Based on the paper by Caleb Gould (GBR) presented at the first IOF Controllers' Clinic but illustrated by examples from World Cup 2001.

There is a range of different problems which can be set by Trail-O planners in their selection of control sites and the placing of flags to give Elite Trail-O competitors the necessary variety and technical level of challenge.

This paper attempts to identify and categorise the different types of problem which can be constructed for elite competition. At elite level, control sites usually present a combination of types of problem.

All the examples listed here are from the three days of the World Cup Trail-O held in conjunction with this clinic.


Distance estimation

This is usually one element of a more complex solution where distance estimation of features or marker flags in the terrain is compared with distances measured on the map to assist in distinguishing between similar marker positions. Where distance estimation is necessary the '25% rule' applies (See Controlling notes).

Example: 1E4 Re-entrant. The correct re-entrant is identified but has two flags fitting the description. Distance estimation from the viewing point has difficulty in separating them. Distance estimation from the boulder pair further up the re-entrant is conclusive. Day 1 map Day 1 solutions

Bearing estimation

a) Coarse bearing. This may be used to help identify which of several well-separated features is that associated with the control circle on the map.

Example: 2E6 Bare rock, west side. With several areas of bare rock and two ring contours there could be uncertainty but the compass quickly confirms which feature is to be concentrated upon. Day 2 map Day 2 solutions

b) Fine bearing. More precise estimation may be required, but the marker flags of interest must be separated by not less than 5 degrees.

Example: 2E2 Spur. All five marker flags fit the description. The angular separation of the flags from the viewing point is less than 5 degrees so a compass cannot be used from that point. However, bearings from the path end opposite the spur have separations of more than 5 degrees and allow the correct marker flag to be identified. Day 2 map Day 2 solutions

Which feature of several?

This is the selection from a number of similar features on the map in a small area. The problem is to identify which feature is the correct one using a choice of techniques, including the description.

Example: 1E10 NE Rock face. All five flags are at the foot of rock faces but the interrelation between them, on the map and in the terrain, allows the correct feature to be readily identified. Day 1 map Day 1 solutions

Example: ME1 Between the middle boulders. This is a very testing problem with boulders everywhere. However, starting with the very large boulders and identifying them on the map, then relating the less large boulders to them until all the mapped boulders have been identified, allows the correct marker flag to be pinpointed. Model event map Model event solutions

Which edge?
A common problem is a single area feature, such as a marsh, and the competitor has to decide which edge is the correct one. The problem can be solved by careful attention to the circle position, description, adjacent features, bearings, etc.

Example: 1E9 Marsh, SW edge. Initial inspection gives three marker flags fitting the description, the correct one being identified by relation to the small spurs nearby. Day 1 map Day 1 solutions

Sometimes a single small feature lends itself to the edge problem.

Example: 2E12 Pit SW edge. All five marker flags are clearly on the edge of the pit. The problem is to deduce from the sighting positions available which flag is most likely to be correct Day 2 map Day 2 solutions

Which side?
Another common problem, useful for NE/NW/SE/SW directions which are more often confused with one another.

Example: 1E14 S Boulder SW side Day 1 map Day 1 solutions

Which part ?
Simple terrain recognition, involving the choice of a number of parts of an area feature. Normally there is another feature to key in the problem.

Example: 1E8 Hill W part. This a flat hill with another ring contour to the north which, from the track, appears to be part of the top of the larger hill. Careful study allows the map to be fitted to the terrain and the correct marker flag selected. Day 1 map Day 1 solutions


Indistinct boundaries and unmapped features

This includes the edges or ends of features which are distinct on the map but much less so on the ground. Examples include the edges of marshes and the ends of tapering ditches and gullies. Use of such features is possible if there are adjacent features which allow the position of the marker flag to be adequately judged.

The use of unmapped features can provide useful problems. These features are legitimately unmapped because they fall below the mapping threshold that the surveyor has set, but there is potential for confusion with similar features which are prominent enough to be mapped.

Example: 2E14 Thicket E edge. Here the terrain is not as clearly defined as the map suggests. There are a number of patches of vegetation which are not on the map and these require elimination to identify the edge of the larger area of thicker vegetation which has been mapped. Day 2 map Day 2 solutions

Example: ME2 Boulder E side. Here, small unmapped boulders have marker flags and have to be eliminated. Model event map Model event solutions

Subtle Shapes

These are sites where the ground shapes are less obvious on the ground than the contours which portray them on the map. Recognition of such ground detail may require some attention.

Example: 2E15 Hill NE side/edge/part. It is not easy relating the mapped contour to the NE section of the hill, hence the difficulty of describing the marker flag position. Day 2 map Day 2 solutions

Complex Ground Shapes

Terrain recognition to a high level is required, with the competitor needing to analyse a complex area of ground shapes not always with the aid of other fixing features.

Example: ME7 NE Bare rock SW part. An area of many spurs and re-entrants with patches of exposed rock. The problem concentrates on the bare rock but the form line contour detail allows the various areas to be identified. Model event map Model event solutions

Floating viewing point

In these cases the position of the viewing point cannot be fixed and, in turn, cannot be used for fixing positions of the flags. These have to be deduced from inspecting the relationships between the features in the terrain.

Example: 2E11 Rock face N foot. The inability to use the floating viewing point to fix the rock faces requires the terrain to be carefully analysed and related to the map, especially as the circled feature, in this instance, is not marked with a flag, giving a zero answer. Day 2 map Day 2 solutions

Parallel similar features

The existence of two or more parallel similar features can be used to set testing problems. The intention is invite misidentification of which feature is which. One version is to have an apparently correctly-sited marker flag on the wrong feature and a wrongly-sited marker on the right feature, leading to a zero result.

Another is to have the correct feature unmarked with the parallel feature(s) bedecked with marker flags.

Example: 1E5 Hill NW side. Three marker flags are clustered on the parallel ring contour to the NE of the correct feature. A further marker is on a boulder to the NE, this boulder possibly being confused as the second ring knoll. Any reference to the pit by the path clearly indicates the correct ring knoll and a zero result. Day 1 map Day 1 solutions

Ground Height

The ground height and shape can be deduced from the visible height of the flags (provided the Planner takes care to ensure that all flags are installed at a standard height).

Example: ME5 Hill SE side. The top of the marker flag is just visible, showing that it is on the SE side of the hill. It is not necessary to seek further confirmation but this is obtained by viewing from a point much further along the track which has a clear line of sight past the hill to the marker. Model event map Model event solutions


Back Marker

This is when the key to the problem is a feature behind the competitor, for example on the other side of the viewing track. This can be used in different ways:

a. to locate the viewing point, which is then used as the base for locating the correct flag;

Example: 1E3 Rock face foot. The pit by the track allows the viewing point to be located which confirms the correct cluster of rock faces. Day 1 map Day 1 solutions

b. to locate directly the correct flag.

Example: 2E2 Spur. The path/track junction acts as a sighting point for compass bearings. Day 2 map Day 2 solutions


This is the extension of a linear feature, typically from the far side of the viewing track, to fix the position of the required flag.

Example: 2E14 Thicket E edge. The path extends directly to the required marker. Day 2 map Day 2 solutions

Leading marks

These are separated mapped features which, when placed in line, point directly to a marker flag or feature. This can be a valuable technique, allowing flags to be precisely located over long distances without nearby confirming features.

Example: 1E7 Spur. The control is on the distant spur which has three markers, two of which are precisely located by pairs of boulders in line. The second of these precisely positioned markers is the correct one. Day 1 map Day 1 solutions

The technique is also useful for hidden or dead ground.

Example: ME6 Ditch/path crossing. Sighting from the viewing point shows all the marker flags to be on the ditch. From a point further along the track the ditch becomes invisible, and the path crossing it, but the position of the crossing can be fixed by interpolation between the boulders and the marker flag identified. Model event map Model event solutions

Secondary Viewing Position

A secondary viewing position gives a clear indication of the correct marker flag. The competitor then has to move (back) to the viewing point to select the letter for this flag.

Example: 2E10 Ditch bend. The bend is a very shallow angle and cannot be determined from the viewing point but viewing along the ditch from a point further along the track clearly shows the bend. Day 2 map Day 2 solutions


Ghost Features

Similar features to that indicated by the control circle appear to be present on the ground but are correctly not shown or shown differently on the map. These 'ghost' features are flagged and the competitor's task is to distinguish between them and the mapped feature.

Example: 2E4 Knoll. There are other knoll-like features on the ground, particularly one on the form line spur. The correct knoll is less visible and some distance away but correctly identified by reference to the nearby vegetation boundary. Day 2 map Day 2 solutions

Wrong Feature

All flags are on a nearby similar feature but, if the viewing point position is reasonably carefully determined, this wrong feature becomes clear and the zero answer is obtained.

Example: 2E7 Knoll. The confusion here is between the correct knoll mapped as a dot and the larger ring knoll to the west. Day 2 map Day 2 solutions

Precision of Description

This is the case where there are many flags close together and the key to the problem lies with careful reading of the description sheet. In such cases it is not possible to determine directly the answer from the map and bearings alone.

Example: 1ET SE Boulder SW side. This is particularly testing because the map segment for the timed control is aligned in the direction of view, in this case SE, but the pictorial description is normal. Although the correct boulder is readily identified, the SW side is not. Day 1 timed control Day 1 solutions

Overlapping Sites

This is where two or more control sites use one cluster of flags. It is good practice for not all of the flags to be visible from each of the viewing points; the disappearance of flags and fresh ones appearing as the competitor moves from one viewing point to the next add interest.

Example: 2E8 and 2E9 Both upper rock face. This is a testing example as one of the marker flags from the E9 cluster is distantly but clearly visible from the E8 viewing point and has to be included as it is on the left hand side of the sequence. Day 2 map Day 2 solutions


This occurs when the flags change their left-to-right identifications when viewed from different positions. This is particularly useful when secondary viewing points have to used to identify the correct marker.

Example: 1E13 NE ditch bend. This site has a tight cluster of marker flags which require identification from a secondary viewing point. The correct marker has to then viewed again from the viewing point to determine its answer letter. Day 1 map Day 1 solutions


For timed controls the problem needs to be clear and not require the competitor to move around to establish the answer. All the marker flags must be clearly visible from inside the viewing window. It must be remembered that competitors only have one minute to choose which flag marks the correct position. Day 1 timed control Day 1 solutions Day 2 timed control Day 2 solutions

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