| Code | Slope angle (degrees) |
|---|---|
| Wheelable | |
| A | 0 - 3 |
| B | 4 - 7 |
| C | 8 - 15 |
| Hilly | |
| D | 16 - 20 |
| E | 21 - 25 |
| Steep | |
| F | 26 - 35 |
| G | > 35 |
22 Soil setting properties
Many of the parameters described in Soil Setting are inherently categorical. Classifying the numeric measurements as well can be useful for efficient communication. However, in the absence of inherent groupings in these data, class boundaries need to be chosen carefully.
22.1 Slope classification
22.1.1 Land Use Capability slope classes
The most commonly-used slope classification in New Zealand is the one associated with the Land Use Capability (LUC) framework. These classes were formulated primarily to support pastoral agriculture, and the class boundaries focus on slope as an activity limiter, rather than an inherent landscape property. This means that the classification can potentially underperform in landscapes where natural slopes are close to the class boundaries. This is acknowledged in the Land Use Capabilty (LUC) Handbook (Lynn et al. 2009) by the inclusion of various options for modifying slope class.
The class boundaries are reproduced from Table 26 of the LUC Handbook in Table 22.1. Some generalised groupings are also included - these are discussed in Appendix 1 of the LUC Handbook but are not explicitly defined.
Tip 22.1: LUC slope limits
Note that the LUC Handbook does not provide fully closed limits for slope classes. For consistency, slope class A should be considered to be 0 - ≤4, B as >4 - ≤8, etc.
22.1.2 Natural Log classifications
Work by Speight (Speight 1971) promotes the use of slope classes with natural-log derived limits on the basis that they come closest to a universal set of ‘characteristic slopes’. The limits proposed are shown in Table 22.2. These classes are commonly used in Australian soil survey, and focus more on splitting up low-slope areas of the landscape. As such, these may be more useful for interpreting low-relief landscapes than the LUC slope classes.
| Code | Name | Slope angle (percent) | Slope angle (degrees) |
|---|---|---|---|
| LE | Level | 0 - ≤ 1 | 0 - ≤ 0.57 |
| VG | Very Gentle | > 1 - ≤ 3 | > 0.57 - ≤ 1.72 |
| GE | Gentle | > 3 - ≤ 10 | > 1.72 - ≤ 5.71 |
| MO | Moderate | > 10 - ≤ 32 | > 5.71 - ≤ 17.7 |
| ST | Steep | > 32 - ≤ 56 | > 17.7 - ≤ 29.25 |
| VS | Very Steep | > 56 - ≤ 100 | > 29.25 - ≤ 45 |
| PR | Precipitous | > 100 - ≤ 300 | > 45 - ≤ 71.6 |
| CL | Cliffed | > 300 | > 71.6 |
22.1.3 UN-FAO Classification
The following classification was offered by the UN-FAO Guidelines for Soil Description (UN-FAO 2006). The classes are highly discriminatory on low slopes, but lump most steeper slopes into only two categories. As such they may be of most use in irrigation planning.
| Code | Name | Slope angle (percent) | Slope angle (degrees) |
|---|---|---|---|
| 01 | Flat | 0 - ≤ 0.2 | 0 - ≤ 0.11 |
| 02 | Level | > 0.2 - ≤ 0.5 | > 0.11 - ≤ 0.29 |
| 03 | Nearly level | > 0.5 - ≤ 1.0 | > 0.29 - ≤ 0.57 |
| 04 | Very gently sloping | > 1.0 - ≤ 2.0 | > 0.57 - ≤ 1.15 |
| 05 | Gently sloping | > 2 - ≤ 5 | > 1.15 - ≤ 2.86 |
| 06 | Sloping | > 5 - ≤ 10 | > 2.86 - ≤ 5.71 |
| 07 | Strongly sloping | > 10 - ≤ 15 | > 5.71 - ≤ 8.53 |
| 08 | Moderately steep | > 15 - ≤ 30 | > 8.53 - ≤ 16.7 |
| 09 | Steep | > 30 - ≤ 60 | > 16.7 - ≤ 31.0 |
| 10 | Very steep | > 60 | > 31.0 |
22.1.4 Data-driven classifications
Where high-precision DEMs are available and the areal limits of landscapes are identified, data-driven slope classification is possible. This cannot be done universally; since the classification is an expression of local geomorphology, the area of interest must first be carefully delineated before proceeding. Classes defined in this way may be very useful in accurately defining landforms, but hard to convert to practical use.
22.2 Aspect classification
The broadest aspect classification available divides the landscape into flat/sunny/shady (@tbl-aspcl-ssh). In New Zealand, ‘sunny’ is roughly ‘north-facing’.
For more detail, aspect data can be classified by cardinal compass direction, in 4, 8, or 16 divisions as needed (N, NEE, ENE, etc).
compass rose image goes here, similar to the WRB’s one
22.3 Relief classification
The following classification system is used globally, and developed over many years (see e.g., Hammond 1954; Löffler 1977; Brabyn 1998; Speight 2009).
Relief classifications are scale-dependant; these are calculated by convention over a 600 m diameter window. The classification can be adjusted to a smaller or larger window size using the formula \(range_{new} = range_{old} × (window_{new} / window_{old})\), where all inputs are in meters.
| Code | Name | Range (m) | Average (m) |
|---|---|---|---|
| P | Extremely low | ≤ 9 | ~5 |
| R | Very Low | > 9 - ≤ 30 | ~15 |
| L | Low | > 30 - ≤ 90 | ~60 |
| M | Moderate | > 90 - ≤ 150 | ~120 |
| H | High | > 150 - ≤ 300 | ~200 |
| V | Very High | > 300 | ~500 |