Mapping Unit: Meaning, Types and Facts
A soil mapping unit can be defined as a collection of soil delineations (on the map) that comprise similar soils or soil combinations (depending on the scale of the survey and the intricacy of the soil pattern) Soil mapping units are designed to efficiently deliver soil information to meet the need of user and for effective management and land use decisions.
Mapping units can appear as individual areas (i.e., polygons), points, or lines on a map. Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map. Each individual area on the map is delineation.
A map unit is a collection of areas defined and named the same in terms of their soil components, miscellaneous areas, or both (components and miscellaneous areas are described below).
Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map.
A map unit description is a written characterization of the component within a map unit and the relationship of one map unit to another.
A delineation of a map unit generally contains the dominant components in the map unit name, but it may not always contain a representative of each kind of inclusion.
A dominant component is represented in delineation by a part of a polypedon, a complete polypedon, or several polypedons.
At the end of this article, students should be able to understand;
· What mapping unit is all about
· Different types of mapping units
Important facts to know
· A soil mapping unit can be defined as a collection of soil delineations (on the map) that comprise similar soils or soil combinations.
·Soil mapping units are designed to efficiently deliver soil information to meet the need of user and for effective management and land use decisions.
·Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map.
· A few delineations of some map units may not contain any of the dominant components named in the map unit description, but contain very similar soils.
· The kinds of map units used in a survey depend primarily on the purposes of the survey and the pattern of the soils and miscellaneous areas in the landscape.
· It must be remembered that soil interpretations are made for areas of land and the most useful map units are those that group similarities.
Meaning of Mapping Unit
A map unit is a collection of areas defined and named the same in terms of their soil components or miscellaneous areas or both. Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map.
Map units consist of one or more components. An individual component of a map unit represents the collection of polypedons or parts of polypedons that are members of the taxon or a kind of miscellaneous area.
Classes of miscellaneous areas are treated the same as soil taxa in soil surveys. A taxonomic unit description describes the ranges in soil properties exhibited in the polypedon for the maps in a survey area that are referenced by that taxonomic unit.
Soil boundaries can seldom be shown with complete accuracy on soil maps, hence parts and pieces of adjacent polypedons are inadvertently included or excluded from delineations.
A few delineations of some map units may not contain any of the dominant components named in the map unit description, but contain very similar soils.
In most survey areas there are a few soils that occur as mappable bodies, but they have very limited total extent. They are normally included with other map units, if, for all practical purposes, interpretations are the same.
Aggregated data capture the ranges of various physical and chemical properties of soil map units as a whole and individual soil map unit components.
They include the descriptions of each soil map unit and map unit component; the detailed physical, chemical, and morphological attributes of each soil; and descriptions of the relationship of one soil map unit to another on the landscape.
Aggregated soil property data generally are the data used to generate interpretive ratings for each map unit and its components.
Read: Types of Soil: Characteristics and Importance
Types of mapping units
The following types of mapping units
a) Single Mapping Unit
· Consociation: A mapping unit dominated by a single soil series and containing less than 25% inclusions of minor soils. b) Compound Mapping Units
· Association: A mapping unit consisting of two or more soil taxa (series) geographically associated in a characteristic recurring pattern. It contains less than 25% inclusions of minor soils.
· Complex:Similar to association but the constituent soil taxa occur in such an intricate pattern or are so small in area that it is not possible to map them separately, even at the scale of the sample area survey. The pattern and proportions of soil taxa are somewhat similar in all areas. A complex contains less than 25% inclusions of minor soils.
· Undifferentiated: A mapping unit consisting of two or more soil taxa e.g. series that are not consistently associated geographically but are included in the map unit because use and management are the same or very similar. Soil taxa occur in variable proportions in various soil delineations representing the mapping unit.
Read: Kinds of Soil Surveys
Aggregated Data in Soil Map Units
Aggregated data are developed by putting together the various pieces of point data that have been collected during the soil survey and referenced to a particular soil map unit or map unit component. Values for a particular soil property are commonly expressed as a range.
Depending on mapping scale, map unit design, and the level of specificity of data needed for the purpose of the soil survey, the upper and lower limits and, in most cases, a representative value (RV) of the range of each soil property need to be stored in the database (e.g., clay content ranges from 18 to 27%, with an RV of 22%).
The representative value is the value most likely to be found for a particular soil property and is useful in computerized interpretive models. The RV can be determined by summarizing the values recorded on the individual pieces of point data.
Tacit knowledge from individual soil mappers can be used to augment recorded point data measurements.
The physical, chemical, and morphological properties of the soils included in the aggregated data generally are most or all of those that are included in the point data.
They should include any properties that are used to generate interpretive ratings.
Values for many physical and chemical soil properties of a particular soil map unit or map unit component commonly vary from one topographic position to another, or from one geographic location to another, within a particular map unit or even a single delineation of a map unit.
Properties can also vary from one time of the year to another, from year to year, and from one land use and/or management system to another. The database must have the capability to record this variability.
Aggregated data may represent map units that cover a particular geographic area at different map scales, for example, 1:12,000 or 1:24,000 and also 1:100,000 or 1:250,000. The differences in scale may represent a “detailed” soil map of the area and a “generalized” soil map of the same area.
Map unit design and the respective map unit components will generally differ between the larger (e.g., 1:24,000) and smaller (e.g., 1:250,000) map scales.
Read: Kinds of Soil Surveys
Mapping units are conceived during the aerial-photo interpretation stage when API units are delineated on the basis of supposedly soil-related characteristics of the photo stereo-image.
The API units are subsequently transformed into regular soil mapping units during the reconnaissance and sample area surveys.
Soil mapping units comprise soils or soil combinations which in turn represent series concepts (by meeting the specific requirements of these concepts). In the literature such defined concepts are often referred to as taxonomic units, taxonomic classes or, shortly, taxa as they all belong to a larger taxonomic framework developed for soil classification purposes.
A delineation of a map unit generally contains the dominant components in the map unit name, but it may not always contain a representative of each kind of inclusion.
A dominant component is represented in delineation by a part of a polypedon, a complete polypedon, or several polypedons. A part of a polypedon is represented when the phase criteria, such as a slope, requires that a polypedon be divided.
A complete polypedon is present when there are no phase criteria that require the subdivision of the polypedon or the features exhibited by the individual polypedon do not cross the limits of the phase. Several polypedons of a component may be represented if the map unit consists of two or more dominant components and the pattern is such that at least one component is not continuous but occurs as an isolated body or polypedon.
Similarly, each inclusion in delineation is represented by a part of a polypedon, a complete polypedon, or several polypedons.
Their extent, however, is small relative to the extent of the dominant component(s).It is important that all major soils of the area are presented in the various mapping units in a way that their geographic setting in relation to the overall soil pattern is clearly shown.
This is one of the main reasons why the structure of the soil map legend is based on physiography and lithology.
