FEDERAL BASE NETWORK (FBN) DEFINITION
AND STATION SELECTION GUIDELINES
APPROVED BY THE
EXECUTIVE STEERING COMMITTEE, NGS
DECEMBER 13, 2000
DEFINITION
The Federal Base Network (FBN) is a very high accuracy network of
permanently marked (monumented) control points at approximately
1-degree-by-1-degree (100-km) spacing throughout the United
States and its territories. FBN contains additional, more
densely spaced stations as needed for safe aircraft navigation,
and in areas of crustal motion. FBN provides the Nation with
spatial reference control having some of the highest accuracies
available today. Developing and maintaining FBN is one of the
primary responsibilities of the National Geodetic Survey (NGS).
The "goal" of FBN is to supply local accuracies of at least:
2 cm (95% confidence level) for geodetic latitudes and
longitudes,
2 cm (95% confidence level) for ellipsoidal heights and
3 cm (95% confidence level) for orthometric heights,
50-microgal accuracy for gravity values, and
1 mm/year relative accuracy for horizontal and vertical
crustal motion.
The interim "goal" of FBN is to supply network accuracies of at
least:
5 cm (95% confidence level) for geodetic latitudes and
longitudes,
5 cm (95% confidence level) for ellipsoidal heights and
7 cm (95% confidence level) for orthometric heights,
100-microgal accuracy for gravity values, and
5 mm/year relative accuracy for horizontal and vertical
crustal motion.
This document provides the guidelines to be used to select
additional FBN stations. These guidelines address the issues of
selection priorities, station spacing, monumentation and station
environment, stability, and accessibility.
SELECTION PRIORITIES
Station selection shall be based on the following priorities,
which are given in the order of highest priority first. Within
each priority category, preference should be given to selection
of appropriate older stations having a known history of prior
stability as indicated by previous observations.
Each FBN station should be selected from:
1. Existing A- or B-order station, where possible. At special
survey sites such as Continuously Operating Reference
Stations (CORS), WAAS, and VLBI, where several high-accuracy
monumented stations exist, only one of the monumented
stations shall be selected as a FBN. In cases where a
protected area exists at the site, the most accessible
station shall be selected as the FBN. Other stations at the
site shall be designated as CBNs, with the appropriate
agency being responsible (FAA, USCG, or State agency).
2. Primary Airport Control Stations (PACS) will be candidates
for FBN station selection. Those PACS that are not selected
as FBNs will be designated as Cooperative Base Network (CBN)
stations, with the responsible agency being FAA. The
Secondary Airport Control Stations (SACS) will be designated
as User Densification Network (UDN) stations; the
responsible agency again being FAA. Where necessary, an
additional nearby station, a CBN, which is easily accessible
by the public, will be established and directly connected to
the airport station.
3. Existing National Spatial Reference System (NSRS) station
with a first- or second-order elevation AND first- or
second-order horizontal coordinates, with higher-accuracy
classification being preferred both vertically and
horizontally (giving vertical accuracy top priority).
4. Existing NSRS station with first- or second-order elevation,
again the higher accuracy classification being preferred.
5. Existing NSRS station with first- or second-order horizontal
coordinates which would require a minimum amount of first-
or second-order leveling (within 10 km) to establish a
precise elevation.
6. New station or existing station not in NSRS, suitable for
GPS observations, set in bedrock, which would require a
minimum amount of first- or second-order leveling (within 10
km) to establish a precise elevation.
7. New station or existing station not in NSRS, suitable for
GPS observations, established by setting a 3D monument,
which would require a minimum amount of first- or second-
order leveling (within 10 km) to establish a precise
elevation.
STATION SPACING
Unless specified otherwise, the overall FBN shall consist of
stations spaced on average approximately 100-km apart. The
actual station spacing shall be flexible enough (from 50 km to
150 km) to allow for optimum station selection. In addition,
existing horizontal NSRS stations should be selected in a pattern
so that overall, these stations are located in each one-degree
block throughout the project. When impossible to meet the above
criteria, an additional first- or second-order horizontal NSRS
station must be recovered that is suitable for GPS observations.
These additional stations shall be selected approximately midway
between FBN stations.
MONUMENTATION AND STATION ENVIRONMENT
The following is a list of considerations for each station in
FBN. The intent is to ensure that station monuments will be
locally stable and remain usable indefinitely. Each of these
considerations is important.
Adequate GPS satellite visibility (unrestricted at 15
degrees and higher above the horizon). Minor obstructions
may be acceptable, but must be depicted on the Visibility
Obstruction Diagram.
Accessible by vehicle (two-wheel drive preferred).
Stability; bedrock mark being most preferred. (See
STABILITY.)
Permanency.
Ease of recovery.
Avoid known multi-path sources.
Appropriate geographic location and spacing.
Location allows efficient use by surveying community.
Accessible by public. (See ACCESSIBILITY.)
No known potential conflict with future development.
Open area for possible aerial-photo paneling.
Avoid electronic interference where possible.
STABILITY
Station monument stability is often difficult to assess in the
field with limited resources. For existing NSRS station
monumentation, the NGS data base contains stability qualifiers
which were assigned for the majority of marks when they were set.
For FBN stations, existing NSRS stations must have a stability
Quality Code of "C" or better. Quality Codes A and B are
preferred. New monuments will have a stability Quality Code of B
or better.
Quality Codes are as follow:
Quality Code A - most reliable; are expected to hold a precise
elevation. Examples: rock outcrops; rock ledges; rock cuts;
bedrock; massive structures with deep foundations; large
structures with foundations on bedrock; or sleeved deep
settings (10 ft or more) with galvanized steel pipe or
galvanized steel, stainless steel, or aluminum rods.
Quality Code B - will probably hold a precise elevation.
Examples: unsleeved deep settings (10 ft or more) with
galvanized steel pipe or galvanized steel, stainless steel, or
aluminum rods; massive structures other than those listed under
Quality Code A; massive retaining walls; abutments and piers of
large bridges or tunnels; unspecified rods or pipe in a sleeve
less than 10 ft; or sleeved copper-clad steel rods.
Quality Code C - may hold precise elevation, but subject to
ground movement. Examples: metal rods with base plates less
than 10 ft deep; concrete posts (3 ft or more deep);
unspecified rods or pipe more than 10 ft deep; large
boulders; retaining walls for culverts or small bridges;
footings or foundation walls of small to medium-size
structures; or foundations such as landings, platforms, or
steps. (See Quality Code C Exception, below.)
Quality Code D - of questionable stability. Examples:
generally, objects of unknown character; shallow set rods or
pipe (less than 10 ft); light structures; pavements such as
street, curbs, or aprons; piles and poles such as spikes in
utility poles; masses of concrete; or concrete posts less than
3 ft deep.
Quality Code C Exception - when selecting FBN stations, only
Quality Codes A and B are recommended. However, concrete posts
may be selected with a C stability if the mark is deemed stable
from review of historical releveling, soil type, and frost
depth. Final selection is subjective, and is based on local
knowledge of soil and frost heave, plus knowledge of how well
the mark has held its horizontal and vertical positions over
the years.
ACCESSIBILITY
Accessible public property should be utilized where feasible. If
the station is located on private property, permission must be
obtained from the land owner for station accessibility. Include
the name, address, and, if public ownership, the telephone number
of the responsible party. Do not include telephone numbers of
private property owners.
FIELD WORK IS UNDERWAY
NGS is conducting field surveys to complete the ellipsoidal
height and horizontal components of FBN stations in order to
attain coordinate accuracies of 2 cm in three dimensions (95%
confidence level). The task began in September 1997.
FOR ADDITIONAL INFORMATION, CONTACT:
NGS, Spatial Reference System Division
ATTN: Mr. Edward J. McKay
Telephone: 301-713-3191; Fax: 301-713-4324
E-mail: Ed.McKay@noaa.gov
NGS Web site: http://www.ngs.noaa.gov