RESTRICTION GRADES:
A MEASUREMENT-BASED GRADING SYSTEM

• by Rory O’Keefe, September 1, 2022.

A restriction is defined as a point in the cave that is too narrow for a pair of divers to swim side-by-side. A major restriction adds the requirement for a diver to manipulate their body position or dive gear to pass the restriction.

In theory, it's a simple concept. In reality, divers vary in size, equipment, comfort levels, and skill. The result is that a minor restriction to one diver may be a major restriction to another.

With the variety of equipment configurations being used by modern cave divers, labeling restrictions as major or minor is no longer practical. By grading restrictions based on their dimensions, these discrepancies can be eliminated with a simple change in how they are labeled on map.

THE GRADING SYSTEM

The Survey Down Restriction Grades table uses incremental measurements to apply a grade to every restriction within an underwater cave system. The concept is to provide divers with a general idea of what to expect when planning a dive. It is not meant to indicate the precise dimensions of a restriction.

The system uses letters (A-F) to represent the width measurement, and numbers (1-6) to represent the height measurement. These grades will appear in place of the traditional “R,” or “X,” on a map. For example, using the table below, a grade of B6 indicates a restriction with a width of 90-120 cm, and a height of less than 40 cm.

The measurements will be based on the usable space within a restriction, rather than the longest possible dimensions of a restriction. Usable space is defined as the area within the restriction that is big enough to be of use to a diver. Additionally, the width and height dimensions are measured perpendicular to each other, however, these measurements are not required to be parallel to the floor or walls of the cave passage.

A higher resolution file of the full table is available here:

THE ASTERISK MODIFIER

Cartographers may use up to two custom asterisks to indicate recurring cave-specific conditions on a map.

A few examples of these circumstances include reduced visibility, a possible need to dig gravel or mud due to volatile flow conditions, long restrictions, and poor cave stability.

As asterisks have no standard meaning, they must be defined in the grading notes of a map. For example, if the asterisk modifier is customized to represent restriction lengths, the grading notes should define these increments as chosen by the cartographer:

• *Indicates a restriction of 3 meters to 6 meters in length.

• **Indicates a restriction of more than 6 meters in length.

The asterisk modifier is optional, and should only be used to represent features that have a significant impact on the usable space of a restriction.

BENEFITS FROM THE CARTOGRAPHERS PERSPECTIVE

1. Greater cartographic consistency.

Cartographers strive for consistency in the style and quality of their maps, and measurement-based restriction grades are an additional means to improve this for both independent and group projects. 

Divers will often notice a difference in what is labeled a minor, major, or no-mount restriction on cave maps made by different cartographers. This is due to factors such as the individual cartographer’s equipment configuration and experience. Additionally, cartographers are not always familiar with the varying equipment configurations used in modern cave diving, making traditional restriction grading challenging.

2. Grading smaller passages that are technically not restrictions.

Cartographers rely on floor-to-ceiling heights, cross-sections, and profile views to highlight challenging areas such as low-ceiling bedding planes that do not fit the traditional definition of a restriction. Unfortunately, these areas are often missed by the casual cave diver as they can appear much bigger than reality on the plan-view of a map. 

A measurement-based grading system can be used to call attention to these misleading passages. Because the grading system has the option to show passage dimensions “Over 120 cm / 48 inches,” cartographers can essentially apply a restriction grade to passages that don’t fit the standard definition of a restriction. Labeling these areas with a restriction grade will highlight that the usable space of a passage is smaller than it appears on the map. 

3. Remove the need for additional explanation.

A cave map that is designed to be used for navigation should include all of the necessary information that a diver needs to safely plan a dive.

With traditional restriction labels, it is common for divers to ask guides and other experienced cave divers about the size of a restriction. This indicates that cartographers should find a solution to provide that information on their maps. By using measurement-based restriction grades, cartographers will eliminate the need for supplemental information regarding the size of a restriction.

BENEFITS FROM THE DIVERS PERSPECTIVE

1. Reliable dive planning.

Measurement-based restriction grades provide divers with the approximate dimensions of every restriction in a cave. There are numerous scenarios in which knowing the usable dimensions of a restriction will benefit a cave diver. Examples of this include:

• Teams can avoid awkward situations where they are forced to turn around in a small passage after reaching a tighter-than-expected restriction.

• Restrictions and tighter passages can be better described on stick maps. 

• Restrictions that are used as a navigational reference, such as scenarios where a jump line is located on the other side of a restriction, can be better described.

• Divers who are trained in multiple equipment configurations can choose the best option for their planned dive. 

• Teams will have the opportunity to decide whether it would be best to reach a section of the cave by an alternative route based on their chosen equipment configuration or training level.

• Stage and DPV usage can be better planned. Teams will also be better informed if these tools are inappropriate for a specific section of the cave. 

2. Reduces peer-pressure.

This system provides each team member the opportunity to form an independent decision regarding a restriction based on their personal training and/or comfort levels. This decision can be made in private, without the risk of peer-pressure, and without relying on the experience and/or memory of another diver to accurately describe the restriction. 

3. Survey notes. 

Divers can quickly write down an estimated restriction grade while surveying for future reference. This is also useful for notes on potential leads through restrictions or quickly describing smaller passage sizes on a survey.

Measurement-based restriction grades will also provide additional insight into the cave when used on published stick-maps.

4. There is no fundamental change to the definition of a restriction.

A measurement-based grading system makes no fundamental change to the definition of a restriction, only how they are labeled.

Divers that prefer the traditional labeling methods such as the classic “R”, or “X”, are reminded that a restriction grade means that there is a restriction or restrictive passage at that point in the cave. These divers can ignore any meaning represented by the grade and be content in knowing the traditional level of information: “There is a restriction here.” Meanwhile, divers who wish to use the grading system can refer to the table and learn the dimensions of the restriction. 

As previously mentioned, any grade of A or 1 which represents a passage dimension of over 120 cm/48 inches, will only be used to highlight passages that are restrictive along one dimension.

CONS TO USING MEASUREMENT-BASED RESTRICTION GRADES

1. Restriction size will become a competition.

Certain personality types will immediately seek the bragging rights of passing an F6 restriction - the tightest possible restriction on the scale. 

It is unfortunate that this competitive mindset exists in cave diving, and as a cartographer, I am often evaluating the balance between withholding information from responsible divers in order to protect the irresponsible divers. 

To prevent competitive restriction-chasing, I chose to stop the scale at “Less than 40 cm/16 inches,” even though it would arguably make sense to implement two additional grades beyond this point. An F6 restriction of 16” x 16” or less is small enough to indicate that partial gear removal will be required, but it is not so small to be considered a competitive challenge. Adding more grades beyond this point will absolutely become a competitive challenge. 

2. Restrictions are not perfect squares.

The reality of the situation is that the cartographer’s opinion on restriction size will not be entirely eliminated. There’s no way to account for every specific scenario, and cartographers will occasionally have to use their best judgment on how to grade restrictions that cannot be easily measured. 

Fortunately, cartographers can at least use the approximate measurements in combination with their opinion to grade unusual restrictions, negating influence from other factors such as body size, equipment configuration, skill, and personal comfort. 

3. The A and 1 grades are too big to be restrictions.

Divers will notice that the first measurement range of “Over 120 cm/48 inches” is larger than what is typically considered to be a restriction. In fact, a grade of A1 is not a restriction. So, why does this possibility exist?

The restriction grading scale starts at “Over 120 cm/48 inches” so that cartographers have the ability to grade passages that are small across one dimension but long across the other. For example, a low bedding plane would fall outside of the standard definitions of a restriction, however, it should still be treated as a restricted passage, particularly by introductory-level cave divers using a sidemount configuration. 

4. It complicates a simple system.

Divers are reminded that the grading system makes no fundamental change to the definition of a restriction. When divers see a restriction grade, it means that there is a restriction or restrictive passage at that location in the cave. Those who do not wish to use the table may simply ignore the grading system and be content in their knowledge that there is a restriction at that point in the cave. 

Conclusion

To summarize the main reasons of why measurement-based restriction grades are useful:

• Creates consistent restriction grading across multiple maps from different cartographers.

• Adds needed explanation of restriction size on a map so that divers can plan appropriately. 

• Divers will not rely on the personal opinion and/or memory of other divers when discussing the size of a restriction.

I will be implementing these measurement-based restriction grades on my upcoming maps, and I welcome other cartographers to do so as well. 

Constructive feedback is welcome and can be forwarded to: info@surveydown.com

This article was updated on October 23, 2024 to include information regarding the use of asterisks to indicate cave-specific conditions.