David Torraca
Using GIS in Local Government: Supporting the National Flood Insurance Program
Abstract
The Federal Emergency Management Agency (FEMA) manages the National Flood Insurance
Program (NFIP). Communities are required to adopt floodplain management ordinances to be
included in the program. The Flood Insurance Rate Map (FIRM) produced by FEMA is used by
private and public sector interests to determine the location of flood prone areas. Locales that have
experienced rapid suburbanization have found the maps to be quickly outdated because of a lack of
current base information and exclusion of recent alterations to the floodplain. By establishing a GIS
floodplain database layer, Loudoun County, Virginia, has been able to make more accurate
delineations and economically maintain the data. When overlayed with other regularly maintained
database layers, the information becomes much more versatile and visible. Products and services
provided with the County's GIS include 1:2400 scale floodplain delineation maps, on-line queries,
environmental assessments, acreage reports, and data transfers to other county agencies.
Introduction
In the not too distant past, development took place in some areas of the United States without regard
for the potential of flooding. Property was often destroyed and life put in jeopardy by rising water
levels. Property owners had no recourse to reclaim repair costs of structures damaged in a flood and
often rebuilt in the same location. Since the late 1960's the federal government has provided a means
of insurance against these catastrophic events. Legislation was enacted that authorized the Federal
Emergency Management Agency to create detailed data on areas most prone to flooding throughout
the nation. Through the use of this data, loss of property is minimized and property owners have
access to a cost effective way of insuring their property against such loss. Communities become
involved by establishing ordinances for development within flood prone areas. Loudoun County,
Virginia is one of these communities. The County realized the need for careful planning as
development began to encroach upon areas most susceptible to flooding. In support of the federal
legislation, Loudoun adopted a General Plan to guide growth in environmentally sensitive areas such
as floodplains, and enacted zoning regulations to control development within the floodplain. To
assist in this effort the County utilizes a Geographic Information System (GIS) to store and maintain
planimetric, environmental, ownership, political, and regulatory data. It has proven to be an
economical way to provide planners, staff, and the general public with current information to make
informed decisions.
FEMA
The Federal Emergency Management Agency (FEMA) is charged with developing flood risk data
for use in insurance rating and floodplain management as part of the National Flood Insurance
Program (NFIP) created in 1968. The NFIP was expanded by the Flood Disaster Protection Act of
1973. The Federal Insurance Administration (FIA) administers the program. The NFIP is designed
to enable property owners in participating communities to purchase flood insurance at reasonable
rates. The definition of "community" includes any state, area, or political subdivision which has the
authority to adopt and enforce floodplain management ordinances. Participation in the program is
available to any community that adopts and administers floodplain management measures. Flood
insurance is mandatory in order to receive federally regulated financial assistance for buying and/or
constructing a building in areas determined by FEMA to be high risk for flooding. Most
communities in the nation with serious flooding potential have joined the NFIP.
FEMA compiles Special Flood Hazard Areas (SFHA) from Flood Insurance Studies (FIS). These
mapped areas depict the 100 year flood, also known as the base flood, and are used as the national
standard on which the NFIP requirements are based. The 100 year flood limit is the area that has
a one percent or greater chance of a flood level being equaled or exceeded in any given year. The
maps delineating these areas are known as Flood Insurance Rate Maps (FIRM). Other floodplain
information included on a FIRM are 100 year flood elevations or depths, flood insurance risk zones,
the 500 year floodplain boundary, and the area known as the "floodway". The floodway includes
the channel of a river and the adjacent floodplain where water would move or flow in a flood event.
FEMA prohibits development that would cause a rise in base flood elevations within the floodway.
Base map information on a FIRM may include major highways, secondary roads, lakes, railroads,
streams, and other waterways. Maps are produced at different scales, include different types of
jurisdictions, and are available in different formats, namely z-fold and flat maps. FIRM's are used
by private citizens, community officials, insurance agents, and lending institutions.
Loudoun County provides FIRM's for viewing by the public at the County's Office of Mapping and
Geographic Information. The flood prone areas of unincorporated Loudoun County are covered by
12 FIRM maps, or panels, at a scale of 1 inch equals 1000 feet. The remainder of the County is
covered by 6 panels at a scale of 1 inch equals 2000 feet. The mapping was initially completed by
FEMA in 1978 and revised in 1985. To determine the flood insurance risk zone for a property you
first need to use the FIRM index for the county to identify the FIRM panel number. The index has
a general map of the county with major highways, streams, and incorporated town boundaries
located. Once you view the panel, you need to identify the general area more specifically by the
secondary roads and waterway features. You may have to refer to other maps showing more detail
for a better general idea of the vicinity. To specifically locate the parcel and any structures, you
should scale the parcel plat or legal description onto the FIRM. Once you have identified the precise
location by this method you should be able to determine the zone where the property is located and,
if necessary, interpolate the base flood elevation. Persons interested in a parcel of land may use this
method prior to purchase or construction. County staff, insurers, and lending institutions verify this
information prior to development.
Locating oneself on a FIRM is often problematic. Persons unaccustomed to working with deeds
and/or plats may have difficulty in adequately locating the area of interest due to the FIRM's lack
of complete base planimetric information (i.e. buildings, roads, fences) and parcel lines. Without
utilizing additional sources containing these features precise locations are often difficult. Also,
features that are mapped on the FIRM are often outdated. In some areas roads may be realigned or
replaced entirely by new development. In rapidly developing areas even the most recently mapped
information can be out of date within days. Scaling may also pose a dilemma for some users. Plats
and deeds come in a variety of measurement systems and a multitude of scales. It is often necessary
to interpolate measurements of distances on a plat at a specific scale, then transfer this information
to the FIRM which may be at a different scale. Accurately transferring these measurements can
prove difficult and is prone to approximations. Even the floodplain information provided on a FIRM
lacks the most recent alteration information. New engineering study alterations will not be included
until a new map is published.
If it appears that the FIRM is incorrect, there are three procedures for changing or correcting the
map. The first is a Letter of Map Amendment (LOMA) which requires the owner to submit
technical data in the form of a certified topographic and hydrographic analysis to establish the
location of the property outside of the designated SFHA. The second is a Letter of Map Revision
(LOMR) submitted through the chief executive officer of the community, to change flood zones,
floodplain and floodway delineations, flood elevations, and planimetric features. The third is a
physical map revision which usually follows a LOMR. It is an official republication of a FIRM.
Geography
Historically, Loudoun County, Virginia has been an agricultural community. Today, agricultural
activity occurs mainly in the valleys of the western half of the County. The eastern portion of the
County is rapidly being urbanized. The growth in the east is attributable to its geographic proximity
to the Washington DC metropolitan region and the development of Dulles International Airport.
Of 23,033 residential building permits issued between 1985 and 1994, 69 percent (15,826) were
located in the east. Development has been in the form of suburban housing, commercial centers,
light industry, and infrastructure improvements.
The Potomac River forms the northern border of Loudoun with the State of Maryland. The
boundary is defined as the mean low water mark. At the northwestern limit of the County the
Shenandoah River joins the Potomac and cuts through the Blue Ridge near Harpers Ferry, West
Virginia. The river traverses forty-five miles before reaching Loudoun's eastern border, 15 river
miles northwest of Washington, DC.
Loudoun County encompasses two physiographic provinces within 521 square miles. The Blue
Ridge, in the west, is composed of three prominent ridges running north to south with valleys of
gently rolling to hilly topography. This region contains 60 percent of Loudoun's land area. The
highest elevation in the County, 1796 feet above sea level, is found on the westernmost ridge. The
Piedmont Plateau, in the east, accounts for the remaining 40 percent of the County's area. It is
characterized by broad undulating to nearly level ridges with some steep areas adjacent to major
streams. Rolling to hilly topography is found in the northeastern portion. Soils in this portion are
generally very poorly drained. The lowest elevation in Loudoun County, 180 feet above sea level,
occurs along the Potomac River at the County's northeast border.
Loudoun is divided into fifteen (15) major watersheds all of which are part of the Potomac River
Basin. The southernmost, Bull Run, accounts for 5 percent of the land area (18,304 acres) and is the
only drainage area that does not flow northerly to the Potomac River. Other major drainage systems
include Broad Run, Goose Creek, and Catoctin Creek, and several minor tributaries to the Potomac
River, encompassing the remaining 95 percent (315,194 acres). Perennial drainage features vary
from small streams to creeks 20 to 30 feet in width.
One hundred year floodplain comprises nine percent of the County's land area. Approximately half
of the total floodplain land area is located in the eastern third of the County. The County maps
floodplain based on watershed size. Floodplain areas are divided into minor, major, and Potomac
River floodplain, based on the 100 year floodplain limit. Minor floodplain is floodplain associated
with a stream that drains less than or equal to 640 acres. At least three percent of the land in
Loudoun is defined as minor floodplain. Major floodplain is floodplain associated with a stream that
drains an area greater than 640 acres. Five percent of the land area in the County is major floodplain.
Potomac River floodplain is the 100 year floodplain area adjacent to the river and amounts to only
one percent of the County's land area.
General Plan
The County Planning Commission is responsible for preparing elements of Loudoun County's
Comprehensive Plan under the Virginia State Code. The Choices and Changes General Plan is the
official document created to support the County's Comprehensive Plan. Developed as a guide for
growth and adopted in 1991, it does not contain regulatory limits, but it does serve as the basis for
ordinances enacted to regulate zoning, subdivision, and site development. The Plan also serves as
the basis for specific land use plans known as "area plans" for particular areas or communities. Area
plans provide more detailed land use guidance by maintaining distinct urban and rural areas. The
General Plan calls for increased urbanization of the eastern portion of the County, while retaining
the rural character of the western portion with some additional development adjacent to incorporated
towns.
The Plan attempts to identify, designate, and protect key resources using refined data and planning
techniques. Identification includes grouping elements of the environment that share common
characteristics. For example, the water related resource group includes water quality, floodplains,
wetlands, scenic rivers, Potomac shoreline, and stream corridors. Identification also includes the
physical location of these features and their interrelationship. Loudoun uses GIS to define
environmentally sensitive areas, analyze impacts, and show location. Maps and databases
constructed to assist in these determinations include floodplain overlays, stream buffer zones,
watershed boundaries, and hydric soil locations.
Floodplains are specified in the Plan as ideal environments for recreational facilities and wildlife
habitat. Since increased urbanization increases impervious surface area and consequently the
volume of runoff, floodplain policies for developing areas need to be updated periodically. Policies
include:
- storm water management strategies to minimize increased demand for flood storage;
- recognizing the interdependency of floodplains and open space, wetlands, wildlife habitats, and
scenic vistas and corridors;
- prevention of the channelization and diversion of streams in areas draining 640 acres or more;
- discouraging stream channel diversions along streams that drain less than 640 acres;
- encouraging the restoration of natural floodplains; and
- the subtraction of floodplain draining more than 640 acres from developable acreage totals.
Zoning Ordinance
The Zoning Ordinance is the regulatory document that implements the County's Comprehensive
Plan. The goals of the Ordinance, as such, are designed to promote the health, safety, and welfare
of the residents of the County. Administration, interpretation and enforcement of the Ordinance is
the responsibility of the County's Zoning Administrator. Originally adopted in 1972 and revised
in 1993, the Ordinance establishes zones for specific land use activities and regulations for
development within those zones. The Zoning Ordinance references the Facilities Standards Manual
(FSM) for Loudoun County which specifies design and construction standards for improvements and
procedures related to subdivisions and site plans.
Regulations within the Ordinance are classified by zoning district. There are three categories of
districts; rural, urban, and special/overlay. Each district is divided into sub-districts designed for
guiding future land use specific to existing natural and man-made conditions and is composed of
regulations that define permitted uses, special exceptions , lot requirements, building requirements,
and use limitations.
Regulations related to floodplains are defined under the Environmental Impact Overlay section as
the floodplain Overlay District (FOD). The intent of these regulations, as specified in the Ordinance,
is to 1) to protect life and prevent or minimize property damage; 2) to reduce public costs for flood
control, rescue and relief efforts; 3) to conserve the natural state of watercourses and watersheds and
minimize the environmental impact on the County's water sources; 4) to comply with Federal and
State law in regards to floodplain management and protection; and 5) to qualify Loudoun residents
for the insurance and subsidies provided by the NFIP.
The Zoning Ordinance distinguishes between allowable uses in major floodplain (floodplain
associated with streams draining greater than 640 acres) and minor floodplain (floodplain associated
with streams draining less than 640 acres). Permitted uses within major floodplain include:
agriculture; fisheries; public or private recreation; storm water management improvements; access
and easements for low density development; incidental structures not exceeding 840 square feet of
floor area; and repair, reconstruction, or improvement of existing residences. Improvement of
existing residences is limited to fifty percent of the market value and no increase in the footprint of
the structure is allowed. Permitted uses within minor floodplain include all of those previously
mentioned, plus farm ponds, recreation courts, swimming pools and alterations. When calculating
permitted floor area and number of residential units within a district, the area included in the major
floodplain zone is excluded.
Applications for uses within a floodplain, except those not requiring the erection of structures, are
required to include grading plans and/or construction plans and profiles and must conform to the
Uniform Statewide Building Code. Alterations of the floodplain include any development action
that changes the cross section, and increases the erosive velocity or the height of flood waters. These
include but are not limited to clearing, grading, excavating, transportation and filling of land. Any
applications for proposed alterations are subject to providing detailed engineering studies in
accordance with the FSM. These studies are reviewed by County staff for conformance by modeling
the proposed change with hydrologic modeling programs. Staff uses HEC1 and HEC2 programs
approved by FEMA to calculate flow rates and watershed impacts of proposed development. These
studies, if approved, are required to be submitted to FEMA prior to the County's record plat approval
and may initiate a change in the FIRM.
The administration of this ordinance relies on an accurate location of the floodplain. Acceptable
sources of floodplain delineations are the 1985 FIRM of Loudoun County at a minimum; flood
hazard studies by the United States Department of Agriculture (USDA) Soil Conservation Service
(SCS), Corps of Engineers, or Geological Survey (USGS); and detailed site specific studies
performed by engineering firms or government agencies. The Zoning Ordinance also states that
minor and major floodplain delineations and elevations shall appear on the floodplain map of
Loudoun County. Loudoun County created this map by plotting SCS and other detailed engineering
study floodplain elevations on a 1:2400 scale topographic base with five foot contour intervals. The
County uses the floodplain map as the official determination of the limits of the FOD in the Zoning
Ordinance.
GIS Development
Loudoun County's modern mapping program began in 1979. The initial effort was to develop an
accurate base map with current planimetric, cultural, and topographic (PCT) features. The process
involved taking aerial photographs of the County at 1:12000 scale and stereoscopically mapping the
features, (buildings, roads, fences, vegetation, topography, etc.), on mylar. Maps were created at
1:12000 (block) scale and 1:2400 (PCT) scale with topography at five foot contour intervals. These
services were provided by contractors under the direction of the County's Department of Natural
Resources. The process for the entire County was completed in 1985 and resulted in the creation
of 26 block maps (A to Z) and 691 PCT maps tied to the Virginia State Plane Coordinate System,
1927 datum. The PCT's also served as a base for all other mapped features. Initially County staff
developed floodplain and soils maps manually using various source data. Detailed engineering data
from SCS or specific engineering studies provided the source material for initial floodplain mapping.
The accuracy of the base map also provided an opportunity for further analysis and refinement of
source data.
The County purchased ArcInfo software in 1986 to develop a parcel overlay based on the PCT.
Data was added to the system by converting existing hardcopy maps to digital form. Among the first
maps to be converted were the tax maps for the County. Other data layers converted included
watersheds, drainage, water bodies, floodplains, soils, wells and pollution sources. Conversion of
the environmental data set was completed by 1990. Conversion of the PCT, including several
updates to specific highly developed areas, was completed in 1995.
Today the County operates its GIS in the Office of Mapping and Geographic Information (OMAGI).
The system resides on a Hewlett Packard 9000 series processor and has 10 gigabytes of storage
capacity. There are approximately 50 full time users of the system. Terminals are located at the
front counters of various County departments and agencies to display data and facilitate queries by
the general public. Data layers maintained on the GIS include floodplains, soils, parcels, addresses,
landfill mapping, zoning, and wells and pollution sources. Staff has developed processes on the GIS
to accurately produce and maintain this data by digitizing and inputting data from development
plans, record plats, and other hard copy sources as they are received by the County.
Data Management
Data on the GIS is stored using ArcInfo's Librarian function. All corporate data is contained in
libraries specific to a data set. For example, the library "pctbase" contains all the layers of
information for the base map. Individual layers exist for buildings, roads, fence lines, trees,
topography, etc. Each library is set up to store data in sections known as tiles. Loudoun's libraries
include data in countywide, block, and base map tiles. Base map tiles correspond to PCT numbers.
Procedures for updating data layers are documented by GIS staff and are assigned by a Steering
Committee to an individual or team responsible for the data. Most procedures are supported by Arc
Macro Language (AML) programs. Librarian provides a mechanism called a transaction to manage
data updates to the library. This helps to guarantee that updated data are not overwritten. A
transaction is created whenever a tile or tiles are removed from Library for updating. This flags the
tile(s) as owned by the transaction. Updates are made to the files (coverages) removed from the
library. These coverages are checked for errors, returned to the Library, and the transaction is ended.
New overlays are produced (ink on mylar) from the library data using a Calcomp plotter. A copy
of the new overlay is generated and distributed to various County departments and agencies.
The "environment" library contains the data layers for floodplains, soils, drainage, and wells/
pollution sources, and is stored in base map tile format. Floodplain maps are composed of the layers
"flood" and "cross". The flood layer contains the data depicting the floodplain boundary and is
composed of arcs and polygons. The cross layer contains the cross section data documented at
specific points of the floodplain and is composed of arcs and annotation. Items in these databases,
defined by the County, may be found in the Appendix.
Updates to the floodplain layers occur when detailed engineering studies are approved as part of the
land development process. These areas are digitized by Planning Department staff using the library
procedures. Initially coded as proposed alterations, these delineations will be used to create new
floodplain limits after construction of the alteration. The procedures used to update floodplain data
on the GIS allow for the availability of accurate and timely floodplain information through services
such as computer queries and maps produced on the system.
Use of Floodplain Data
One of the ways staff utilizes the GIS is to determine the location of environmentally sensitive areas.
These environmental assessments utilize floodplain and other data to develop maps and reports.
Planners use this information in their land development application review. The query program is
designed with AML to create a color environmental assessment map for a specific site and a report
of the located features. A basic knowledge of the GIS and its functions are required to produce this
product. The AML uses an input screen to prompt the user for a project name and a series of pull
down menus to assist in site selections. Based on this information a workspace is created and
coverages are clipped from the libraries containing parcels, floodplains, soils, and wells/pollution
sources. The map is generated using lookup tables to identify areas of minor and major floodplain
and other data such as hydric soils, mountain colluvium, steep slopes, and well and pollution source
locations. Output scale is specified by the user and the map is plotted on a Calcomp electrostatic
plotter. The program also generates a report detailing information contained on the map including
floodplain and soil classifications with computed acreage, and well/pollution source data including
well depth, yield, elevation, and source type among others.
Staff is also frequently called upon to use the GIS to produce assessments of larger areas containing
specific data. Assessments have been used for determining the feasibility of new regulatory districts,
such as scenic stream buffers, or for determining the impact of new transportation corridors, such
as Highway Route 50 Realignment Study, the Washington Outer Beltway Corridor Study, and the
Dulles Greenway Tollroad Study Area. These assessments use a multitude of GIS data to determine
the feasibility of the plan in question.
The GIS was also crucial for the floodplain reassessment of the Broad Run watershed. This area
contains a subdivision located at the confluence of Broad Run with the Potomac River. Homes
built here, prior to adoption of the Zoning Ordinance floodplain regulations, are frequently inundated
by floods. The County initially contracted for an updated engineering study to be performed on the
floodplain within the watershed. The study area was mapped at 1:2400 and staff digitized this
information registered to the base map. Existing floodplain data and the new data were joined with
current parcel boundaries. Calculations of floodplain acreage changes for all parcels affected by the
proposed revisions were performed. A map and report were generated detailing the changes for each
property owner. Upon review by the County the revised delineation for a portion of the lower
watershed of Broad Run was accepted. These delineations were then used to update the existing
floodplain data layer, increasing floodplain acreage in this area by fifty acres.
Services
Data is shared electronically between the GIS and the County Commissioner of Revenue's
assessment database used for taxation. The GIS is used to generate all new parcel or tax numbers
(MCPI) and maps. Data on the GIS pertaining to these parcels is captured and used to update the
assessment records on a daily basis. Data transfer includes new or current MCPI numbers,
subdivision information, addresses, legal and computed acreage, zoning districts, floodplain
acreage, census tracts, and soils data. This data is subsequently used in the appraisal and assessment
process.
Most GIS products, including floodplain overlays, are available for sale to the general public at
OMAGI. The County charges the cost of reproduction as provided by the Freedom of Information
Act. The cost for a PCT map is presently $39.00, with each additional overlay costing $13.00. Also
available are small area maps, designed to fit an 8.5 by 11 piece of paper. These maps are site
specific for PCT's and their overlays. In addition to the 1:2400 series maps and overlays, there are
1:12000 scale maps and other plotted maps. Plotted maps are available in a variety of scales, the
most common being 1:100,000 and 1:50,000. Both show the entire area of the County and are priced
at $5.00 and $10.00 respectively. User selected GIS data is also available on 8mm and 4mm tape
and are priced accordingly.
Floodplain data is available on many GIS produced maps. The most common for site specific
determinations are the 1:2400 scale overlay, small area maps, and Floodplain Acreage
Determinations (FAD). The FAD is a map produced in black and white via a xerox plotter
specifically for a parcel or parcels. It is generated using a menu system written in AML by GIS staff.
Printed at 8.5 by 11, it contains parcel information for the area of interest with the subject parcel in
bold, and building footprints and floodplain delineations in shades of grey. Two reports are
contained on the page. The first contains a parcel description, detailing grantee, MCPI, scale, legal
acreage, and date of production. The second report contains the floodplain acreage calculated by the
GIS, broken down into minor, major, Potomac, non floodplain, and proposed floodplain. With this
report customers know the location of floodplain on their property and the amount of developable
land not within the floodplain regulatory limit.
The public can also determine the location of a property in relation to the floodplain by viewing data
on the GIS. Access is available through an AML, named Logis, produced by GIS staff. This
program is designed as a series of pull down menus for displaying data or generating output. The
program accepts tax or MCPI numbers, tile numbers, or locations identified on a generalized map
of the County to define the area of interest. Layers of data, such as buildings, roads, fences, parcels,
and floodplains, can be displayed within the area identified by using a point and click feature on the
menu to turn data "on" or "off". Feature values such as soil type and property owner information,
(such as name and address), can also be displayed by clicking on a query option from the menu.
Conclusion
Participation in the NFIP enables property owners in flood prone areas to insure their property.
When communities, such as Loudoun County, enact stringent regulations, the potential for flood
damage and the future economic losses due to flooding are minimized. In addition, the County's
floodplain regulations require low intensity, flood compatible land uses such as recreation, wildlife
habitat, and open space.
In rapidly developing areas, the need for accurate and up-to-date information when determining
floodplain locations is critical for guiding development. By operating under the guidelines of the
NFIP, and working in cooperation with FEMA, data generated by the County has proven to be an
invaluable tool for the decision making process. Ease of access, availability of current information,
and the versatility to produce maps and perform data integration with the County's land records
management system make the GIS a productive and cost effective means of communicating
floodplain information.
Acknowledgments
My thanks to the following County of Loudoun staff for providing information to make this paper
possible: Sally Biesecker OMAGI; Irish Grandfield, Dept. of Planning; Karen Lanham, Dept. of
Building and Development; Larry Stipek, OMAGI. Appendix
Library Environment Layer Flood
.AAT Item Definition Description .AAT Value
FLOOD 3, 3, I less than or equal to 640 acres drainage 1
Greater than 640 acres drainage 2
Potomac River floodplain 3
Town Boundary 4
Proposed Floodplain Boundary 5
.PAT Item Definition Description .PAT Value
FLOOD 3, 3, I less than or equal to 640 acres drainage 1
Greater than 640 acres drainage 2
Potomac River floodplain 3
Incorporated Towns 4
Outside of floodplain 5
Proposed floodplain area 6
Library Environment Layer Cross
.AAT Item Definition Description .AAT Value
CROSS 3, 3, I less than or equal to 640 acres drainage 1
Greater than 640 acres drainage 2
Potomac River floodplain 3
Proposed floodplain area 6
CROSS-ID 12, 12, C Cross Section ID SCS (number)
(SCS# or engineering study #) (AXS-84-0001)
CROSS-ELEV 7, 7, N, 2 Cross Section Elevation Elevation in Feet
(XXXX.XX)
CROSS-AC 4, 4, I Watershed acreage between cross sections Acreage (XXXX)
CROSS-DPTH 4, 4, N, 1 Cross section depth Depth (XX.X)
Annotation Description Annolevel
Cross Section ID for drainage > 640 acres 1
for drainage <= 640 acres 2
Cross Section Elevation for drainage > 640 acres 3
for drainage <= 640 acres 4
Cross Section Acreage for drainage > 640 acres 5
for drainage <= 640 acres 6
Cross Section Depth for drainage > 640 acres 7
for drainage <= 640 acres 8
Cross Section Station No. for drainage > 640 acres 9
for drainage <= 640 acres 10
References
Federal Emergency Management Agency FIA-2 (1992), Answers to Questions About The National
Flood Insurance Program.
Federal Emergency Management Agency FEMA (1994), How to use a Flood Map to Protect Your
Property.
Loudoun County Department of Economic Development (1995), 1994 Annual Growth Summary,
Loudoun County, Virginia.
Loudoun County Department of Environmental Resources (1993), Interpretive Guide to Geology
and Groundwater, Loudoun County, Virginia.
Loudoun County Planning Commission (1991), Choices and Changes General Plan, Loudoun
County, Virginia.
Loudoun County Planning Commission (1993), Zoning Ordinance, Loudoun County, Virginia.
Author Information
David Torraca, GIS Analyst
Loudoun County Office of Mapping and Geographic Information
1 Harrison Street, 2nd Floor
PO Box 7000
Leesburg, VA 20177
phone: (703) 777-0367 fax: (703) 771-5075