ss_03_landscape.qmd

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output: html_document
editor_options: 
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# Geomorphology {#sec-gmph}

```{r}
library(dplyr)
library(tibble)
library(gt)
library(gtExtras)
```

Geomorphology limits the range of soils than can occur at specific locations on the basis of slope, relief, aspect, and drainage. Geomorphology can be described at multiple scales, which break down into progressively simpler shapes. The data points described in this chapter help the pedologist to do that consistently.

## Province {#sec-gmph-prov}

Landscape Provinces identify major geomorphic zones in New Zealand, expressive of large-scale lithological associations, tectonic regime, and climate. @tbl-ss-lftp lists their names and @fig-ss-lftp shows their general distribution.

|   | Northern Provinces |   | Southern Provinces |
|:-----------------|:-----------------|:-----------------|:-----------------|
| [NN]{.ceg} | Northland | [SI]{.ceg} | South Island axial mountains |
| [NL]{.ceg} | Northern Lowlands | [SR]{.ceg} | Front Ranges and inland basins |
| [NW]{.ceg} | North-western sedimentary hills | [SO]{.ceg} | Otago basin and range |
| [NV]{.ceg} | Central Volcanic Zone | [SF]{.ceg} | Fiordland |
| [NT]{.ceg} | Stratovolcanoes and ringplains | [SW]{.ceg} | West coast piedmont, hills and mountains |
| [NI]{.ceg} | North Island Axial Mountains | [SN]{.ceg} | Northwest Nelson |
| [NS]{.ceg} | Southern and Eastern Sedimentary Hills | [SS]{.ceg} | North Bank and the Marlborough Sounds |
| [NM]{.ceg} | Marine Terraces, Plains and Dunelands | [SM]{.ceg} | Marlborough Mountains |
| [NA]{.ceg} | Accretionary wedge and old sedimentary hills | [SP]{.ceg} | East Coast Peninsulas |
| [NC]{.ceg} | Chatham Islands | [SE]{.ceg} | East Coast Plains and Downs |
|  |  | [SL]{.ceg} | Southland mountains, hills and plains |
|  |  | [ST]{.ceg} | Stewart Island |

: Landscape provinces of New Zealand {#tbl-ss-lftp .hover .striped tbl-colwidths="\[5,45,5,45\]"}

![Landscape provinces of New Zealand - indicative spatial distribution.](data/NZ_provinces.png){#fig-ss-lftp .lightbox width="85%" fig-alt="A map of New Zealand depicting the boundaries of landform Provinces."}

::: {#nte-prov .callout-note collapse="true" title="Province descriptions" appearance="minimal"}
Northern:

| Name                                             | Description |
|:-------------------------------------------------|:------------|
| **Northland**                                    |             |
| **Northern Lowlands**                            |             |
| **North-western sedimentary hills**              |             |
| **Central Volcanic Zone**                        |             |
| **Stratovolcanoes and ringplains**               |             |
| **North Island Axial Mountains**                 |             |
| **Southern and Eastern Sedimentary Hills**       |             |
| **Marine Terraces, Plains and Dunelands**        |             |
| **Accretionary wedge and old sedimentary hills** |             |
| **Chatham Islands**                              |             |

Southern:

| Name | Description |
|:-----------------------------------|:-----------------------------------|
| **South Island axial mountains:** | Steep to precipitous, high, glaciated mountains of the South Island Main Divide and associated ranges with strong structural and lithological control of the macro-topography. |
| **Front Ranges and inland basins** | Steep to very steep dissected mountain ranges with extensive scree, sharp crested peaks and ridges, and deep colluvial and moraine mantled footslopes, separated by wide braided river valleys and fault controlled inland basins with extensive glacial outwash terraces, moraines, and lakes. Predominantly Torlesse greywacke sandstone and mudstone, and tz II schist of acid-intermediate quartz feldspathic composition. |
| **Otago basin and range** | Prominent steep-sided and flat to rolling broad-crested ranges with tors, separated by wide deformation controlled flat-floored sediment-filled basins and valleys with extensive glacial moraine, outwash terraces, and recent valley fill. Predominately schist (tz IIa – tz IV) with minor Torlesse sedimentary rocks of acid-intermediate quartzo- feldspathic composition. Tertiary aged sedimentary soft rock inliers incorporating minor alkali to basaltic volcanics underlie the floor floors, with remnants present on ridge crests as outliers and fringe the coastline in places. Extensive loess mantles lower elevation slopes. |
| **Fiordland** | Steep to very steep to precipitous alpine and valley slopes with cirque basins, U-shaped valleys, rolling tops, and coastal fiords developed on hard, massive, coarse-grained, crystalline acid to basic igneous and metamorphic rocks, minor Paleozoic sedimentary, calcareous, and ultrabasic rocks all with very shallow, weakly weathered regoliths. Minor Tertiary sedimentary lithologies and valley floor in-fill. |
| **West coast piedmont, hills and mountains** | Dissected, extensive fluvioglacial outwash terraces, moraine ridges, and marine beach terraces; Recent broad valley floor floodplains, terraces, and fans infilling abandoned glacial troughs; extensive lowland moderately steep to steep hillscapes; and steep, glaciated and intensely fluvially dissected mountain ranges of granite and gneiss, or ancient sedimentary rocks (greywacke) rocks with a drape of less well indurated Cretaceous-Tertiary cover beds in places. |
| **Northwest Nelson** | Steep to very steep extensively glaciated mountain ranges with an exhumed peneplain and lithologically controlled topography underlain primarily by hard to very hard heterogenous Paleozoic sedimentary and acid (granitic) igneous and altered volcanics rocks, marble, and the remnants of hard and soft Cretaceous-Cenozoic cover beds. The region includes the deeply weathered Pleistocene gravels of the Moutere depression. |
| **North Bank and the Marlborough Sounds** | Steep to very steep dissected mountains with angular ridges and narrow valleys dominated by indurated quartofeldspathic sedimentary and lower grade schist rocks \[tz IIa, IIb\]; a belt of ultramafic and associated volcanic rocks characterised by irregular, hummocky topography; and limestones, volcaniclastics, and basalts to the west. Structural and lithological control of the landscape is evident with thick sandstone sequences forming ridge crests, and valley profiles showing strong asymmetry with prominent dip slopes reflecting the regional schistosity. |
| **Marlborough Mountains** | Steep to very steep fault splintered structurally controlled intensively dissected sharp crested mountain ranges with extensive bare rock and scree, rock glaciers and minor cirque basins, and extensive steep to very steep dissected lower mountain and colluvial footslopes, separated by long, straight intermittently terraced valleys and minor intermontane basins. Underlain by acid-intermediate quartz feldspathic Torlesse greywacke rocks, with minor basic igneous plutons, dikes, and sills; with remnants of Cretaceous-Pliocene covering strata preserved in fault-angle depressions and along the coastal margin. |
| **East Coast Peninsulas** | Strongly rolling, moderately steep and steep, dissected collapsed intermediate to basic volcanic peninsulas with bedrock, mixed volcanic bedrock and loess, and loess-dominated colluvium. |
| **East Coast Plains and Downs** | Extensive, coalescing, broad outwash piedmont fans, aggradational and degradational terraces, lower fan, and low-gradient coastal plain depositional sequences; associated interior basins, and the rolling to strongly rolling to moderately steep hills and downlands underlain by moderately soft and hard young non-calcareous and calcareous rocks, greywacke, and weathered older gravels, with or without a significant loess mantle. |
| **Southland mountains, hills and plains** | Steep to very steep glaciated mountains shedding volcanic and basic-quartzofeldsphathic colluvium, moderately steep to steep hills with prominent strike ridges developed on andesitic sedimentary rocks, with an intermittent loess mantle; moderately steep to steep hill country developed on deeply weathered plutonic rocks, and the alluvial landscapes sourced from the Fiordland and Wakatipu glaciers; moraines and extensive coalescing and overlapping alluvial aprons, and suites of outwash terraces, fans and floodplains; coastal Recent and older sand dunes, gravel beach ridges, alluvial lagoonal deltaic flats and extensive peat bogs; and encompassed hard and soft rock cored downlands and hills. Deep loess mantles a significant proportion of all topographies but is thickest on the terraces and rolling downlands. |
| **Stewart Island** | Steep to very steep mountain slopes with minor cirques and moraines; and strongly rolling to moderately steep, dissected, hill and plateau slopes developed on coarse-grained deeply weathered crystalline granitic- to dioritic plutonic rocks; with steep cliffs interrupted by beaches backed by large sand dune complexes extending inland on the western coastline and the drowned valleys of Paterson Inlet, Port Adventure, Lords River and Port Pegasus on the east; extensive sand plain deposits form gently east-dipping flights of terraces throughout the Freshwater River catchment. |
:::

Given their very broad extent, region does not need to be recorded in the field. However, the province boundaries are not currently strictly defined so an understanding of their conceptual basis (@nte-prov) is required before a confident allocation can be made.

## Landscape {#sec-gmph-lsc}

The landscape around a soil observation comprises a distinct geomorphological pattern with repeating elements. The scale of observation varies but commonly extends from a few hundred to a few thousand meters [@heck2017]. Landscapes nest within Provinces but are not exclusive to one or another.

### Parameterising landscapes {#sec-gmph-lscpar}

A landscape can be described in terms of a short list of parameters: relief, characteristic slope, drainage pattern, and the landforms contained within the landscape.

#### Relief {#sec-gmph-lscrel}

Relief is the degree of separation between high and low points in a landscape. As described in [@nationalcommitteeonsoilandterrain2009, pp. 45], relief can be visualised by picturing two surfaces roughly parallel to the land surface - one passing through local crests and one through local depressions. The median vertical distance between the two surfaces is the relief.

[add a diagram here]{.diags}

Relief is usually best determined from a DEM, but can be estimated in the field. Record in whole meters if estimating, e.g. [55 m]{.ceg}. Relief classes may be applied to the recorded data; see @sec-reliefclass.

#### Characteristic slope {#sec-gmph-lscslp}

The characteristic slope is the most common range of slopes within a regional landscape. These are often controlled by local geology.

Characteristic slope is usually best determined from zonal analysis of a DEM-derived slope dataset, but can be estimated in the field. Estimate in the field as a median and range, in whole degrees, e.g. [25° (20-28)]{.ceg}. Slope classes may be applied to the recorded data; some options are presented in @sec-slopeclass.

::: {#nte-multislopes .callout-note appearance="simple" collapse="true" title="Multiple characteristic slopes"}
Some landscapes may have two or more distinct characteristic slopes, e.g. the fronts and tops of alluvial terrace elements. In this case the median will describe the dominant surface while the range will capture the steeper areas. Recording multiple characteristic slopes at landscape scale is not considered useful, as adequate context is provided by naming the landscape and its component landforms.
:::

#### Stream channels {#sec-gmph-lscchn}

Natural stream channel patterns are relevant to understanding landscape development, but are best observed remotely. They do not need to be recorded in the field. If desired, assess the landscape drainage pattern using available data sources and define it using the stream channel attributes given in @nationalcommitteeonsoilandterrain2023a, p. 26-29.

#### Landscape description {#sec-gmph-lscdsc}

Few controlled vocabularies for landscapes exist. The schema given below is provisional and based on terms seen in common use in New Zealand environmental datasets.

```{r}
#| label: tbl-hr-lsnames
#| tbl-cap: "Common Landscape terms"

dat_hr_lsnames <-
  tribble(~Code, ~Name, ~Description, ~Synonyms,
          # natural
          'Mt', 'Mountains', 'Steep to very steep, high relief (>300 m), fixed and well connected stream channels, often deep', 'Alps, Volcanoes, Mountain country',
          'Hi', 'Hills', 'Gentle to very steep, moderately high to high relief (90-300 m), fixed and well connected stream channels, shallow to deep', 'Hillscapes, hill country, hilly land',
          'Up', 'Uplands', 'Gentle to very steep, low relief (30-90 m) but relatively high elevation, fixed and well connected stream channels, shallow valleys and plateaus, located among hills and mountains', 'Piedmont',
          'Ll', 'Lowlands', 'Gentle to very steep, low relief (30-90 m) and elevation, fixed and well connected stream channels, shallow to deep', 'Downlands',
          'Te', 'Terraces', 'Former plains, uplifted and sometimes covered by loess. Flat to gently undulating upper surfaces bounded by distinctive drop-offs on their downhill end, often backing on to lowlands, hills or mountains at their uphill end.', 'Marine benches',
          'Pl', 'Plains', 'Level to very gentle surfaces created by fluvial and/or airfall tephra deposition.', 'Floodplains, ringplains',
          'Sc', 'Sand country', 'Extensive dune and sandplain areas. Low relief and elevation, gentle to steep slopes, and active stream channels, often with interrupted connectivity', 'Dunefields',
          'Cs', 'Coasts', 'On- and near-shore features created by the interaction of the ocean with the land, including beaches, deltas, lagoons and estuaries', 'Coastlines'
          )

tbl_hr_lsnames <- gt(dat_hr_lsnames) |>
   tab_options(
    column_labels.font.weight = 'bold', 
    heading.title.font.weight = 'bold',
    table.align = 'center',
    table.width = '85%'
  ) |>
  tab_style(style = list(cell_text(color  = 'red',
                                   weight = 'bold',
                                   align  = 'center')),
            locations = cells_body(columns = 1)) |>
  cols_label(Synonyms = 'Synonyms and subtypes') |>
  cols_width(Code ~pct(10), Name ~pct(20), Synonyms ~pct(20))

tbl_hr_lsnames

```

#### Composition {#sec-gmph-lsccmp}

A breakdown of the major landforms expected within the landscape should be included (see @sec-gmph-lfm below) along with an estimate of their relative dominance (e.g. [Cs: Bc 80%, Du 10%, Cl 10%]{.ceg} for a coastal landscape comprising steep cliffs fronted by a narrow beach system with some limited dune development in places where the beach is wide enough to allow it).

## Landform {#sec-gmph-lfm}

A landform is a component of the landscape, and again can have a variable extent - usually 10's to 100's of meters. A landform has a relatively simple shape and is not itself composed of multiple repeating elements in the way that a landscape is. A landform has a characteristic range of slopes and aspects. Landforms also have distinctive composition in terms of geology/parent materials and are usually restricted to particular locations within a landscape.

### Naming Landforms {#sec-gmph-lfmnm}

Few controlled vocabularies for landforms exist. The schema given below is provisional and based on terms seen in common use in New Zealand environmental datasets and related publications.

```{r}
#| label: tbl-hr-lfnames
#| tbl-cap: "Common soil-bearing landforms of New Zealand (adapted from @milne1995, @johnson2004, @tielidze2021, and @nationalcommitteeonsoilandterrain2023a)"

dat_hr_lfnames <-
  tribble(~Code, ~Name, ~Description, ~Synonyms,
          # natural
          'Ba', 'Bar', 'A ridge-like accumulation of sand, gravel, or other alluvial material formed in the channels, along the banks, or at the mouth of a stream where a decrease in velocity induces deposition', 'sandbar, channel bar, meander bar',
          'Bc', 'Beach', 'Low-slope, shorefront accumulation of unconsolidated sediment (usually sand to cobble sized)', '',
          'Bo', 'Bog', 'Rainfall-fed, peat-accumulating freshwater wetland with low pH and nutrient availability and restricted water flow', 'peat bog, domed bog',
          'Ca', 'Caldera', 'Large hollow in the top of a volcano', 'volcanic crater',
          'Cl', 'Cliff', 'Steep to vertical slope with more than 10 m local relief', '', 
          'Cn', 'Cone',  'Moderate to steep, usually isolated hill built up by volcanic activity', '', 
          'Cr', 'Crest', 'A locally high area, from which the surface slopes downwards in opposite directions', 'Apex, Summit',
          'Dl', 'Delta', 'A fan-shaped accumulation of alluvial sediment, usually with several water channels, at a river or stream mouth.', 'River mouth',
          'Du', 'Dune', 'Accumulation of windblown sand into an elongated ridge or crescent shape', '',
          'Ew', 'Ephemeral wetland', 'Wetland subject to pronounced seasonal drying. Non-peaty, with neutral pH and moderate nutrient status. Usually in a closed depression', '',
          'Fl', 'Flat', 'Level area created by deposition', 'Tidal flat, intertidal flat, supratidal flat, infill',
          'Fn', 'Fan', 'Gently sloping, flat to convex depositional slope, often at the foot of a steeper area and built up by a stream or by colluvial action', '',
          'Fe', 'Fen', 'Rainfall and groundwater-fed peaty freshwater wetland with low to moderate pH and nutrient availability, and slightly fluctuating water table', '',
          'Go', 'Gorge', 'A valley 10 m or more deep with over three-quarters of its sides sloping at more than 25°, and half or more of its sides formed by cliffs, and less than 5 times as wide as it is deep over half or more of its length, and everywhere less than 10 times as wide as it is deep', 'Ravine', 
          'Lv', 'Levee', 'An embankment of flood alluvium built up alongside a river and typically with lower-lying land behind', '',
          'Ma', 'Marsh', 'Wetlands with large periodic water table fluctuations, relatively high pH and nutrient content, and mainly mineral substrate', '',
          'Mh', 'Hummocky moraine', 'Low, roughly undulating deposits of glacial till', '',
          'Mo', 'Mound', 'A small hill with moderate to steep sides, often somewhat isolated', 'Lahar mound, knoll, hillock',
          'Mr', 'Moraine ridge', 'Low ridges comprising glacial till', '', 
          'Pk', 'Pakihi and gumland', 'Wetland in highly leached soils with low pH and nutrient availability. Sometimes periodically dry and usually without peat', '',
          'Pl', 'Plain', 'Level to gently undulating area built up by alluvial, aeolian, or volcanic deposition', 'Alluvial plain, backplain, sand plain, outwash plain',
          'Pt', 'Plateau', 'Any comparatively flat extensive and elevated area, often dissected and with a steep dropoff on at least one side', 'Tableland',
          'Ri', 'Riser', 'The vertical or steeply sloping surface of one of a series of natural, step-like landforms, as those of a glacial stairway or of successive stream terraces', '',
          'Rt', 'Tread', 'The flat or gently sloping broadly planar surface of one of a series of natural step-like landforms, as those of a glacial stairway or of successive stream terraces', 'Bench',
          'Sa', 'Saddle', 'A low point (dip) along a ridge or the lowest point between two adjacent elevated areas. In one axis along the ridge or between the elevated areas the land slopes up in both directions and in the other axis, typically at right angles to the first, the land slopes down in both directions', '', 
          'Se', 'Seepage', 'Sloped area carrying a slow but steady flow of groundwater', 'Flushes',
          'Sm', 'Swamp', 'Rainfall and groundwater-fed, somewhat peaty freshwater wetland with high pH and nutrient availability, and seasonally fluctuating water table', 'Backswamp',
          'Sw', 'Swale', 'A low-lying depression between adjacent ridges', 'Interdune, dune slack',
          'Sl', 'Slope', 'Very gentle to very steep inclined surfaces', 'Backslope, midslope, toeslope, footslope, dip slope, flank, headslope, talus slope', 
          'St', 'Saltmarsh', 'Wetlands with saline water supply, usually in tidally influenced areas', 'Estuary, mudflat',
          'Vl', 'Valley-bottom', 'Open depression between parallel facing slopes', '',
          # human:
          'Cs', 'Cut-surface', 'Flat excavated by human activity', '',
          'Cf', 'Cut-face', 'Slope excavated by human activity', '',
          'Em', 'Embankment', 'Ridge or slope built up by human activity', '',
          'Fi', 'Fill-top', 'Flat built up by human activity', ''
          )

tbl_hr_lfnames <- gt(dat_hr_lfnames) |>
   tab_options(
    column_labels.font.weight = 'bold', 
    heading.title.font.weight = 'bold',
    table.align = 'center',
    table.width = '85%'
  ) |>
  tab_style(style = list(cell_text(color  = 'red',
                                   weight = 'bold',
                                   align  = 'center')),
            locations = cells_body(columns = 1)) |>
  # consider replacing this with a column in dat_hr_lfnames, maintenance sucks
  tab_row_group(label = 'Natural', rows = 1:31, id = 'n') |>
  tab_row_group(label = 'Human-induced', rows = 32:35, id = 'a') |>
  row_group_order(groups = c('n', 'a')) |>
  tab_style(style = list(cell_text(weight = 'bold')),
            locations = cells_row_groups()) |>
  cols_label(Synonyms = 'Synonyms and subtypes') |>
  cols_width(Code ~pct(10), Name ~pct(20), Synonyms ~pct(20))

tbl_hr_lfnames

```

### Describing landforms {#sec-gmph-lfmdsc}

Where available landforms are unsatisfactory or a more detailed parameterisation of the landform is desired, the landform can optionally be described in terms of its slope, orientation and element.

#### Slope {#sec-gmph-lfmslp}

Record the characteristic slope of the major part of the landform as median and range in whole degrees, e.g. [5° (4-6)]{.ceg} for a low-angle fan. Slope classes may be applied to the recorded data; some options are presented in @sec-slopeclass.

#### Orientation

For simpler sloping landforms, record orientation as the characteristic aspect of the major part of the landform. Record median and range in whole degrees, e.g. [315 (300-330)]{.ceg} for a roughly northwest direction. For complex landforms with a range of aspects, record the orientation of the landform's longest axis, looking downslope (e.g. the direction in which a valley bottom is descending). Some landforms may not have an identifiable orientation (e.g. cones, saddles), in which case record [NA]{.ceg}.

Aspect classes may be applied to the recorded data; some options are presented in @sec-aspectclass.

#### Landform element {#sec-gmph-lfmelem}

The following is adapted from [@nationalcommitteeonsoilandterrain2009, pp. 19-26].

A landform element is the simplest component of a landform, and some landforms are made up of only one element. Landform elements are first assigned to a morphological type - one of a series of simple shapes - at the scale of observation. These are:

```{r}
#| label: tbl-hr-elemmorph
#| tbl-cap: "Landform Element Morphologies"

dat_hr_elemmorph <-
  tribble(~Code, ~Name,
          'CR', 'Crest',
          'RI', 'Ridge',
          'SL', 'Slope',
          'FL', 'Flat',
          'DO', 'Open depression',
          'DC', 'Closed depression')

# TODO add some little graphics in a third column

tbl_hr_elemmorph <- gt(dat_hr_elemmorph) |>
   tab_options(
    column_labels.font.weight = 'bold', 
    heading.title.font.weight = 'bold',
    table.align = 'center',
    table.width = '85%'
  ) |>
  tab_style(style = list(cell_text(color  = 'red', 
                                   weight = 'bold',
                                   align  = 'center')),
            locations = cells_body(columns = 1)) |>
  cols_width(Code ~pct(10))

tbl_hr_elemmorph

```

Note that these are paired mirror images, e.g. a closed depression is the opposite of a crest. Each morphological type extends to a break of slope or point of 0-curvature.

Slopes can be further refined by including information about their relationship to their neighbouring elements. Up and down slope, four options are available.

```{r}
#| label: tbl-hr-slopeinc
#| tbl-cap: "Slope element relative inclination"

dat_hr_slopeinc <-
  tribble(~Code, ~Name, ~Description,
          'X', 'Waxing', 'Element upslope is gentler, element downslope is steeper.',
          'N', 'Waning', 'Element upslope is steeper, element downslope is gentler.',
          'A', 'Maximal', 'Element upslope is gentler, element downslope is gentler.',
          'I', 'Minimal', 'Element upslope is steeper, element downslope is steeper.')

tbl_hr_slopeinc <- gt(dat_hr_slopeinc) |>
   tab_options(
    column_labels.font.weight = 'bold', 
    heading.title.font.weight = 'bold',
    table.align = 'center',
    table.width = '85%'
  ) |>
  tab_style(style = list(cell_text(color  = 'red', 
                                   weight = 'bold',
                                   align  = 'center')),
            locations = cells_body(columns = 1)) |>
  cols_width(Code ~pct(10), Name ~pct(20))

tbl_hr_slopeinc

```

Three more are available across the slope.

```{r}
#| label: tbl-hr-slopeenc
#| tbl-cap: "Slope element relative enclosure"

# I'm stepping into new territory here D:

dat_hr_slopeenc <-
  tribble(~Code, ~Name, ~Description,
          'L', 'Enclosed', 'Adjacent slope elements face towards each other',
          'E', 'Exposed', 'Adjacent slope elements face away from each other',
          'O', 'Open', 'Adjacent slope elements face the same direction')

tbl_hr_slopeenc <- gt(dat_hr_slopeenc) |>
   tab_options(
    column_labels.font.weight = 'bold', 
    heading.title.font.weight = 'bold',
    table.align = 'center',
    table.width = '85%'
  ) |>
  tab_style(style = list(cell_text(color  = 'red', 
                                   weight = 'bold',
                                   align  = 'center')),
            locations = cells_body(columns = 1)) |>
  cols_width(Code ~pct(10), Name ~pct(20))

tbl_hr_slopeenc

```

This combination of properties gives rise to more complex shapes. For example, an open hollow could be described as an enclosed maximal slope ([SL A L]{.ceg}). A shoulder would be an exposed waxing slope ([SL X E]{.ceg}). Adding further information about slope and aspect can define a complete element. For instance, for a gently sloping, northeast facing fan - [5 40 SL N E]{.ceg}.

[TODO add graphics]{.diags}

## Other terrain parameters {#sec-gmph-other}

The following terrain parameters may be useful in particular contexts.

### Slope length {#sec-gmph-slplgt}

Slope length is used in assessments of erosion vulnerability. Length is measured from the point at which a slope becomes steep enough to lose soil to erosion processes (see @sec-pr-erosion), to where it levels out enough to receive soils (or to a drainage channel). This is usually impractical to measure in the field, but can be determined computationally given an appropriately detailed DEM. Alternatively, the slope length can be manually traced in GIS.

### Mesoscale topographic index {#sec-gmph-mti}

The Mesoscale Topographic Index (MTI) [@hurst2022, p. 15] is a measure of how sheltered or exposed a location is within a landscape. To measure in the field, use a compass and clinometer to measure angle to the horizon at the 8 cardinal compass bearings. Record positive values if looking up and negative when looking down. The average of the eight values defines the index, with high values signifying a sheltered area and low values signifying exposure.

::: {#tip-automti .callout-tip collapse="true" appearance="minimal" title="Automating the MTI"}
The closest automated equivalents of the MTI appear to be the 'Morphometric Protection Index' (MPI) as implemented in SAGA-GIS [@yokoyama2002; @conrad2015], and the 'intensity' output of the 'Geomorphons' module as implemented in GRASS-GIS [@jasiewicz2013; @grassdevelopmentteam2023]. In the case of geomorphon intensity, the output also reports the mean relative elevation in meters of the 8 defining points rather than their average angles above or below the central point.
:::

::: {#wrn-automti .callout-warning collapse="true" appearance="minimal" title="Field vs Computer MTI"}
Field and computer-based measurement of MTI may differ slightly. The in-field version measures against magnetic north rather than 'map north', which will vary with spatial data projection.
:::

### Terrain shape index {#sec-gmph-tsi}

Terrain shape index (TSI, @hurst2022, p. 15) is a localised version of MTI, where slope angles are measured at cardinal compass points 20 m from the point of observation, rather than to the horizon.

::: {#tip-tsi .callout-tip collapse="true" appearance="minimal" title="Automating the TSI"}
The TSI can also be calculated using the automated tools mentioned above, but will require a DEM with high vertical accuracy and cell size no more than 10 m.
:::