4.0 FIELD MAPPING METHODOLOGY

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4.1 Determination of the Map Unit

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As stated in A Soil Mapping System for Canada: Revised (Mapping Systems Working Group, 1981) "map units are established by examining the range of soils or nonsoils that represent a real segment of the soil landscape whose size and properties are relevant to the objectives of the survey". Map units should have the following properties:

The two common types of map units are referred to as Simple and Compound. A simple map unit consists of predominantly one soil or nonsoil. A compound map unit contains predominantly two or more soils or nonsoils (or a combination of both) whose proportions within the map unit may vary depending upon the location of the map unit.

Figure 4.1 shows a portion of a soil survey map which contains both simple and compound map units. As can be seen, not all combinations of a map unit will contain exactly the same proportions of each component that makes up a compound map unit. While the proportions of each component may vary from delineation to delineation, it is the map unit as a whole (the aggregate of all delineations) that has these limits. For example, in the example shown in Figure 4.1, there are the following map units:

1. GN1c 2. GN82 - MN22

Gm:E Gft - T/R: Ff

3. CP9b - GN1b

Figure 4.1 Portion of a Soil Survey Map Illustrating Simple and Compound Map Units
(From Young, et al. 1992 - Soils of the Ashcroft Map Area)

Example 1 is a simple map unit composed of soil association GN. Examples 2 and 3 are compound map units with dominant and minor proportions respectively of soil association GN in combination with soil association CP and soil association MN. In these examples, symbols in the denominator of the map unit designator refer to landform and slope, and numbers provided as superscripts in the numerator refer to relative decile percentages of the proportion of the soil association in the map unit.

Whether or not the map unit is simple or compound, the actual occurrence of soils in nature require the recognition of inclusions in the map unit. Inclusions are areas of soil or nonsoil that are not identified by the map unit name (or symbols) given to a map delineation. Inclusions usually occupy areas that are too small to be delineated as a different map unit. Inclusions that are named as part of the map unit reduce purity while unnamed inclusions reduce the precision of the map units.

At all times, it is important to design map units that can be consistently identified and delineated in the field and that meet the objectives of the survey. In the example above, if soil association GN, which is an Eluviated Dystric Brunisol, has inclusions of Orthic Humo-Ferric Podzol, then this may or may not be a problem for the user of the soil map. The similarities between these two taxa may be so close for the purpose of the survey that their separation doesn't matter. However, if a user of the map needs to recognize the pedological differences between a Brunisolic soil and a Podzolic soil due to perhaps differences in pH or organic matter content that may influence the plant community and wildlife habitat, then the map unit has important restrictions that must be recognized.

There are four terms used to describe and define map unit components. These are described below.

1. Similar Components

2. Dissimilar Components

3. Nonlimiting Components

4. Limiting Components

When any population is classified and its distribution mapped, be it plants, animals or soils, it is necessary to provide a means of designating the oddities that always seem to occur and which occupy limited areas in the survey. There are three terms used to provide this flexibility and they are normally applied at the Series level.

1. Variant

2. Taxadjuncts

3. Phases

Phases are used to differentiate soils on the basis of predictions about use and management and as such are often a very useful mapping tool for separating soils for the purpose of interpretations. An example might be a Howarth Soil, moderately eroded phase which is differentiated from Howarth Soil due to 25 to 75% of the A horizon being lost from most of the area due to water erosion.

Soil mapping in B.C. has and should continue to utilize the concept of a Soil Series and Soil Association for the purpose of soil map units. The majority of the mapping in the Province has been at SIL 3 and 4, whereas some areas, due to the intensity of land use (e.g. the Fraser Valley and Okanagan Valley) have been mapped at SIL2.

A Soil Association is a map unit that is usually made up of components that are defined relative to a central concept for the identified soil (Table 4.1). In this way the central concept for the Soil Association is a Soil Series and the components of the Soil Association are Phases or Taxadjunts or Series that are the products of slightly different ecological and pedological variables (e.g. aspect, drainage, erosion, texture, etc.). A Soil Series, however, has a more narrowly defined set of properties that are outlined in the Canadian System of Soil Classification (1987a).

Table 4.1 Definition of Soil Association Components Recommended for Use

Soil Component

  

1

This component consists only of the modal or most commonly occurring soil in the association. It is the central concept of the association and has usually developed on well drained, deep materials in a mid-slope position.

2

The less common soil is drier than the most common soil, due to either somewhat coarser textures, southern exposures, or being located in a slightly drier climate (e.g. south aspects). This component usually occurs at the lower elevation of the soil association elevation range.

3

The less common soil is moister than the most common soil, due to somewhat finer textures, northerly aspects, or being located in areas which receive slightly more moisture than component 1. The increased moisture often causes deeper soil weathering sufficient to produce a different taxonomic subgroup or order. Typically this soil component occurs at the upper parts of the elevational range of the soil association.

4

The less common soil varies from the modal due to ecological differences. The less common soil differs from the modal due to a historical alteration of vegetation (i.e. land clearing), or the present vegetation pattern is highly contrasting for the area. For example, clearings in some forested areas have developed Chernozemic-like soil profiles whereas the profiles under forest are Brunisolic.

5

The less common soil is shallow (lithic, less than 100cm thick) overlying bedrock.

6

Lithic soils (soils less than 100cm thick) overlying bedrock are most common and bedrock outcrops occupy a significant portion of the map unit.

7, etc.

Additional components of the Soil Association can be added and defined as a function of the survey objectives and the characteristics of the landscape being mapped.

The process of soil survey is one of landscape stratification, based on a number of pedological, ecological, and edaphic factors. This type of landscape stratification is recommended for use by all soil surveys. At SIL 3 and 4, the soil surveyor usually makes use of a number of landscape elements that can assist in broadly differentiating large areas of the landscape. In B.C., these landscape elements have usually been a physiographic region and a vegetation zone or biogeoclimatic subzone. The physiographic region provides broad differentiation of bedrock types and elements of landscape formation, and the vegetation zone and biogeoclimatic subzone is a surrogate used to reflect macro-climate.

Landform or terrain materials (soil parent materials) are also used to stratify the landscape. For example, the soil surveyor will determine the distribution of surficial geologic materials such as glacial till, fluvial and lacustrine sediments and their landform characteristics, and use them to define broad map units within the physiographic regions and vegetation or biogeoclimatic zones. Soil characteristics of these broad landscape units (such as horizonation, drainage, texture) are determined in the field and used to finalize the map units. The soil mapping units (soil association or soil series) are then confined within these broad zones so that a named soil map unit does not occur outside of the defined area. For example, the Allamore Soil will only occur in the Engelmann Spruce-Subalpine Fir Biogeoclimatic Zone and not in the Interior Douglas-fir Zone.

The manner by which soil polygon boundaries are determined is usually a process of proceeding from the general to the specific, depending on the objectives of the survey. This process entails:

1. Delineate broad physiographic regions or bedrock types.

2. Delineate biogeoclimatic units within physiographic regions.

3. Delineate landform and soil parent material types within biogeoclimatic units.

4. Delineate soil map units that occur on the various landform and soil parent material types.

By proceeding from the general to the specific, the soil surveyor is able to put relevant bounds on the geographic distribution of the map unit and, as a result, provide a stratification of the landscape that is of value for land use and management interpretations. At larger scales (e.g. SIL 1 and 2) the soil survey area is often contained within one or perhaps two physiographic regions and vegetation zones such that there may not be a need to recognize them as differentiating landscape characteristics.

The process of soil mapping and hence the recognition of a soil body in nature is one of landscape stratification based on a number of defined variables with specific class limits. For the purpose of defining soil map units, and the field work that goes into recognizing them as segments of the earths surface that lines can be drawn around, it is necessary to define a number of terms that represent or contain units of soil that are classified. These are defined below and are illustrated in Figure 4.2.

Exposure

This is the exposed face of a soil pit or a cut-face of a soil exposed by a road cut or on an eroded terrace front. It is usually the smallest part of the soil landscape that the soil surveyor will recognize and describe during the process of field mapping and it represents a pedon or polypedon.

Site

The term site is used to define the characteristics of the external landscape that is associated with a soil exposure. Examples include a plant community, slope, aspect, and elevation.

Soil and Nonsoil

This is the concept of the "mapping individual". Through the process of examining many exposures and their associated sites, portions of the landscape are recognized as similar (depending on the purpose of the survey). A soil belongs to only one class of the Canadian System of Soil Classification (the level within the classification will depend on the purpose and intensity of the survey). Nonsoils are recognized and named (e.g. bedrock, gravel pits, water, made-land) according to the requirements of the survey. They are identified as polygons or through the use of on-site symbols. When on-site symbols are used, it is necessary to establish a convention in order for the map user to become familiar with the symbols. It is recommended that the same on-site symbols used in Terrain Mapping be employed wherever practical.

Map Units

A soil map unit represents a grouping of recurring map delineations and is a conceptual portion (or area) of the soil landscape that is mappable and has attributes that vary within the limits established by the intensity and use of the survey. A map unit contains one or more soils and/or nonsoils plus sometimes a small proportion of inclusions. Map units are usually repetitive throughout the soil landscape that is being mapped. Symbols or names are given to map units by naming them for geographic locations at which the soil was first described.

For example, a map unit may be described as the Espinoza 4 map unit which contains two soils: Duric Humo-Ferric Podzol and Orthic Humo-Ferric Podzol. These two soils are essentially the same except for the presence of a duric layer.

Map Delineation (polygon)

This is the soil map polygon. It is the line placed on the map that delineates the boundaries of a segment of the soil landscape that is recognized by the soil survey. A delineation is a real example of a map unit as denoted by the symbol placed in the polygon. A map unit, on the other hand is conceptual and comprises all delineations that contain exactly the same symbol.

Figure 4.2 The Building Blocks of Soil Mapping (after Agriculture Canada, 1981)

4.2 The Use of Aerial Photography and Remote Sensing

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Prior to actual field work, and certainly during the conduct of the field survey, aerial photography and other forms of remote sensing are invaluable aids in understanding and mapping the soil landscape.

In most soil surveys, the mapper conducts what is commonly referred to as "pretyping." This is a process of examining aerial photography (stereoscopically) to delimit soil landscape patterns that are recognizable. Other remotely sensed information such as satellite images can also be used for this purpose but are not conducive to stereoscopic examination. These other remotely sensed images, and the digital data that comprises them, are often of value in determining broad landscape and cultural patterns.

Throughout most of B.C., the most useful pretyping of aerial photography involves the recognition of landforms or terrain boundaries that can be drawn on the photo (depending on the study area, this information may already be available). The most useful system for this purpose is the Terrain Classification System for British Columbia - Revised Edition (B.C. Ministry of Environment, 1988). This system provides the means of recognizing and mapping surficial geological materials, landform, texture, stratigraphy and geomorphological processes that have or are affecting the landscape. For the purpose of the soil survey, it is also very useful to recognize other landscape elements during the pretyping process. These might be patterns of vegetation, drainage, aspect and cultural patterns. This process of landscape stratification enables the soil surveyor to recognize broad landscape or ecological parameters that can be used in separating and mapping soil landscape elements.

The pretyping of aerial photos is recommended for all soil surveys. However, the cluttering of the photos with lines and symbols that are only conjecture should be avoided. Once the aerial photos are pretyped, the soil surveyor has a preliminary understanding of the distribution of broad soil landscapes. This information greatly assists in the field mapping exercise by providing the delineation of areas that require intensive field checking and as a kind of map that assists in field location.

Aerial photography should be chosen that is recent and at a scale that is similar to the scale or intensity of mapping. New aerial photography should be flown if the scale, date and quality of the existing photos are not adequate to meet the needs of the survey.

All lines and symbols placed on the aerial photos must be matched between the edges of the adjacent photos in the same flight line and between flight lines. The soil mapper should "box" the aerial photos so that the area on the photo within which lines and symbols are placed contains minimal distortion.

During the field mapping exercise, the pretyped map polygons provide a first approximation of the map units that will be recognized by the soil survey. Both the map polygons themselves and the boundaries between them are validated by exposing a number of soil pits and the boundaries are adjusted or new ones added to reflect the location of the soil map units. In addition, preliminary symbols are placed in the map unit to indicate the name of the soil that is being delineated. The location of soil exposure sites are marked on the photo and referenced to the field sheets that describe the soil and site at that location (An example of field sheets are contained in Appendix B).

4.3 Plotting Soil Boundaries

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During the field mapping exercise, the soil mapper determines routes and transects that cross as many soil boundaries (as determined by the aerial photo interpretation) as possible. This work consists of a series of predictions and verifications as the route is traversed. The predictions are where the soil boundaries are and the sets of soil properties within them. The verification is the examination of soil pits and exposures used to confirm, modify or reject the prediction. Throughout this process, the mapper is constantly examining his field notes and aerial photography to refine the concept of the map unit. Preliminary (pre-typed) lines and symbols placed on the aerial photography are changed to reflect the information gathered in the field (Figure 4.3). Once in a preliminary map unit delineation, the soil is examined to verify the prediction, often in many locations within the delineation (Figure 4.4). This information is used to determine the map unit (based on the preliminary legend) which best identifies the area. If such a map unit is not currently recognized, then a new one should be defined. The soil mapper then selects a location within the map unit that is the best expression of the features that are being represented and describes the site and soil in detail. In addition, the surveyor determines inclusions within the map unit that must be expressed but are beyond the range of the taxon.

As this process proceeds, the soil mapper prepares lines and symbols on the aerial photography or other base maps that are close to the final lines and symbols that will be shown on the soil map (Figure 4.5). In addition, the soil map legend is altered to encompass the characteristics of the map units that are being depicted. The map unit boundaries are often transferred from the aerial photo to a topographic base map during the field mapping exercise in order to visualize the full extent of the soil distribution in the map area. Plotting the soil map unit boundaries is often accomplished by utilizing plotting devices such as a Zoom-Transferscope or Epidiascope, or can be done by hand. If TRIM aerial photography is used, the soil map units can be directly transferred (digitally) to a map polygon file and plotted since the photographs are digitally corrected for distortion. In many instances, these preliminary map units are re-examined in the field in order to confirm their position and characteristics after they have been tentatively drawn on the base map. However, final confirmation of the map unit boundaries and their characteristics must often wait for the completion of laboratory analysis of sampled soils and a more detailed examination of the data gathered in the field and recorded on the site and soil description forms. Figure 4.6 illustrates a completed soil map that is ready for publication.

Figure 4.3 Preliminary Map Unit Delineations Checked and Verified in the Field


Figure 4.4 Numbered Sample Sites within the Map Unit Delineations

Figure 4.5 Soil Map Units with Labels that are explained in the Map Legend


Figure 4.6 Part of a Published Soil Map; Map Scale 1:100,000; from Soils of the Taseko Lakes Area; Soil Survey Report No. 36

Example Symbol

4.4 Sampling Design and Data Collection

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During the conduct of a soil survey, it is necessary to examine all soils and their landscape characteristics in order to determine the nature and extent of the distribution of soil individuals and their variability. The soil surveyor must sample the population of soils that occur in the map area in order to determine their full range of properties and thereby organize classes of the population (types of soil) and delineate the areas containing those classes (map units). The sampling design, or the manner by which locations are chosen for sampling, will influence the quality of the final map. A sample may be simply a soil inspection where notes are taken on field data collection sheets or it may be an actual sample, where soil is taken from the soil excavation for laboratory analysis. The appropriate sampling design will depend on the purpose and intensity of the survey, the nature of the landscape, the amount of soil information already available from other sources and the experience of the surveyors. The sampling design will play a crucial role in determining the precision of the soil survey.

The majority of soil mapping in B.C. has been undertaken by what is referred to as authoritative or "free survey". This technique utilizes known and inferred soil-landscape relationships to predict soil characteristics. The soil surveyor chooses his sample site and then extrapolates the information gathered at this location to other nearby areas with the same soil-landscape relationships without necessarily examining the other areas in the field. By definition, the sample is biased and not random, thereby limiting the use of statistical methods to determine estimates of error and objective conclusions about the soil population.

The use of "free survey" sampling is appropriate for small scale surveys, particularly where large areas of inaccessible terrain must be covered in a short time within a limited budget. However, where detailed surveys are undertaken, the most appropriate techniques are those that employ a degree of randomness. The random transect method (Wang, 1980) is an example of this approach. For all soil surveys in B.C., it is important that a systematic sampling procedure be employed. The random transect method is recommended as the basic sampling technique. Depending on the type of landscape being mapped, this should take the form of the line intercept method or the point intercept method (these are described in Bartelli and De Ment, 1970). In the application of either method, each soil delineation should be adequately sampled with one or more transects in order to provide unbiased coverage. The number of observations (sample size) will be determined by the desired accuracy and the use of standard statistical methods.

Data collection at sample sites will vary with the survey objectives and purpose. However, for all soil surveys in B.C., the data forms contained in the manual Describing Ecosystems in the Field: Second Edition (Luttmerding, et.al. 1990) for recording site and soil characteristics can be utilized. The surveyor is able to choose those parameters that are most appropriate for the survey and the forms are organized for ease of input to digital data files. Examples of these data forms are contained in Appendix B.

The following minimum data set is recommended for all soil inventories regardless of scale or purpose. Definitions for all of these items are contained in Luttmerding, et al (1990).

Site Description Form

Soil Description Form

4.5 Map and Data Reliability

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Previous sections of this manual have discussed the concept of reliability (accuracy). This term provides a measure of how close the information contained on the map and in associated data bases and reports conforms to actual soil conditions in the field. The users of the map and report require some expression of reliability in order for them to appropriately use and apply the information.

A recommended manner to generally convey reliability to the user is through the concept of a Survey Intensity Level. By having the survey conform to the precision inherent in each of the intensity levels, the user is assured a certain level of reliability.

In order to actually measure the degree of reliability, when required, it is recommended that the random transect method be used following completion of the survey. It is rarely possible to have a constant level of reliability throughout the map area. Consequently, it is often valuable to provide a discussion about this in the soil report or perhaps on the map. Often, this is best expressed by illustrating the degree of sampling, accessibility and landscape complexity on a small scale map that is contained in the soil report or on the soil map.

The reliability of laboratory data is usually well controlled by established procedures. However, it is recommended that some duplicate and/or standard samples be submitted for analysis in order to provide a check on quality control. The Manual on Soil Sampling and Methods of Analysis (CSSC, 1978) and Canadian Society of Soil Science (1993) provide methods and techniques for conducting most soil analyses.

4.6 Field Verification, Quality Control and Correlation

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Quality control and quality assurance are two terms that are currently much in vogue. They are necessary elements of any soil survey program and require someone or some organization to assume the responsibility of maintaining consistency in the methods, terminology and conventions used for describing and defining soils.

In soil survey, this responsibility usually rests with the "soil correlator". This individual (or more than one person if necessary) is usually appointed to the soil survey team by the government agency responsible for the survey.

The correlation activity occurs throughout the survey, from the initial planning phase to the publication of the map and report. As a consequence, this requires both formal and informal meetings between the soil survey project leader and the soil correlator as well as field reviews at specified stages of the survey. The final correlation involves a review of the draft soil map and report in order to verify the precision with which the map and legend portray the soils and landscapes they represent and meet the objectives of the survey. The soil correlator compares the descriptions of the soils (including laboratory data) to ensure that the names used for the soil map units are correct if they have been previously defined and used, and to verify the taxonomic classification.

All systematic soil surveys require correlation prior to releasing the survey results to user groups. From a quality control perspective, it is recommended that this requirement not be over-looked.

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