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Soil Survey Manual - Chapter Four (Part 2 of 4)Mapping TechniquesTable of Contents
MapsImagery to Aid Field OperationsAerial photographs are used as the mapping base in most soil survey areas in the United States today1. With few exceptions aerial photographs are by far the most practical mapping base for field use by soil scientists. Several kinds of aerial photography are available. Conventional panchromatic (black and white) photography is sensitive to approximately the visible portion of the electromagnetic spectrum (wavelengths of 0.38 to 0.78 micrometer). Color photography covers a similar range. Infrared photography, which covers radiation of somewhat longer wavelengths, is also available. The main kinds of aerial photography are described in the following paragraphs. Single-lens aerial photographs.—The two basic types of aerial photographs are vertical and oblique. Single-lens vertical photographs are the best for soil mapping, although oblique or multiple-lens photographs can be used when rectified. USDA specifications for single-lens aerial photography require an overlap in line of flight of about 60 percent and a sidelap between adjacent flight lines of an average of 30 percent. With this overlap, all ground images appear on two or more photographs exposed from different air positions, providing stereographic coverage. Two consecutive photographs within a line of flight are called a stereographic pair. If every other photograph in a continuous line of flight is removed, the remaining photographs provide alternate coverage. Adjoining photographs of alternate coverage in the same line of flight are called alternate pairs. Alternative pairs overlap about 20 percent—too little to permit stereoscopic study of the entire area. Using alternate coverage, instead of full stereographic coverage for mapping, leads to problems with relief displacement during map compilation. Alternate coverage is inadequate for constructing maps by photogrammetric methods based on complete stereographic coverage. Photographs are exposed on film at a predetermined scale and fixed negative size. The scale depends on purpose. Most USDA aerial photographs are taken with a 153 millimeter lens. Scale ranges from 1:38,000 to 1:80,000. Satisfactory enlargements up to 1:7,920 can be made from 1:40,000 negatives. Most aerial cameras currently in use expose an image of about 23 by 23 centimeters. Photographs made directly from the original negatives at the same scale are called contact prints. In contact printing, errors cannot be rectified and the scale cannot be changed. Contact prints are economical to make and have better resolution than enlargements. Photographs can be readily enlarged or reduced; this is one of their advantages. The process is slower and more expensive than contact printing. Some detail is lost in the preparation of enlargements, but the loss is small when skilled operators use modern processing equipment and the original negatives. Enlarging has certain advantages. All prints for an area can be brought to a nearly uniform scale. Tilt, which causes displacement of objects and scale distortion, can be rectified. Such operations require more time than simple enlarging, but later savings may more than offset the cost of bringing photographs to a common scale. If the photograph is enlarged more than 5 times, prints are usually unsatisfactory. Enlargement increases the size of the photograph as well as the scale. The size of sheet varies with the enlargement. If the contact print at a scale of 1:20,000 is 23 cm square, an enlargement to 1:15,840 will be 29 cm square and an enlargement to 1:7,920 will be 58 cm square. Photo indexes are inexpensive and should be obtained when available. They are useful for determining the number and location of individual photographs within an area. They are also useful for schematic mapping and for preliminary studies. The greatest advantage of aerial photography in soil surveying is the wealth of ground detail shown. Field boundaries, isolated trees, small clumps of bushes, rock outcrops, and buildings are visible and assist in orienting the mapper and in plotting the soil boundaries and other features. Both the speed and accuracy of the work are increased by using photographs. Base maps for publication can be constructed from aerial photographs economically and in a reasonable time. Showing all of the intricate cultural and physical details, a stereographic series provides a relief model of the area. Aerial photographs also have some disadvantages and limitations in soil surveying. Elevations are not shown. Scale is not precisely uniform. Differences of scale between adjoining photographs create some minor difficulties in matching and transferring soil boundaries. Distances and directions cannot be measured as accurately as on topographic maps or some other kinds of photographs because of distortions caused by tilt, image displacement, and other inherent errors. Finally, although far more detail is shown on aerial photographs than on most maps, the detail is not always as legible and more skill is required to interpret the photograph. Nevertheless, the advantages of aerial photographs generally greatly outweigh the limitations. Photographic indexes are available for most of the photography available from Federal agencies. Indexes are prepared by fastening together the individual prints of an area. The images are matched, and the photographs are overlapped so that all marginal data are visible. The assembly is then photographed at a smaller scale. Most indexes prepared by the United States Department of Agriculture have a scale of 1:63,360 or 1:126,720. Once a survey has been scheduled, photographs should be ordered as soon as possible. The order gives the exact boundaries of the survey area, the scale of photography needed, desired coverage (stereographic or alternate), and the date that fieldwork is to begin. Any special requirements, such as weight or finish of paper, are stated. Low-shrink paper is recommended for most field-mapping. Panchromatic photography records all colors in varying shades of gray. Most modern black and white photography is of excellent quality. Because of their quality and economy, photographs made from panchromatic film are the most widely used for soil surveys. Color photography records features of the surface in colors of the visible spectrum. The colors on the print are about the same as the colors of the features when the photograph was taken, but the colors of the ground features may be different at other times. The color of a soil also may differ, according to such factors as sun angle, atmospheric conditions, delays between flights, and moisture state of the surface. The cost for obtaining color photography is about 11/2 to 2 times as much as panchromatic photography. Color prints cost 2 1/2 to 4 times as much as black and white prints. Excellent black and white prints can be made directly from color negatives at the same cost as prints from a panchromatic film. With high-altitude photography, fewer photographs are required to cover an area. Contact prints of the original negatives can be used for stereographic coverage. Enlarged stereographic coverage can be prepared from selected stereographic pairs. Special stereoscopes are helpful when viewing the larger prints in field offices. For some soil surveys, photobase maps are printed from high-altitude photography and low-shrink paper. In other surveys the photobase maps are printed on transparent film with a matte surface. Normally, field mapping on outdated photographs is transferred to film prints, and paper prints are used for new field mapping.2 Obtaining high-altitude photography and preparing photobase maps nearly always cost less than constructing photobase maps from a controlled aerial mosaic. High-altitude photographs have better image quality than controlled mosaic ones. Infrared photography records a portion of the spectrum that is not visible to the human eye. Infrared film is also sensitive to part of the visible spectrum, but true infrared photography is exposed through a deep red filter so that only the infrared radiation is recorded. Prints from infrared film have distorted shades of gray in comparison to prints from panchromatic film. Bodies of water and areas in shadow appear black. Broad-leaved trees appear very light, as though covered with frost. Foliage of coniferous trees appears distinctly darker. Roads are dark, instead of very light as on panchromatic prints. These characteristics are useful for detecting patterns of soil moisture states, identifying forest types, and detecting vegetation under stress from disease or other causes. Infrared aerial photography is especially valuable in areas having atmospheric haze because the film is not sensitive to the blue portion of the spectrum that is normally associated with haze. Infrared photography costs about 10 percent more than panchromatic photography. Modified infrared photography is a compromise between true infrared photography and panchromatic photography. The images have some of the characteristics of each. At first glance, a modified infrared photograph looks very much like a photograph made from panchromatic film. It shows more contrast between some kinds of vegetation and records differences in soil wetness in more distinctive patterns than panchromatic photographs. Modified infrared photography costs about 10 percent more than panchromatic photography. Prints from infrared negatives cost the same as prints from panchromatic film. Color infrared photography is sensitive to the green, red, and infrared portions of the electromagnetic spectrum. It produces false colors for most objects. The prints are spectacular; the colors are often brilliant and contrasting. This type of photography is especially useful for the study of vegetation. Vigorously growing vegetation appears brilliant red. Color infrared photography costs about the same as conventional color photography. Remote sensing.—refers to the full range of activities that collects information from a distance. It includes photography, which has been the most widely used remote sensing technique for many years. The range of the electromagnetic spectrum that can be sensed from a distance, however, is much greater than that covered by conventional photography. Other techniques have been devised to use part of this range3. Nonphotographic sensors can perceive the parts of the electromagnetic spectrum from ultraviolet (wavelengths less than 0.38 micrometers) through microwave to the upper wavelength of 100 cm. The extent to which some of the newer remote sensing techniques can be used in soil surveys has not been fully explored. Field work cannot be eliminated, but how much it can be reduced is not clear. Soils must be examined to a depth of about 2 m or to solid rock—beyond the present reach of most remote sensors or combination of sensors. At least some clues to many soil properties are provided by surface features. It is these clues, many of them quite subtle and obscure, that are sought and used in drawing soil boundaries. These clues also assist the making of accurate soil maps without excessive digging or probing. Remote sensing contributes greatly to soil surveys by revealing these clues. The imagery extends hard data about soils and their formation to new areas. In areas of the country where it can be used, ground penetrating radar (GPR) and statistical analysis of the radar data can be a useful aid to soil mapping and can provide an effective and efficient method to characterize variability within soil map units. GPR has the advantage of observing a linear transect of the soil continuum across a landscape. The prospect of using more than one set of imagery is important. Such a set might be made up of two or more kinds of photography made at the same time—multiband photography—or two sets of one kind of photography made at different times of the year, or some combination of these. Although several sets of photographs and other imagery are likely to yield more clues about soils than one set, the extra cost for the additional clues would have to be justified. Photograph-like images can be made by nonphotographic sensors of any part of the electromagnetic spectrum. Hence, outputs from the sensors can be viewed and treated like photographs. An example is side-looking radar, which can penetrate clouds and can be used at night as well as in the daytime. The radar can produce prints that resemble photographs, although the images are not as clear as panchromatic photographs. Side-looking radar is useful where continuing cloud cover prevents conventional photography. For use with computers, impulses from side-looking radar and other nonphotographic sensors can go directly into automatic data processing systems for storage or analysis. Space exploration has added a new dimension to remote sensing. Earth-orbiting satellites can be equipped with several kinds of sensors, including cameras. Imaging from space has the same problems as imaging from aircraft and the additional problem of transmitting the data to earth. Imaging from space has two important advantages. First, large areas; thousands of square kilometers—can be examined from a single point in orbit. Second, any area can be repeatedly examined on a regular schedule. Base MaterialMore than one kind of cartographic material suitable as a mapping base could be available for an area. The choice of base material depends on the relative advantages of available material for all aspects of the job, including map compilation and reproduction as well as fieldwork. Selecting the mapping base.—The quality of the cartographic material used in mapping and for publication affects the accuracy of map unit boundaries and soil identification, the rate of progress, the methods and costs of map construction, and the quality of the published map. The assembly of cartographic materials should begin as soon as an area is selected for survey. For most surveys, purchasing new or recent photography and preparing field sheets at the dimension and scale that will be used for publication is an economically sound practice. Some of the costly steps of map compilation are eliminated. High-altitude aerial photographs are particularly suitable, as are orthophotographs. Such photography is precise enough to eliminate the preparation of a costly controlled mosaic. Plans for the survey must consider all costs of map construction—fieldwork, compilation, finishing, and publication. Plans must be made in advance of field operations, especially if contracts are to be let for new photography. Completion of aerial photography contracts can be delayed for a long time by adverse weather conditions. In ordering new photography, time must be allowed for preparing specifications, awarding contracts, photographing the area, and inspecting before accepting the work. The cost of original aerial photography varies greatly. Enabling aerial photography contractors to keep their equipment and personnel busy throughout the year and taking advantage of favorable seasonal conditions reduces the cost of aerial photography. Such factors as geographic latitude and solar altitude must be considered in scheduling flights to reduce or eliminate objectionable shadows. Trees should be bare and other vegetation at a minimum for the best results. This requirement further limits the flying season in the northern half of the United States. Moisture conditions are important in revealing soil patterns. In areas of the central United States where annual row crops are the main type of crop, the lack of ground cover and soil-moisture conditions are nearly optimum for indicating soil pattern sometime between late April and the end of June. For economy, scheduling requires close study of regional weather patterns in a survey area in order to forecast the number of "photographic days" (no more than 10 percent cloud cover) in each month. Orthophotographs.—An orthophotograph is an aerial photograph with nearly all the image displacement and scale errors removed. Aerial photographs are converted to orthophotographs by simple rectification for low-relief terrain or by differential rectification for high-relief terrain. Orthophotography is prepared by methods designed to meet National Map Accuracy Standards. Various accuracy tests are performed to verify that 90 percent of the well-defined points tested are within 12.19 meters of true horizontal position—the horizontal accuracy standards for a 1:24,000 scale. An orthophotoquad is an orthophotograph formatted to the same size and scale as any of the USGS topographic quadrangles. Orthophotographs portray an abundance of detail and have correct scale and positional accuracy that is not found in conventional aerial photography. Production costs of orthophotographs compare favorably with controlled mosaic production costs. Orthophotographs of varying scales are used as base maps for soil surveys, land-use planning, resource studies, and topographic maps. Orthophotography can be enhanced with such cartographic features as contours, political boundaries, highways, and principal places to provide maps designed to meet the general need of most users. Aerial mosaics.—Aerial mosaics are made by matching and assembling individual photographs to form a continuous image of an area. Several methods of assembly are used, and the resulting mosaics vary widely in accuracy and usefulness. The two general types of aerial mosaics are uncontrolled and controlled. An uncontrolled mosaic is made by simply matching like images on adjoining photographs without geographic control of the positions of the features. A controlled mosaic, displays photographs that are very close to uniform scale and rectified to reduce tilt and displacement. Features on the mosaic are close to their correct positions on the map grid. The accuracy of a controlled mosaic approaches that of a good planimetric map. Between the uncontrolled mosaic and the controlled mosaic are a wide variety of semicontrolled mosaics for which different degrees of ground control are used. Mosaics vary greatly in accuracy and must be carefully checked before being used in soil mapping. Because an aerial mosaic covers a larger area than a single photograph, fewer photobase sheets need be matched and the chance for error is reduced. A mosaic can be made to cover a specific area, such as a township or a drainage basin. Topographic maps.—Topographic maps are not photographs. A topographic map represents horizontal and vertical positions of physical features by using standard symbols. Published maps usually show cultural features such as roads, railroads, and buildings in black; drainage features in blue; and contour lines in brown. Some also show additional features, such as vegetation in overprints of green or other colors. Most topographic maps published by the U.S. Geological Survey and other Federal agencies comply with national standards of map accuracy. The standards for horizontal accuracy require that not more than 10 percent of the tested points be in error by more than a specified distance on the map. This distance is 0.85 mm for maps published at scales larger than 1:20,000 and 0.50 mm for maps published at 1:20,000 or smaller. These limits apply to positions of such well-defined points as roads, monuments, large structures, and railroads that are readily visible and can be plotted on the map within 0.25 mm of their true positions. Standards for vertical accuracy require that not more than 10 percent of the tested elevations be in error by more than one-half of the contour interval. Because of the prescribed standards of accuracy, topographic maps published by different agencies differ little. Some variation may be noted in format, scales, sheet boundaries, and classification and selection of planimetric detail—variations due primarily to the need to meet specific requirements. Standard topographic maps are published in quadrangles bounded by lines of latitude and longitude. Generally, topographic quadrangles cover 30 minutes, 15 minutes, 7 1/2 minutes, or 3 3/4 minutes of latitude and longitude. Scale varies with topography and contour interval. The most common publication scales are 1:24,000 (the largest generally available), 1:25,000, 1:31,680, 1:48,000, 1:62,500, and 1:63,360. Coverage at 1:250,000 compiled from larger scale maps is distributed by the Geological Survey for the entire country, and a new series of maps at scales of 1:50,000 and 1:100,000 is available for certain areas. The smaller scale maps are useful as the bases for general soil maps, for reference, and for schematic soil maps. Topographic maps can be used as the base for detailed mapping if recent large-scale maps are available for the whole survey area. The accuracy of standard topographic maps gives them definite advantages in measuring distances and directions. The topographic pattern is helpful in understanding soil and studying drainage, irrigation, and hydrology. The detail on the maps relieves soil scientists of part of the task of recording the location of ground features while mapping. As a base for soil mapping, topographic quadrangles lack the details—field boundaries, isolated trees and bushes, fences, and similar features—that are shown on photographs. The small scale of many topographic maps is a disadvantage. The topographic maps of recent years made from aerial photographs by photogrammetric methods are much more accurate than old topographic maps which may not be accurate and may need too many revisions to be useful. In the United States, most standard topographic maps are published by the U.S. Geological Survey. The cartographic staffs of the Soil Conservation Service receive new lists and new quadrangles as they are published and can supply information about work in progress, expected dates of completion, and the topographic mapping program. Topographic maps needed for a soil survey can be ordered directly from the Geological Survey. Preliminary proofs or copies of manuscript material frequently can be obtained in advance of publication if the need is urgent. Topographic maps of standard accuracy are expensive to construct and publish, but the published maps can be purchased for a small price per sheet. Besides serving as the mapping base in some areas, they are useful references. Maps and data-base requirements.—The demand for natural resource data in SCS and the Federal sector has increased. In the past these data were displayed on various base maps that generally did not meet national map accuracy standards. Soil Conservation Service could not feasibly digitize the resource data because the use by SCS and other agencies is limited by the inaccurate bases being used. Using accurate uniform scale orthophotographs and planimetric base maps, resource data will be digitized and available for automated mapping procedures and repeated manipulation in providing various inventories and interpretative maps at great cost reduction. Repeatability of use of digitized data by SCS and other agencies, including exchanging of digitized resource data by agencies, precludes a duplication of effort in the Federal sector and results in savings in Federal mapping programs. Selecting Mapping ScaleThe best map scale for a survey is determined by many factors. The purposes of the map are the main consideration. Soil maps in areas of intensive land uses are designed for predictions about soil use, management, and behavior in relatively small areas. The scale must be large enough to permit delineation of most areas significant for such predictions. The scale does not have to be large enough to include all property lines, cultural features, works, and structures for detailed plans to be plotted directly. A large scale increases the number of map sheets, the amount of joining of sheets, and the cost of compilation, reproduction, publication, and storage. Most soil surveys are made at a scale of 1:24,000 or 1:12,000. A scale of 1:24,000 commonly is used for surveys in areas of less intensive land use. Scales of 1:12,000 are needed for highly detailed surveys. Generally, the scale of mapping depends on the intricacy of the soil pattern in relation to the expected intensity of soil use. The patterns of soils are very complex in many areas where potentials do not justify a mapping scale large enough to show the patterns in detail. Where the purposes of the survey do require that small areas be delineated, the scale must be large enough to permit delineating and labeling the areas. Part of a survey area may have high value or intensive land use that justifies a scale larger than that of the rest of the area. Two publication scales can be used in such an area if the needs justify the extra costs. Legibility of the maps is very important. Many potential users will not use maps that they can not read easily. Figure 2-4 illustrates differing legibility of the same map at different scales. Map C is clearly illegible. Map B can be read with difficulty. Map A is reasonably legible. If the map is to be published at scale B, detail that is legible only at scale A should not be delineated. Table 2-2 gives a general idea of the smallest areas that can be shown legibly at different scales. These sizes are for isolated areas within much larger delineations. If numerous intermingled areas of the smallest size are delineated, the map will be difficult to use. If the field sheets are made at the planned publication scale, the amount of detail that should be drawn in the field is limited to that judged adequate for the purposes of the published survey. Using the publication scale also eliminates the necessity of transferring the field mapping to a different scale. If mapping scale is larger than publication scale, the surveyor should try to visualize what the map will look like at publication scale. A reducing lens can be used. Reference MapsMany types of maps are published by public and private agencies. They range from small-scale road maps prepared by oil companies and county highway maps prepared by State highway departments to the large-scale detailed maps used in city planning. Most reference maps are designed, constructed, and reproduced to meet a special purpose. Necessary details are emphasized and others are subordinated. On small-scale road maps, for example, highways, highway numbers, towns and cities, points of interest, and mileages are prominently shown; drainage, railroads, pipelines, powerlines, and public land lines are omitted or subordinated. Aeronautical charts.—These are designed and constructed specifically for air navigation. The scale is small so that large areas can be shown on a single sheet. Ground features that are prominent from the air are emphasized in bold and simple symbols. Other features of equal importance on the ground but less noticeable from the air are subdued or omitted entirely. Elevation is shown by gradient tints. Navigational data are shown by bright overprinting. Plats.—These are prepared from public land surveys and are designed to present land survey data. They usually cover a survey unit, such as a township. The scale is large. Courses and distances, subdivisions of sections, acreage figures, and other data from the survey are shown. Cultural and drainage features are reduced to a minimum and are accurate only on the survey lines. Special-purpose maps have little value as bases for detailed soil surveys. Such maps are very useful for references, however, and they may be the best base maps available for surveys of remote areas. Aeronautical charts, for example, are useful for rapid small-scale surveys of large areas. Many other kinds of special maps are available for some areas. These include maps of published soil surveys, maps of geology, maps of forest or other vegetative cover, coast and harbor charts, census maps, U.S. Postal Service maps, and highway maps. County highway planning maps are available for many areas and are good references. Some State highway or transportation departments make good small- to medium-scale highway planning maps for internal use that can be reproduced with special permission. Maps protected by copyright cannot be reproduced without permission. Index Maps for Field SheetsAn index map is prepared to show approximately the location of each field sheet. A useful scale is about 1:125,000. Many States have county highway maps at about this scale, and many of these are good bases for preparing the index. The mapping limits of each field sheet are drawn on the index map, and the field sheet number is written in each area. The preferred position for the label—"Index to Field Sheets"—is at the top center. Indexing by column and row makes the sheets easy to use. For example, if the northwest sheet is 1-1 (column 1, row 1), then the next sheet south is 1-2 (column 1, row 2), and the sheet east is 2-1 (column 2, row 1). The index accompanies the completed survey when it is submitted for map assembly. Footnotes
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