Map Puzzles

Control: Edge of the Meadow

Submitted by IntoTheWoods

on Mon, 11/21/2011 - 23:55

Control Desciption: A-D open land, northern edge, viewing station facing northeast

I'm not much of a fan of distance estimation and compass bearings for trail orienteering, but sometimes you have to use them as a last resort, and this control appears to be a case in point. At first, I thought the circle might be equidistant from the corner of the field on the left and the bush that extends into the meadow, but it's not quite centered. My approach to the control would be as follows. First, I would take bearings from the trail intersections on either side of the viewing station. It is important when taking a bearing to do so from a point feature. In this case, while I might take a bearing from the tip of the white woods along the trail to the right, I would not use it as the primary way to identify the control, only as possible confirmation, because where the white woods ends is difficult to determine reliably.

Tip: Trail orienteering does not require bearing estimation to be better than five degrees. If controls are bunched closer than this, you should find another way to solve the puzzle.

Next, I would place my compass with one corner (you are using a baseplate compass right?) at the trail junction to the right of the viewing station, aligned as

shown. You can then measure from the left edge of your compass to the center of the circle, in this case approximately 36mm, convert that to on-the-ground distance, pace count along the trail, align yourself perpendicular to the trail and should expect to see the correct control directly in front of you. Hopefully, by this point, you have convinced yourself that either A or B is the correct answer. You could at this point try to walk back and forth along the trail using the parallax effect to determine that B in this case is actually away from the edge of the field, but an easier way to make that determination is simply to walk around to the trail on the top right part of the map. From that vantage, it should be clear that B is not at the edge of the meadow.

Tip: Trail orienteering does not require distance estimation to be better than twenty five percent. If controls are bunched closer than this, you should find another way to solve the puzzle.

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Control: Cliff Identification and Marker Placement

Submitted by IntoTheWoods

on Sun, 11/20/2011 - 18:40

Control Desciption: A-D (D is not visible in the photo), cliff, foot, northern part, viewing station facing east

The first challenge here is to match the features of the map snippet to the features in the terrain. At first, you might think that the features aren't mapped correctly as there appear to be three cliffs in the photo and only two on the map. However, the ISOM (International Specifications for Orienteering Maps) standards indicate that the minimum height of a mapped cliff should be one meter. In other words, only the portions of a cliff that are one meter or taller should be mapped. Unfortunately, orienteering maps in the United States don't always meet this exact standard. Mappers and field checkers often make

subjective decisions that deviate from ISOM standards and maps are invariably imperfect. This is why it is important to read and identify as many features along the course as possible - to get a feel for how the mapper thinks - even though this may not directly impact the various controls along the way.

In this case, however, the rounded cliff is quite clear in the center of the photo (taken from the viewing station) which means that the long straight cliff face in the map is the one on the right side of the map, and the shorter cliff on the left side of the photo is unmapped.

This eliminates markers A, B, and D. (Marker D is not observable in the photo.) Next, we must determine whether marker C is in the correct position along the cliff, in particular on the northern part (left side from our vantage point) of the foot. (Tip: If the control description just states "cliff" without any qualifier, the marker should be located in the center of the cliff at its foot as per the IOF standard.) This is harder to tell from the photo, but notice that although the cliff extends quite a bit farther north (left) than the marker is placed, it is less than 1m in height for a good portion of this length. A better vantage point for making this determination is from further along the path, facing the cliff head on so that the face is equidistant and perpendicular to your viewing direction.

Solution: C

Tip: To help estimate where the edges of a mapped cliff are in the terrain (i.e. where the cliff is at least 1m in height), use the height of the control as a reference. A control is always 0.3m x 0.3m in dimension.

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Control: Bend in Stone Wall

Submitted by IntoTheWoods

on Tue, 10/25/2011 - 00:45

This is a fairly typical beginner level control in trail orienteering. The challenge here is simply to identify the feature, in this case the bend in the stone wall, and determine whether there is a flag at the feature. In the photo, markers A, B, and C are visible, but marker D is cut off at the right edge of the photo. From the vantage point of the photo, taken at the decision point, it may not be obvious which is the correct choice. This is why it is always very important to move back and forth along the trail, as how the markers and wall move in relationship to each other lets us understand where the markers are placed. In this case, marker C will appear to move faster than the wall as you walk along the trail because it is closer to you. This effect is called parallax and can be very useful in making decisions in trail orienteering. For this control, one of the best places to study the problem from is along the site line of the wall. From this vantage point, near the right side of the map snippet, you would be able to determine that markers A and B are against the wall and that A is too far back to be correct. You would also be able to eliminate choices C and D.

If you have time, try to look at other features in the area even if they aren't important for solving your current problem. In this case, see if you can find the small cliff and boulder in the map snippet. Identify the light and medium green vegetation. More experienced trail orienteers will want to also mentally place the contour lines from the map into the terrain. Where would the index contour emerge from the road? How would it define the small spur? Trace the contour from the boulder and see how it shapes the upper part of the spur.

Solution: B

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