ETL 1110-1-183
1 Apr 98
deviations between closely spaced points would be
vertical control within the project area will provide a
expected. The regional trend or slope of the geoid at a
check on the solution heights of the unknown stations.
given point will not be highly sensitive to local gravity
The ideal condition would be to have all benchmarks with
anomalies especially in non-mountainous areas.
high order vertical and horizontal control surrounding and
within the project area.
See EM 1110-1-1003,
A-5. GPS - NAVD88 Transformation
NAVSTAR Global Positioning System Surveying, for
Techniques
assistance in setting up a network.
a. The GPS relative positioning and processing
(3) Geoid heights at the reference stations are
concepts used for NAVD88 elevation determination can
determined from a published geoid model such as
be outlined as follows:
GEOID93 or GEOID96. The geoid height is added to the
published orthometric height at the GPS reference station
(1) GPS baselines are observed to determine
to determine its ellipsoid height to the accuracy level of
ellipsoidal height differences across a network of stations.
the geoid model.
This step corresponds to a standard GPS network survey
using whatever observation techniques are best suited for
(4) Once the reference stations' ellipsoidal,
the project accuracy requirements (i.e. static, rapid static,
orthometric and geoidal heights have been fully
kinematic, On-The-Fly (OTF), or other geodetic quality
determined according to the above methods, elevations
GPS surveying process). GPS precise ephemeris should
are transferred from the reference stations to the
be used in place of the broadcast ephemeris during
remaining points in the network according to the
baseline data processing. Repeat baselines, for all
following relation:
baselines, should be performed for all control surveys
Given,
established with DGPS. The average ellipsoid height
from the repeat observations will be closer to the truth,
(where i is the station of
Hi = hi NI
with a few exceptions, than the ellipsoid height value
unknown height)
from a single observation. Baselines should be
reobserved on different days with significantly different
(where ref is the station of
Href = href - Nref,
satellite constellations. For topographic and location
known heights)
surveys (using kinematic or OTF techniques), repeat
with measured difference in ellipsoid height ()h = hi -
occupations should be performed where feasible. It is
important that the positions be adjusted on NAD83 since
href) from a DGPS survey, and computed difference in
Geoid height ()N = Ni - Nref) from a known Geoid model,
most geoid models (GEOID93, GEOID96) are also based
on NAD83.
then,
(2) At least two or more established NAVD88 first
Hi = Href + (Hi - Href)
order benchmarks should be occupied to serve as the GPS
reference stations where accurate vertical coordinates will
Hi = Href + (hi - Ni) - (href - Nref)
be fixed for the network adjustment. It is suggested that
at least one (preferably 2 or more) of these benchmarks
Hi = Href + (hi - href) + (Ni - Nref)
are also High Precision Geodetic Network (HPGN)/ High
Hi = Href + ()h - )N)
Accuracy Reference Networks (HARN) stations to ensure
accurate geoid modeling. First order accuracy standards
for geodetic leveling ensure the relative vertical position
where Hi is the orthometric height of the i-th station, the
quantity )h is determined from the measured GPS
of these reference monuments will agree. Redundant
A-3
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