angles of 5° or 10° either side of track through departure point and destination are most useful for quick estimation of track error and for estimating heading alterations.
Estimation of Track Error
As mentioned earlier, track error lines are useful for estimating alterations of heading quickly.
In the example above, the aeroplane position can be seen to be along the 3° line. The angle between planned track and track made good is therefore 3°.
Correction for Track Error
There are various geometric rules which can be used to correct for track error. It should be remembered that all methods assume that the drift will not change after small alterations of heading:
- When track error is measured from the departure point end of track heading should be altered towards the planned track by double the track error. When the planned track is regained an appropriate alteration is made to parallel track.
- When track error is measured relative to the destination, it is usually sufficient to alter heading towards the destination by the amount of track-error.
- When track error is measured from both ends simultaneously, alteration should be made towards the destination by the sum of the two measured track errors.
The 1 in 60 Rule
The 1 in 60 rule is another method of correcting for track error and is based on the fact that one nautical mile subtends an angle of 1° at an approximate distance of 60 nm, so:
- 3 nm subtends 3° at 60 nm
- 5 nm subtends 5° at 60 nm
In applying the rule, the triangle relevant to the problem is identified and the ratio of the long side 60 is established. This ratio may then be applied to the angle to reveal the length of the short side. Conversely, the ratio may be applied to the short side to determine the angle it subtends.
If the distance off track is known the track error can be calculated. In the example below:
5nm = 5 / 30 x 60 = 10°
60 / 30 x 5 = 10°
Remember that the 1 in 60 calculations are approximations.