The District of Columbia

Soils (SoilPly)

Soils (SoilPly)

by U.S. Department of Agriculture, Natural Resources Conservation Service on 09/14/2006

About: soil survey, soils, Soil Survey Geographic, SSURGO, environment
Taking place at: District of Columbia, Washington West Quadrangle, Washington East Quadrangle, Alexandria Quadrangle, Anacostia Quadrangle


This data set is a digital soil survey and generally is the most
detailed level of soil geographic data developed by the National
Cooperative Soil Survey. The information was prepared by digitizing
maps, by compiling information onto a planimetric correct base
and digitizing, or by revising digitized maps using remotely
sensed and other information.

This data set consists of georeferenced digital map data and
computerized attribute data. The map data are in a soil survey area
extent format and include a detailed, field verified inventory
of soils and miscellaneous areas that normally occur in a repeatable
pattern on the landscape and that can be cartographically shown at
the scale mapped. A special soil features layer (point and line
features) is optional. This layer displays the location of features
too small to delineate at the mapping scale, but they are large
enough and contrasting enough to significantly influence use and
management. The soil map units are linked to attributes in the
National Soil Information System relational database, which gives
the proportionate extent of the component soils and their properties.


SSURGO depicts information about the kinds and distribution of
soils on the landscape. The soil map and data used in the SSURGO
product were prepared by soil scientists as part of the National
Cooperative Soil Survey.

Access constraints


Use constraints

The U.S. Department of Agriculture, Natural Resources Conservation
Service, should be acknowledged as the data source in products
derived from these data.

This data set is not designed for use as a primary regulatory tool
in permitting or citing decisions, but may be used as a reference
source. This is public information and may be interpreted by
organizations, agencies, units of government, or others based on
needs; however, they are responsible for the appropriate
application. Federal, State, or local regulatory bodies are not to
reassign to the Natural Resources Conservation Service any
authority for the decisions that they make. The Natural Resources
Conservation Service will not perform any evaluations of these maps
for purposes related solely to State or local regulatory programs.

Photographic or digital enlargement of these maps to scales greater
than at which they were originally mapped can cause misinterpretation
of the data. If enlarged, maps do not show the small areas of
contrasting soils that could have been shown at a larger scale. The
depicted soil boundaries, interpretations, and analysis derived from
them do not eliminate the need for onsite sampling, testing, and
detailed study of specific sites for intensive uses. Thus, these data
and their interpretations are intended for planning purposes only.
Digital data files are periodically updated. Files are dated, and
users are responsible for obtaining the latest version of the data.

Point of contact

in U.S. Department of Agriculture, Natural Resources Conservation Service

USDA - Natural Resources Conservation Service
Annapolis MD, 21401-5534,

phone: 443-482-2913

Data accuracy


Attribute accuracy is tested by manual
comparison of the source with hard copy plots and/or symbolized
display of the map data on an interactive computer graphic system.
Selected attributes that cannot be visually verified on plots or
on screen are interactively queried and verified on screen. In
addition, the attributes are tested against a master set of valid
attributes. All attribute data conform to the attribute codes in
the signed classification and correlation document and amendment(s).

Logical consistency

Certain node/geometry and topology GT- polygon/chain relationships
are collected or generated to satisfy topological requirements
(the GT-polygon corresponds to the soil delineation). Some of these
requirements include: chains must begin and end at nodes, chains
must connect to each other at nodes, chains do not extend through
nodes, left and right GT-polygons are defined for each chain
element and are consistent throughout, and the chains representing
the limits of the file are free of gaps. The tests of logical
consistency are performed using vendor software. All internal
polygons are tested for closure with vendor software and are checked
on hard copy plots. All data are checked for common soil lines (i.e.,
adjacent polygons with the same label). Edge locations generally do
not deviate from centerline to centerline by more than 0.01 inch.


A map unit is a collection of areas defined and named 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
each map unit has a symbol that uniquely identifies the map unit
on a soil map. Each individual area, point, or line so identified
on the map is a delineation.

Soil Scientists identify small areas of soils or miscellaneous areas
that have properties and behavior significantly different than the
named soils in the surrounding map unit. These minor components
may be indicated as special features. If they have a minimal effect
on use and management, or could not be precisely located, they may
not be indicated on the map.

A map unit has specified kinds of soils or miscellaneous areas
(map unit components), each with a designated range in
proportionate extent. Map units include one or more kinds of soil
or miscellaneous area. Miscellaneous areas are areas that have little
or no recognizable soil.

Specific National Cooperative Soil Survey standards and procedures
were used in the classification of soils, design and name of map
units, and location of special soil features. These standards are
outlined in Agricultural Handbook 18, Soil Survey Manual, 1993,
USDA, NRCS; Agricultural Handbook 436, Soil Taxonomy, 1995,
USDA, NRCS; and all Amendments; Keys to Soil Taxonomy,
(current issue) USDA, NRCS; National Soil Survey
Handbook, title 430-VI,(current issue) USDA, NRCS.

The actual composition and interpretive purity of the map unit
delineations were based on data collected by scientists during
the course of preparing the soil maps. Adherence to National
Cooperative Soil Survey standards and procedures is based on
peer review, quality control, and quality assurance. Quality
control is outlined in the memorandum of understanding for the
soil survey area and in documents that reside with the Natural
Resources Conservation Service state soil scientist. Four kinds
of map units are used in soil surveys: consociations, complexes,
associations, and undifferentiated groups.

Consociations - Consociations are named for the dominant soil.
In a consociation, delineated areas use a single name from the
dominant component in the map unit. Dissimilar components are
minor in extent. The soil component in a consociation may be
identified at any taxonomic level. Soil series is the lowest
taxonomic level. A consociation that is named as a miscellaneous
area is dominantly that kind of area and minor components do not
significantly affect the of the map unit. The total amount of
dissimilar inclusions of other components in a map unit generally
does not exceed about 15 percent if limiting and 25 percent if
nonlimiting. A single component of a dissimilar limiting inclusion
generally does not exceed 10 percent if very contrasting.

Complexes and associations - Complexes and associations consist
of two or more dissimilar components that occur in a regularly
repeating pattern. The total amount of other dissimilar components
is minor extent. The following arbitrary rule determines whether
complex or association is used in the name. The major components
of an association can be separated at the scale of mapping. In
either case, because the major components are sufficiently different
in morphology or behavior, the map unit cannot be called a
consociation. In each delineation of a complex or an association,
each major component is normally present though their proportions
may vary appreciably from one delineation to another. The total
amount of inclusions in a map unit that are dissimilar to any of
the major components does not exceed 15 percent if limiting and
25 percent if nonlimiting. A single kind of dissimilar limiting
inclusion usually does not exceed 10 percent.

Undifferentiated groups - Undifferentiated groups consist of two
or more components that are not consistently associated
geographically and, therefore, do not always occur together in
the same map delineation. These components are included in the
same named map unit because their use and management are the same
or very similar for common uses. Generally they are grouped together
because some common feature, such as steepness, stoniness, or
flooding, determines their use and management. If two or more
additional map units would serve no useful purpose, they may be
included in the same unit. Each delineation has at least one of the
major components, and some may have all of them. The same principles
regarding the proportion of minor components that apply to
consociations also apply to undifferentiated groups. The same
principles regarding proportion of inclusion apply to
undifferentiated groups as to consociations.

Minimum documentation consists of three complete soil profile
descriptions that are collected for each soil added to the legend,
one additional per 3,000 acres mapped; three 10 observation
transects for each map unit, one additional 10 point transect per
3,000 acres.

A defined standard or level of confidence in the interpretive
purity of the map unit delineations is attained by adjusting the
kind and intensity of field investigations. Field investigations
and data collection are carried out in sufficient detail to name
map units and to identify accurately and consistently areas of
about 10 acres.

Horizontal positional accuracy

The accuracy of these digital data is based upon their
compilation to base maps that meet National Map
Accuracy Standards at a scale of 1 inch equals 1,000
feet. The difference in positional accuracy between the
soil boundaries and special soil features locations in the
field and their digitized map locations is unknown. The
locational accuracy of soil delineations on the ground varies
with the transition between map units.

For example, on long gently sloping landscapes the transition
occurs gradually over many feet. Where landscapes change
abruptly from steep to level, the transition will be very
narrow. Soil delineation boundaries and special soil features
generally were digitized within 0.01 inch of their locations on
the digitizing source. The digital map elements are edge matched
between data sets. The data along each quadrangle edge are
matched against the data for the adjacent quadrangle. Edge
locations generally do not deviate from centerline to centerline
by more than 0.01 inch.

Lineage / Sources

  • Soil Survey of District of Columbia
    paper, information for soil and map unit delineation, special soil feature location and data on soil properties
    by U.S. Department of Agriculture, Natural Conservation Service , 1976
  • DLG-3 files
    digital, digital information containing area and special features
    by U.S. Department of the Interior, unpublished material
  • National Aerial Photography Program (NAPP)
    stable-base material, base used for field mapping
    by U.S. Geological Survey, 2005
  • ARC GIS GEODATABASE files for the soil survey of District of Columbia
    CD-ROM, digital information containing area and special soil features for evaluation
    by U.S. Department of Agriculture, Natural Resources Conservation Service , unpublished material
  • National Soil Information System (NASIS) database for District of Columbia
    online, map unit legend used for comparison to spatial map unit labels
    by U.S. Department of Agriculture, Natural Resources Conservation Service , 2006
  • National Soil Information System (NASIS) data base
    database, attribute (tabular) information
    by U.S. Department of Agriculture, Natural Resources Conservation Service , 2006
Lineage / Processes


Visit the above mentioned Internet Web Site, select state or
territory, then select individual soil survey area of interest.
Spatial line data and locations of special feature symbols are in
ESRI ArcGIS (ArcView,ArcInfo) shapefile, coverage and interchange
(i.e., export) formats. The National Soil Information System
attribute soil data are available in variable length, pipe
delimited, ASCII file format.

Distribution liability
<br />Although these data have been processed successfully on a computer<br />system at the U.S. Department of Agriculture, no warranty expressed<br />or implied is made by the Agency regarding the utility of the data<br />on any other system, nor shall the act of distribution constitute<br />any such warranty. The U.S. Department of Agriculture will warrant<br />the delivery of this product in computer readable format, and will<br />offer appropriate adjustment of credit when the product is determined<br />unreadable by correctly adjusted computer input peripherals, or<br />when the physical medium is delivered in damaged condition. Request<br />for adjustment of credit must be made within 90 days from the date<br />of this shipment from the ordering site.<br /><br />The U.S. Department of Agriculture, nor any of its agencies are<br />liable for misuse of the data, for damage, for transmission of<br />viruses, or for computer contamination through the distribution of<br />these data sets. The U.S. Department of Agriculture (USDA) prohibits<br />discrimination in all its programs and activities on the basis of race,<br />color, national origin, sex, religion, age, disability, political<br />beliefs, sexual orientation, or marital or family status. (Not all<br />prohibited bases apply to all programs.)<br />

Distributed by

in U.S. Department of Agriculture, Natural Resources Conservation Service, National Cartography and Geospatial Center

501 West Felix Street, Building 23, P.O. Box 6567
Fort Worth Texas, 76115,

phone: 800 672 5559
fax: 817 509 3469


DCGIS.SoilPly / Agricultural Handbook 18, Soil Survey Manual, 1993, USDA, SCS.

Special Soil Features represent soil, miscellaneous area, or landform
features that are too small to be digitized as soil delineations
(area features).


    Soil Type Detailed
  • Shape / ESRI
    Feature geometry.
    MU KEY
  • GIS_ID
    OCTO GIS Sequential Identifier
  • TYPE
    Soil Type
  • DESC_
    Soil Type Description
    Soil Type Expanded
    Internal feature number.
    Feature geometry.
    Slope Description


FGDC Content Standards for Digital Geospatial Metadata / FGDC-STD-001-1998 as of 06/26/2008

Provided by
REQUIRED: The person responsible for the metadata information.
in U.S. Department of Agriculture, Natural Resources Conservation Service

USDA - Natural Resources Conservation Service
Annapolis MD, 21401-5534,

phone: 443-482-2913