What Board Members Need to Know
section of the website is not yet complete, so please check
back for updates! We will be featuring additional resources
on topics such as writing
and how to link local actions to water quality.
are perhaps the single most useful natural feature available
to predict water flow and pollutant pathways. For example, soils
that are sandy generally allow rapid infiltration of rainwater,
unlike silty soils which channel rainwater towards streams and
rivers allowing little infiltration. Below is a basic overview
of how to interpret soils data.
to the top
Soils in the United States were
first mapped by the Soil Conservation Service, the predecessor
to the National Resource Conservation Service (NRCS). The most
recent soils data for Rhode Island are contained in the Soil Survey
of Rhode Island, published in 1981 but based on field work completed
in the 1970's. In this document the soils of Rhode island are mapped
and classified into 43 different soil series based on features
such as texture and drainage characteristics. Soil series are commonly
named for the place where that soil series was first mapped, such
as Narragansett silt loam, which was first identified in Narragansett,
In the Soil Surveys each soil series
is categorized in a series of tables that provide information on
development suitability, drainage characteristics, how well the
soil will support crops, hydrogroup and many other characteristics.
Soil hydrogroup is an important consideration for development suitability
analysis. There are four hydrogroups: A, B, C and D. These groups
indicate soil permeability and the potential for rainfall to either
seep into or run-off the ground surface. A is the most permeable
and D is the least permeable (usually wetlands soils).
Soil Symbols (NaA, NeA, NP, NfB)
The first letter, always a capital,
is the initial letter of the soil name. The soil name indicates
the soil series. For example, NaA represents Narragansett silt
loam, 0-3% slope, while NeA stands for Newport silt loam 0-3% slope.
The second letter is a small letter unless the mapping unit includes
a broad range of characteristics, then it is a capitol letter.
For example, NP is the symbol for Newport -Urban land complex.
A soil complex that includes two or more soils that have different
characteristics but occur together in such an intricate pattern
that they cannot be mapped separately at the scale used in Soils
Survey of Rhode Island. The soil name also indicates soil texture
and stoniness. For example, Nf B is the symbol for Newport very
stony silt loam, 3-8% slopes. The third letter, always a capital
A, B,C, D, or E, indicates the slope. The slope categories vary
based on soil type but generally range as follows: slope A = 0
to 3 %; B = up to 8%; C = up to 15%; D = greater than 25 %
of Soil Types
It is important to note the Soil
Survey of Rhode Island is designed as a planning tool and is
not intended for parcel-level analysis. A site-specific soil
is needed to determine actual soil conditions on a particular
the proportion and location of soil constraints is valuable in
predicting pollution risks and
in selecting pollution controls. But the effect of soil type in
estimating runoff impacts becomes less important in urban areas
with extensive drainage "improvements". Stormwater drainage
systems, channelized streams, and artificially drained fields and
building sites all bypass natural rainfall storage and infiltration
processes quickly diverting runoff to downstream discharge points.
These artificial improvements are not identified on soil maps and
must be field-inventoried.
to the top
of Soil Hydrogroup Descriptions and Development Risks
downloadable table includes descriptive information
on soil hydrogroup as well as possible risks to
surface and groundwater associated with development
of each soil group. Management actions to mitigate
those risks are also included in the table. (Soil
Hydrogroup Description and Development Risks Table,
to the top
of Rhode Island Soil Characteristics and Soils Fact
Summary of Rhode Island Soil Characteristics table provides
a summary of Rhode Island soil characteristics
such as flooding duration and hydrogroup by soil
of Rhode Island Soil Characteristics, PDF).
The Soils Fact Sheet provides a review of basic
soils information as well as an explanation of
the codes used in the Summary of Rhode Island Soil
Characteristics table. (Soils Fact Sheet, PDF).
to the top
Soils Data in Project Review
Soils data are available in electronic format from both the Rhode Island Geographic
Information System (RIGIS) as well as the Soil Survey Geographic (SSURGO) database.
The RIGIS soils coverages are based on the SSURGO data and are a rich resource
for development suitability analysis. The basic RIGIS soil data table alone
describes nineteen different characteristics ranging from slope to farmland
value to hydrologic characteristics. These data are appropriate for an initial
site review, but a site specific soil survey should always be undertaken before
development because the RIGIS/SSURGO data are not extremely accurate at the
scale of a small development. STATSGO (State Soil Geographic) data is also
available from the NRCS, however STATSGO data are designed for large-scale
planning activities making these data a poor choice for looking at projects
on the county or local level.
The approach advocated by RI NEMO
for mapping of soils characteristics is associated with the MANAGE
method for site assessment. For more information about the MANAGE
method please click here (MANAGE).
Soils mapped using the MANAGE approach are classified based on
Seasonal High Water Table (SHWT) depth and hydrogroup. When soils
are mapped in this manner likely pathways for water flow and pollutant
movement are revealed. Sandy soils (Hydrogroup A) with deep water
tables are zones of groundwater recharge where pollutants can easily
infiltrate to deeper groundwater. In contrast, slowly permeable
soils (hydrogroup C and D) have low infiltration rates and tend
to have high water table. These areas naturally collect surface
water and generate runoff. High water table areas are almost always
connected to small streams, wetlands and intermittent drainage
ways, forming an extended drainage network. As a result of these
connections, pollutants generated in these areas can move rapidly
to surface waters or to shallow groundwater. Septic systems constructed
in these slowly permeable soils are also more likely to fail, especially
where a dense compacted "hardpan" soil layer restricts
downward flow of water.
Soil Risk Map Legend and Description
The legend provided below is the
standard legend used to describe soils in the MANAGE method. This
legend highlights soils with the greatest potential for transporting
pollutants to surface water bodies (slow and slow/wetland soils)
or to groundwater (Very rapid soils).
The legend above is based upon
the soil characteristics exhibited in the table below:
A, Deep Water Table
or loose soils, very permeable
Rapid, >6 ft)
B, Deep Water Table (>6')
texture and permeability; few development constraints; may
have excessively permeable subsoil
|| (Moderate, >6
B, Shallow Water Table (1.5’ – 3.5’)
texture and permeability, seasonal high water table
C, Deep Water Table (>6')
soils slowly permeable with restrictive layer
C, Shallow Water Table (1.5’ – 3.5’)
soils slowly permeable with restrictive layer and seasonal
high water table
1.5 – 3.5)
C, Very Shallow Water Table (0 – 1.5’)
D, Very shallow Water Table (0 – 1.5’)
soils with restrictive layer and seasonal very high water
hydric soils, essentially wetlands
to the top
Geographic Information Systems (GIS) and Understanding
The two self-guided
Power Point presentations provided below are meant to serve as
brief introductions for those who are unfamiliar with Geographic
Information Systems (GIS). If you would like additional information
about mapping and GIS, please visit the link to Mapping
Links and Resources.
brief Power Point presentation provides
an overview of what Geographic Information Systems are
and some of their capabilities (graphics and some text
provided by Paul Jordan, RIDEM Office of Management Information
Easy Way to Access Rhode Island GIS Data
This brief Power
Point presentation provides information on websites where Rhode
Island GIS data are available for use without specialized software.
A tutorial on the use of the Rhode Island Environmental Resources
Interactive Data Viewer is included.
the Accuracy of Scale
Scale is one
of the most often misunderstood aspects of GIS and maps in general.
All map data is created at a specific scale; in the case of RIGIS
mostly 1:24,000 or 1:12,000. For example, a scale of 1:24,000
means that 1 inch on the map equals 24,000 inches (or 2,000 feet)
on the ground. When we zoom in to a larger scale (ie: 1:10,000),
the data become less accurate and obvious errors begin to appear.
This is because at 1:24,000 scale one inch equals 2,000 ft but
at the 1:10,000 scale, one inch equals only 10,000 feet, incorrectly
implying that there is more data then at the 1:24,000 scale.
GIS and other non site-level data are useful for general assessment
but the user must keep scale limitations in mind and request
more refined data when needed.
A good way
to visualize error is "variation around a line” as
shown below. For example, when a road section is mapped there
is some error inherent in the equipment used to map the road.
The errors will fall randomly along either side of the line making
the line appear wider then it really is. These types of inaccuracies
become more apparent as the user zooms in.
talk about large and small scale maps. This comparison is a statement
regarding the ratio of the map units. A map with a scale of 1:100,000
inches (1 inch equals 100,000 inches or 1/100,000) is smaller
scale then a map with a scale of 1:5,000 (1/5,000) inches. A
large scale map is more detailed and generally shows a smaller
area then a small scale map, generally used to provide an overview.
different scale maps:
Larger Scale Map
This map of a subdivision is a larger scale than the map of
the Town of Cumberland, Rhode Island
This map of the Town of Cumberland Rhode Island is much larger sclae
than the map of the entire State of Rhode Island (shown to the right)
This map of the State of Rhode Island exhibits very little detail and
is a much smaller scale than the map of the Town of Cumberland (shown
to the left)
typically use equivalent scales such as 1" = 400'. Switching
from one type of scale to the other can be confusing. The table
below presents corresponding ratio and equivalent scales.
RIGIS data are designed for use at this scale
GIS data are
produced at many different scales, each with its own level of
error. A good rule of thumb is to treat all
GIS/general data as approximate. It is acceptable to use
these different scale data together, but be aware that inaccuracy
can be compounded. Accuracy is related to the Minimum Mappable
Unit – the smallest area that was collected as data. For
example, if the minimum mappable unit for wetlands is ¼ acre
then the smallest amount of wetland that will be placed on the
map will be ¼ acre. Smaller areas of wetlands will not
be collected. Metadata for each RIGIS data layer are extremely
useful in determining the accuracy of data.
to accuracy, data for Rhode Island fall roughly into seven groups
as presented in the table below. Please note that this table
does not include all RIGIS data but is solely an overview.
Map/Date of Production
RIGIS Data, Topographic Maps, and NOAA Navigational Charts(1)
7.5 Minute Topographic Quadrangle/ NOAA Charts (various years)
- 50’ Note: Land use/cover has1/2 acre min. mappable
Soils (RIGIS/SSURGO) (2)
Survey of Rhode Island (1976-81)
- 40’ (delineation boundaries may also have error)
Note: 1/4 acre min. mappable unit.
Wetlands (RIGIS/ NWI) (3)
- 75’ Note: 1/4 acre min. mappable unit.
RIGIS Roads, Hydrography, and Orthophotography (4)
Parcel Data (5)
or digitized plat maps
Quality Parcel Data
or digitized plat maps based on high resolution planimetric
- The 1:
24,000 USGS Topographic map has 10’ contours and is of
limited value for site assessment. A detailed site engineering
survey with 2’ contours is much more useful.
- The 1988
wetlands data were created from aerial photographs at a scale
of 1: 24,000. Standard error associated with this scale data
(+ or - 50’). But because of the uncertainty associated
with remote delineation, potential error is considered + or
- 75’. In addition, only wetlands ¼-acre and larger
were mapped. This means that the wetlands line on a GIS-derived
map should only be considered a general indicator- never an
accurate delineation. Field delineation should be done as early
- RIGIS soils
data were created at a scale of 1: 15,840. While adequate for
getting a general sense of soil conditions, it is not as accurate
as a site-specific soil survey. The soil series boundaries
can easily be 40’ off. A site-specific soil survey will
be needed to confirm that soils are accurately mapped and to
identify soil features where the map unit is actually a complex
of soils with dramatically different characteristics. If initial
evaluation eveals very complex soil constraints, it may be
prudent to have the developer commission a site-specific soil
survey early in the review process.
are aerial photographs that have been processed for use with
GIS data. They are available at 1: 5,000 (half-meter pixel)
and 1: 12,000 (one-meter pixel) scales. This means that each
pixel on the map is equal to 1 meter for the 1:12,000 scale.
The smaller the pixel size the more accurate the photograph
and the closer you can zoom in before the picture becomes fuzzy.
data vary greatly because of the different quality base maps
used to produce it. Consult metadata to determine intended
to the top
Reading Maps and
Have you ever
found yourself wondering what some of those symbols and bars
Reading Maps and Plans (available
as a PDF, 3051 KB) is a concise look at the fundamentals of those
types of documents.
Reading 101: Using and Reading Maps and Plans (available
as a Power Point Slide Show, 23,252 KB) is a PowerPoint style
presentation that takes the user through
a quick tour of how to read and use maps.
to the top