ETL 1110-1-183
1 Apr 98
orthometric elevations on national vertical control
procedures can be found in EM 1110-1-1003.
benchmarks in the North American Vertical Datum of
d. Recent advances in geoid modeling have also led
1988 (NAVD88) height system are established with
respect to the geoid, a model of the earth based on gravity
to more accurate conversions between NAD 83 GPS and
measurements. A determination of a NAVD88 elevation
NAVD 88 orthometric height systems. Accuracies of 30
using GPS measurements at a given point requires a
mm or better have been obtained when converting
transformation between ellipsoid and geoid based height
ellipsoid heights from GPS surveys, based on NAD 83
systems. The conversion between the NAD83 GPS
control, to NAVD 88 orthometric heights using the latest
ellipsoid and NAVD88 orthometric height is made using
geoid model (GEOID96). The initial GPS survey data
the geoidal undulation (also referred to as geoid height)
must be valid for the elevation transfer method to be
value that represents the geoid-ellipsoid separation
effective. Guidance for GPS survey accuracies and
distance.
procedures can be found in EM 1110-1-1003,
NAVSTAR Global Positioning System Surveying.
b. DGPS may provide an efficient and cost-effective
e. Appendix A presents the basic methodology for
means of densifying elevation data over large, extended
project areas when compared to conventional differential
using GPS to determine NAVD88 elevations. GPS
leveling. Height measurement accuracy that meets most
positioning techniques, coordinate systems, and vertical
USACE mapping requirements can be successfully
datum concepts are introduced and discussed along with
achieved from several different GPS surveying
operational requirements and computational schemes
techniques.
However, DGPS vertical elevation
used to obtain NAVD88 elevations from GPS
techniques may not be sufficiently accurate for
coordinates. These operational requirements are based on
construction control or may not be cost-effective for small
field test results conducted by U.S. Army Topographic
project areas.
Engineering Center (CETEC) and the National Geodetic
Survey (NGS) using several different GPS surveying
c. GPS relative vertical positioning and calculated
methods and comparing these results to conventional
geoid height differences for the determination of
differential leveling networks.
NAVD88 orthometric heights may be used when an
6. Proponency and Technical Assistance
accuracy no better than 30 mm is required. This GPS
height accuracy satisfies feature elevation tolerances
specified for most USACE engineering mapping
The HQUSACE proponent for this technical letter is
activities. However, it may not be sufficiently accurate
CECW-EP. Technical assistance in performing GPS
for hydraulic engineering studies or construction
surveys may be obtained by contacting the U.S. Army
activities. Guidance for GPS survey accuracies and
Topographic Engineering Center, ATTN: CETEC-TD-G,
7701 Telegraph Road, Alexandria, VA 22315-3864,
FOR THE DIRECTOR OF CIVIL WORKS
(703) 428-6767.
Appendix A
STEVEN L. STOCKTON, P.E.
Determination of Elevations
Chief, Engineering Division
with GPS Surveying Techniques
Directorate of Civil Works
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