Section 8. Resection
What is Resection?
Resection is a navigation technique used to determine your position on a map from features you can identify in the landscape.
In the previous section, you learnt that taking a bearing to a known landscape feature and plotting its back bearing onto the map produces a line. Your position must lie somewhere along that line, but a single bearing alone does not normally tell you exactly where along it you are.
Resection solves this by taking bearings to two or more known landscape features and plotting their back bearings onto the map. Each bearing produces another line on which your position must lie. Where the lines intersect indicates your position.
This is the reverse of following a bearing. When following a bearing, you know where you are and use a bearing to travel towards another location. With resection, you can identify features around you but want to use them to determine where you are on the map.
Modified Resection using a Known Linear Feature
Sometimes you do not need a second landscape bearing because you already know that you are standing somewhere along a recognisable linear feature, such as a track, ridge, river or road.
The linear feature already provides one line on which your position must lie.
You therefore need only one bearing to a known landscape feature. Convert that bearing to a back bearing and plot it from the identified feature onto the map. Where the plotted bearing line intersects the linear feature indicates your position.
In plain terms:
Known linear feature plus one plotted bearing equals your position.
This is called modified resection because the known linear feature takes the place of an additional bearing line.
When Should You Use Resection?
Resection is useful whenever you are uncertain of your exact location but can clearly identify prominent features around you.
You might use resection to:
• confirm your position on an unfamiliar ridgeline
• locate yourself after leaving a track
• check your progress during a bush traverse
• confirm your position before committing to difficult terrain
• resolve uncertainty before continuing your route
Understanding the Principle
Each bearing transferred onto the map creates a bearing line, also known as a line of position. Your position lies somewhere along that line.
A single plotted bearing tells you that your position lies somewhere along that line. While this may be sufficient to identify a feature or determine your position on a known track, ridge or spur, it will not usually pinpoint your exact location if you are not on some linear feature.
A second bearing from another known landscape feature creates another line of position. Where these two lines intersect represents your estimated location.
A third bearing can be used to confirm your position and improve confidence in the result.
Choosing Suitable Features
The accuracy of a resection depends largely on selecting suitable landscape features.
Choose prominent, well defined features that can be confidently identified both in the landscape and on the map. Isolated summits, distinctive rock outcrops, prominent saddles and other clearly recognisable landmarks generally provide the most reliable results.
Avoid broad ridgelines, indistinct hilltops, dense vegetation or vague skyline features, as they are more difficult to identify accurately and may reduce the reliability of your result.
Feature Separation
For the most accurate resection, choose landscape features that are well separated from one another. It is not enough for a feature simply to be prominent and easily identified. The angle between the bearings also has an important effect on the accuracy of your estimated position.
If the features are close together when viewed from your position, the plotted bearing lines will intersect at a shallow angle. Even a small error in either compass bearing can then cause a much larger shift in where the lines intersect on the map.
The most reliable results are usually obtained when the angle between the bearings is approximately 60° to 120°, with about 90° providing the best geometry. As the angle becomes much smaller, or approaches 180°, the effect of small bearing errors becomes increasingly significant.
The example image is intended primarily to demonstrate the effect of angular separation. The bearings to Isongerup North Peak, Third Arrow and Ellen Peak provide very different angular separations. Isongerup North Peak and Third Arrow are separated by only 24°, while Third Arrow and Ellen Peak are separated by 32°. By comparison, Isongerup North Peak and Ellen Peak are separated by approximately 56°, making them the strongest pair of the three.
In this particular example, however, your position is already known to be somewhere along the Ellen Track. Because the track provides a known linear feature, a single bearing to a correctly identified landmark can be sufficient to establish your position: where the plotted bearing line intersects the known track indicates your position.
The geometry is strongest when the bearing crosses the track at or close to a right angle (90°). As the bearing becomes increasingly parallel to the track, small bearing errors can produce a much larger displacement of the estimated position along the track.
The additional bearings shown here are therefore for teaching purposes, allowing the differences in angular separation to be compared rather than suggesting that three bearings would normally be required to locate yourself on this section of track.
More generally, before taking bearings, look around the landscape and, where possible, deliberately select identifiable features that are widely separated rather than simply choosing the most obvious features clustered together in one direction.
In practice, you will not always have the luxury of choosing perfectly positioned features. Terrain, vegetation and the surrounding landscape may limit what is visible from your location, making it difficult to find two well separated landmarks. Finding three suitable features can be even more challenging and is often impractical.
In these situations, use the best available features and interpret the resulting intersection alongside the surrounding terrain, tracks, contours and other known features rather than relying solely on the intersection of the bearing lines.
Understanding Accuracy
Resection provides an estimated position rather than an exact point.
Small inaccuracies are normal and may result from slight errors when taking a bearing, identifying a landscape feature, or plotting the bearing onto the map. Local magnetic interference can also affect compass readings.
Always compare the plotted position with the surrounding terrain. Your estimated location should agree with the ridgeline, spur, saddle, valley or other terrain features visible around you.
Plotting a Bearing from a Known feature to an Unknown Position on a Linear Feature
Once you have taken a bearing to a known landmark in the field, you need to transfer that bearing onto the map. The resulting line represents the direction between the known landmark and your unknown position.
There are two ways to plot this line.
Using a back bearing
Convert your field bearing to a back bearing by adding or subtracting 180°. Set this bearing on the compass, then place the edge of the compass through the known landmark on the map. Rotate the entire compass until the orienting lines are parallel with the map's north–south grid lines and the orienting arrow points towards Grid North.
The Direction of Travel arrow now points from the known landmark towards your unknown position. Draw a line from the landmark in this direction.
Where the bearing line intersects the known linear feature indicates your position.
Using the original bearing
You do not have to calculate a back bearing. Leave the original field bearing set on the compass and place the compass through the known landmark. Orient the compass correctly to Grid North as before.
This time, the Direction of Travel arrow points in the opposite direction, so extend the bearing line from the known landmark away from the Direction of Travel arrow.
This produces exactly the same line and therefore the same intersection with the linear feature.
In both methods, the principle is the same:
Known feature → plotted bearing line → intersection with known linear feature → your position.
Using a back bearing
Step 1:
Identify the known feature on the map from which you took the bearing. There is no need for the map to be orientated. In this example the known feature is Arthur’s Knob. Your unknown position is on a linear feature. In this example your are on Old Kojaneerup Track.
Step 2:
Ensure the bearing recorded in the field is converted to a back bearing. Compass bearing taken in field standing somewhere on Old Kojaneerup Track was 260° Magnetic bearing. Back bearing is 260° minus 180° = 80°. Subtract 1° West for Magnetic Declination. Set bearing on your compass to 79°.
Step 3:
Place one long edge of the compass on the map so that it passes through the mapped position of the known feature, Arthur’s Knob. Use either of the bottom corners of compass baseplate as the pivot point
Rotate the entire compass, ensuring baseplate edge is pivoting around known feature, until the orienting lines are parallel with the map's north–south grid lines and the orienting arrow points towards Grid North.
Step 4:
Draw the bearing line from the known feature in the opposite direction of the “Direction of Travel Arrow”. If baseplate is not long enough to reach your linear feature,(Old Kojaneerup Track”), use a ruler or possibly edge of the map folded. The point where the bearing line intersects the linear feature is your position.
2. Using the original bearing
You do not actually need to calculate a back bearing. Keep the original bearing on the compass, position the compass correctly through the known landmark, and extend the line from the landmark in the opposite direction to the Direction of Travel arrow. This produces exactly the same line of position.
Step 1 is the same as using a back bearing.
Step 2:
Compass bearing taken in field standing somewhere on Old Kojaneerup Track was 260° Magnetic bearing. Subtract 1° West for Magnetic Declination. Set bearing on your compass to 259°.
Step 3:
Place one long edge of the compass on the map so that it passes through the mapped position of the known feature, Arthur’s Knob. Use either of the top corners of compass baseplate as the pivot point
Rotate the entire compass, ensuring baseplate edge is pivoting around known feature, until the orienting lines are parallel with the map's north–south grid lines and the orienting arrow points towards Grid North.
Step 4:
Draw the bearing line from the known feature in the direction of the “Direction of Travel Arrow”. If baseplate is not long enough to reach your linear feature,(Old Kojaneerup Track”, use a ruler or possibly edge of the map folded. The point where the bearing line intersects the linear feature is your position.
Key Points for Resection
Modified resection uses a known linear feature as one line of position, allowing a single bearing to a correctly identified landscape feature to determine where you are along a track, ridge, road or other mapped feature.
Use modified resection when you know which linear feature you are on but are uncertain of your position along it, choosing a distinctive landscape feature that can be confidently identified both in the field and on the map.
Accuracy depends on the quality of the bearing and the angle at which it crosses the linear feature, with a wide intersection angle, ideally approaching 90°, providing a more reliable position than a shallow angle; accurate feature identification, declination correction and careful plotting are equally important.
Resection Using Three Features
Using three features provides an additional check on your estimated position. Rather than relying on the intersection of only two bearing lines, you take bearings to three clearly identifiable landscape features and plot all three onto the map.
In this example, the three features are Isongerup South Peak, Isongerup Main Peak and Isongerup North Peak.
Step 1:Take Bearings to Three Known Features
From the same position, take and record a bearing to each of the three features. The features should be clearly identifiable both in the landscape and on your map.
In this example, the magnetic bearings are approximately:
South Peak — 229°
Main Peak — 270°
North Peak — 323°
Each bearing is taken from exactly the same position. Before plotting them onto the map, convert the magnetic bearings to Grid bearings by applying the appropriate magnetic declination.
The three bearings now provide three independent observations of your position.
Step 2 :Plot the Three Bearings onto the Map
Plot each bearing back from its corresponding known feature towards your unknown position.
In this example, the resulting Grid bearings are shown as approximately 228°, 269° and 322°.
If every field bearing and every plotted line were perfectly accurate, all three lines would intersect at exactly the same point. In practice, this rarely happens.
Small differences can be introduced when sighting the feature, reading the compass, applying magnetic declination, identifying the exact point represented by the feature on the map, or plotting the bearing line.
As a result, the three lines may intersect at three slightly different points and form a small triangle.
Use Back Bearings (if preferred)
To plot each bearing back from the known peak towards your position, calculate its back bearing by adding or subtracting 180°.
Set the back bearing on the compass and place the compass edge through the known peak. Orient the compass to Grid North so that the Direction of Travel arrow points away from the known peak and towards your unknown position.
The back bearing is the opposite direction to the original Grid bearing, allowing the line to be plotted from the known peak back towards your position.
Triangle of Error
Any two bearings will intersect at a single point. With only two bearings, that intersection may appear to give an accurate position, but there is no third bearing to independently check the result.
In the enlarged example, each pair of bearings produces a different possible position: A, B or C. Each intersection could appear correct if only those two bearings had been taken.
The third bearing provides the additional check. When all three bearings are plotted and do not intersect at exactly the same point, the three intersections form a triangle of error.
Your actual position is likely to be somewhere within or close to this triangle. In this example, the triangle has sides of approximately 38 m, 45 m and 58 m, enclosing an area of approximately 884 square metres.
A small triangle indicates that the three bearings are consistent with one another and generally gives greater confidence in the estimated position.. A large triangle indicates greater disagreement and should prompt you to check the bearings, conversions, plotted lines and identification of the features rather than simply assuming that the centre of the triangle is your position.
The triangle of error is therefore useful information: it reveals uncertainty that would not be apparent from only two intersecting bearings.
Why the Angle Matters
The accuracy of a resection depends not only on how accurately the bearings are taken, but also on the angle at which the bearing lines intersect.
Bearings that cross at a strong angle produce a much more reliable position fix than bearings that meet at a shallow angle.
As a general guide, an angle of approximately 60° to 120° between bearings is desirable, with an angle close to 90° being particularly effective.
When two features lie close together in the same direction, even a very small bearing error can move their intersection a considerable distance across the map.
This is why the best landmarks for resection are not simply the most obvious landmarks. Where possible, choose features that are widely separated around your field of view.
Three well separated features provide three independent lines of position. When those lines converge into a small triangle, you have a strong visual indication that your estimated position is reliable.
Key Points
The lines do not have to intersect perfectly.
Small differences are normal when taking and plotting real compass bearings.
A small triangle is generally a good result.
It shows that the independent bearings are producing similar estimates of your position.
A large triangle is a warning.
One or more bearings may contain a significant error.
Do not automatically assume the centre of the triangle is your position.
Use the surrounding terrain, contours, altitude, tracks, ridges, gullies and other known information to refine your position.
Check suspicious bearings.
If one line is noticeably inconsistent with the other two, retake that bearing and check that you have identified the correct feature.