Soil Survey Manual - Chapter Four (Part 1 of 4)

Mapping Techniques

Table of Contents

Page 1
Documentation
    Descriptive Legend
    Soils Handbook
    Supporting Data

Page 2
Maps
    Imagery to Aid Field Operations
    Base Material
    Selecting Map Scale
    Reference Maps
    Index Maps for Field Sheets

Page 3
Field and Office Activities
    Preliminary Research
    Preparing the Mapping Legend
    Field Operations
    Completing Field Sheets
    Cultural Features

Page 4
Equipment
    Tools for Examining the Soil
    Mapping Equipment
    Transportation

Documentation

Descriptive Legend

The descriptive legend is the basic document of a soil survey and is composed of four parts: (1) description and classification of the soils, (2) identification legend, (3) conventional and special symbols legend, and (4) general soil map and legend.

Description and classification of the soils.

The descriptive legend includes descriptions of the taxa as they occur in the survey area and descriptions of map units delineated on field sheets. These descriptions form the primary reference document for identifying kinds of soils and miscellaneous areas and provide the information needed for proper classification, correlation, and interpretation. They also provide the information needed to recognize the map units in the survey area. Descriptions of the taxa and the map units, including the ranges in characteristics within the survey area, ensure that all members of the field party classify and map the soils consistently. Creating a clear, concise, accurate, and complete set of descriptions of the soils is a difficult and important job.

An up-to-date record of what has been learned about the soils is especially important when members of a survey party change. If the party leader leaves before completion of the survey area, an up-to-date descriptive legend of how the soils have been classified and mapped ensures continuity in survey operations.

The party leader organizes the information that has been gathered about the soils in an area. While preparing the descriptions, the party leader may discover matters that need clarification or supporting field data. Field studies can then be planned to clarify concepts and improve knowledge of the soils.

Guidelines for describing soils in chapter 3 emphasize individual pedons and polypedons. The soil descriptions in the descriptive legend give the properties of pedons and polypedons plus the extent of the components in each map unit, the variations in properties and in extent of components from one delineation to another throughout the survey area, and the geographic relationships of components within each map unit and of map units to each other. The descriptions are made from detailed descriptions of pedons and polypedons, brief notes about internal properties and surface features, and summaries of transects.

As the descriptions of the soils are prepared, every map unit description is compared with the standard definition of the soil for which it is named and with the descriptions of closely related soils. The classification of the soils must be consistent with the descriptions of the soils in the map units and also with the standard definition of series or other taxa.

A table of classification is included in the descriptive legend and shows how soils in the survey area fit in the national system of soil classification as discussed in Soil Taxonomy. Where soil series are used in naming map units, the series can be listed alphabetically followed by the classification, or they can be arranged under the appropriate families, subgroups, and so on.

The nature, kind, position, and amount of inclusions are also described for every map unit. The extent, position, and significant differentiating characteristics of soils that are dissimilar to the major components of the map unit are particularly important. The extent and nature of inclusions that are similar to the major components should also be determined.

Written descriptive records of the soils are references for an ongoing soil survey. The properties of a soil commonly vary from one part of a survey area to another and may be evaluated differently as a result of increased experience in the area.

The soil descriptions are continually revised and updated as mapping progresses. During mapping, new map units and taxonomic units are commonly added and units that are found to be of limited extent are discontinued.

As mapping progresses, kinds of soil are often discovered that do not fit any map units in the legend. If the kind of soil is extensive and uniquely different from the soils in other map units, it is added to the legend after it has been defined by a party member and approved by supervisory soil scientists of the cooperating agencies. Some new kinds of soil can be accommodated best by redefining existing map units, and others can be accommodated as inclusions. New, approved map units must be listed in the legend promptly and defined to enable all members of the party to use them correctly.

Some soils are so limited in extent that they should be included in other map units. Two or more soils that have similar use and management may be best combined in one map unit. Soils that are so closely intermingled that they cannot be delineated separately must be mapped as complexes. Deletions and other changes are not made formally until the supervisory soil scientists have reviewed the proposed legend changes and found them acceptable. If proposed changes are not acceptable, the agency representatives and the party leader resolve any differences they may have. A complete record is kept concerning changes in map units and the disposition of any discontinued map unit. Any changes made between field reviews are recorded in the report of the next field review.

Distinctions between map units must be larger than the ranges that normally occur in measuring diagnostic properties and locating soil boundaries. The soil descriptions must be tested to ensure that the map units are recognized and delineated consistently.

Progressive mapping by the field party is a continuing test of the legend. Inadequacies are evaluated, and any necessary changes are made in the legend. Changes are recorded on all copies of the legend, and each soil scientist in the party must clearly understand the new concepts.

Field notes are summarized periodically and the summary is recorded in the revisions of the soil descriptive legend. If observations are not summarized and recorded promptly, they may be lost or not used by other members of the survey party.

Field reviews also test the legend and its use in mapping to determine whether survey objectives and requirements are being met. Such reviews usually involve supervisory soil scientists and representatives of cooperating agencies.

The final test of a descriptive legend comes during the formal steps in soil correlation. Correlation is a continuing process from the initial descriptions before mapping starts to the final correlation. A map unit can be tentatively correlated as soon as it has been accurately described and mapped. Few changes are needed in final correlation if the descriptions are adequately tested and revised as the survey progresses.

Quality soil descriptions ensure a quality soil survey. The importance of soil descriptions cannot be overemphasized. A good set of descriptions is needed for consistent, uniform, and accurate mapping. The descriptions also provide the basic information needed for complete and accurate interpretation. Working from the soil descriptions, supervisory soil scientists can give maximum help to the survey party.

Soil surveys of lesser detail, made with more widely spaced field observations, traverses, or transects, resemble the preliminary surveys made to prepare the initial set of soil descriptions for detailed mapping. For these surveys also, map unit descriptions are modified as more is learned about the soils. Map units are added only after they have been defined and approved by the representatives of the cooperating agencies.

Identification legend.—A symbol is placed in each delineation on the map to identify it. The identification legend is a list of these symbols and the names of the map units they represent. In some legends the names of the map units are listed alphabetically, followed by their symbols. This list of names is used by soil scientists as they map. In other legends the symbols are listed in order, followed by their names. This list is used by everyone who reads the maps. Usually both lists are prepared. If the symbols are not listed in order, as is common when new map units are added to the legend, associating a symbol on the map with the map unit it represents can be difficult.

The identification legend keys names of map units to delineations on the soil maps through the map unit symbols. Many conventions and systems are used for selecting symbols. The choice of symbols is unimportant provided the symbols are short, each symbol is unique, and the map unit that each symbol represents is named and described.

All symbols must be legible on photographic reproductions of the maps. Long symbols are difficult to place on the map without being made too small to be legible. Long symbols often must be placed outside small delineations and arrowed into them. This increases the chance of error. Experience and tests have shown that map users have great difficulty in reading field sheets that have many symbols placed outside the areas to which they apply. If the symbol is arrowed from a large delineation to a small one, many users assume that it represents the large delineation.

The map symbols serve primarily to identify map units; any connotations of soil properties are incidental. Efforts to go beyond identification and devise connotative symbols usually leads to a legend that fails to achieve its primary purpose. The connotative value of symbols may be offset by decreased legibility of the map. Map users must not assume that connotative symbols or even the map unit names describe all of the important soil properties. The set of soil descriptions (map unit and taxon descriptions) is essential to the purpose of the soil survey and should be used by mappers and by those who need the information while the survey is in progress.

Using the same or similar symbols during the mapping process and on published maps accelerates map compilation because compilers are not required to spend much time converting one set of symbols to another. Errors are reduced. Such symbols have the greatest advantage in areas where soils are well known. Where soils are not well known at the start of the survey, changes during mapping and correlation may reduce the advantages.

The following are parts of two identification legends.

Conventional and special symbols legend.—Conventional symbols on soil maps show many natural and cultural features other than map units and their boundaries. They help users locate delineations. Special symbols identify some areas of soils or miscellaneous areas that are too small to be delineated at the scale of mapping. All symbols must be defined. Definitions of special symbols specify the size of area that each represents.

General soil map and legend.— The general soil map helps the field party in mapping and in organizing field work. The draft of the general soil map prepared during preliminary field studies is refined as more is learned about the soils. The properties, distribution, and extent of the soils in each general area and their suitabilities, limitations, and potentials are described. Significant differences in soil moisture or soil temperature between areas can also be shown on the general soil map.

Soil Handbook

The descriptive legend is the main document that governs field operations, but it is only part of the information compiled during a survey. The descriptive legend and the other information about the soils in the survey area are organized into a soil handbook. The soil handbook is used by the field party and by engineers, agronomists, planners, and others who need information about the soils of the area before the survey is completed.

The handbook contains everything needed for the published soil survey, plus material that is important to the soil scientists who are making the survey. A detailed outline for the text of the published soil survey should guide development of the handbook.

Included in a soil handbook, in addition to the mapping legend are interpretations and general sections covering such topics as climate, physiography, relief, drainage, geology, and vegetation, which relate to the kinds of soil in the area. These characteristics improve the understanding of the properties, distribution, use, and management of the soils.

In addition, a record of the acreage of each map unit is maintained. In some surveys acreage is recorded progressively as the field sheets are completed. In other surveys progressive acreage records of each map unit are kept only until the unit is found to be extensive enough to keep in the legend. The final tally is made after the survey has been completed.

Some items prepared for the mapping legend or handbook may be incorporated into different sections in the publication. For example, the genetic key and classification table could become part of the section on how the soils formed and how they are classified. Some of the diagrams could be used in that section as well as in the section on the general soil map.

The descriptive legend and soil handbook should follow the same format that will be used in the published soil survey. A soil handbook that is kept up-to-date as mapping progresses will require a minimum amount of editing after the mapping has been completed.

Supporting Data

Data collected can be filed in the soil handbook. Separate sections can be added that contain all additional documentation obtained during the course of the survey. In addition, file folders, cross-indexed by soil series and map unit, can be used. Items that require simple filing systems for easy retrieval are transects, field notes, soil keys, laboratory data, special studies, special interpretations, climatic data, geology maps, vegetation maps, research reports, and any other items unique to the survey. A few of these are described below.

A genetic key shows the relationships of the various taxa to factors such as parent material, natural drainage, vegetation, annual precipitation, topographic position, and form, and aspects. The key should emphasize the factors associated with important soil characteristics and differences in characteristics within the survey area.

A table of soil characteristics highlights important properties of the soils. Comparisons can be made easily and quickly. Both the genetic key and the table of soil characteristics are particularly helpful in orienting newly assigned field personnel.

The general soil map helps the field party in mapping and in organizing field work. The draft of the general soil map that is prepared during preliminary field studies is refined as more is learned about the soils. The properties, distribution, and extent of the soils in each general area and their suitabilities, limitations, and potentials are described. Significant differences in soil moisture or soil temperature between areas can also be shown on the general soil map.

Remotely sensed imagery is produced from both photographic and nonphotographic sensors. The use of more than one set of imagery for reference is important. Several sets of photographs and other images are likely to yield more clues about soils than one set. The kinds of remote imagery and their advantages and disadvantages in soil mapping are discussed later in this chapter.

Photographs of soil profiles can be very effective in illustrating some soil features. Photographs or diagrams of landscapes show the relationships of soils to various landscapes. Cross-sectional and three-dimensional diagrams of parts of the survey area are also helpful.

Notes are indispensable parts of the mapping legend. Some notes are used in revising the descriptive legend, which becomes incorporated in the manuscript for publication. Notes help make mapping faster and more accurate. They may record tonal patterns on aerial photographs that are peculiar to a certain map unit, the relationship between minor but key indicator plants, or the surface configurations that have little bearing on use or management but that help the mapper locate significant soil areas. Notes and other information needed in mapping but not intended for publication can be kept on separate sheets after each taxon or map unit description in the descriptive legend.