SURFACE GRAVITY FILES
Surface Gravity Anomalies over Land Areas
The surface gravity data used in this project has come predominately from data held in
DMA's Point Gravity Anomaly (PGA) File. This file contains in excess of 30 million
point values collected and processed by DMA during the last three decades through its
independent collection efforts, reciprocal data arrangements, and cooperative agreements
with foreign governments, academic institutions, and private concerns. The PGA file is
the primary source of gravity anomalies used in statistical techniques that estimate the 30'
mean terrestrial gravity anomalies directly.
Major terrestrial gravity acquisitions since 1990 include aerogravity over Greenland
and parts of the Arctic and Antarctica, surveyed by the Naval Research Lab (NRL), and
cooperative gravity collection projects, several of which were undertaken in conjunction
with the University of Leeds. These collection efforts have improved and densified data
holdings over many of the world's land areas. Some of the notable geographic regions
include Alaska, Canada, parts of South America and Africa, Southeast Asia, Eastern
Europe and the former Soviet Union. In addition to the above gravity collections there
have been major efforts to improve DMA's existing 30' mean anomaly database by mean
anomaly contributions over various countries in Asia.
The details of two major DMA data acquisitions illustrate the significance of the
improved gravity coverage over previously void areas. The former Soviet Union is now
covered by a set of 5'x7.5' refined Bouguer anomalies and the Greenland Aerogravity
Project resulted in complete aerial gravity coverage (200000 km of flight lines) at 4
kilometers elevation with ground densification along many of the coastal regions for
downward continuation and evaluations of the aerogravity.
Important steps in DMA's gravity anomaly pre-processing algorithm include:
1. Gravity anomalies adjusted to IGSN 71 system.
2. Major effort to reference all point gravity anomalies to the WGS-84 horizontal datum.
3. Molodensky free-air gravity anomalies defined on the Earth's surface. The formula
used to compute these anomalies is given by (Heiskanen and Moritz, 1967, eq. 8-9):
a g =g-r.{-2(1+ f+m-2fSin21/J)~· +{ ~·rJ
(1)
where g is the observed value of gravity on the Earth's surface and Yell is the value of
normal gravity on the surface of the reference ellipsoid. The normal height H* of the
gravity station is generally unavailable, so the orthometric height H is used instead. For
the definitions of the quantities appearing in (1) see (Heiskanen and Moritz, 1967).
4. The geometry and the gravitational potential of the reference ellipsoid adopted for this
project were defined as follows:
We adopt the second degree zonal coefficient of the JGM-2 model as:
(C2,o)~~ __ fee = --484.1654767 x 10-6
(2a)
and the transformation:
(J2)ffI~A/DMA = -(C2,O)~g:,-=tee . ..J5 - (-3.11080 x 10- 8 .0.3)
(2b)
yields the 'zero' (permanent tide) 12 value adopted for this project. This value, along
with:
83
Surface Gravity Anomalies over Land Areas
The surface gravity data used in this project has come predominately from data held in
DMA's Point Gravity Anomaly (PGA) File. This file contains in excess of 30 million
point values collected and processed by DMA during the last three decades through its
independent collection efforts, reciprocal data arrangements, and cooperative agreements
with foreign governments, academic institutions, and private concerns. The PGA file is
the primary source of gravity anomalies used in statistical techniques that estimate the 30'
mean terrestrial gravity anomalies directly.
Major terrestrial gravity acquisitions since 1990 include aerogravity over Greenland
and parts of the Arctic and Antarctica, surveyed by the Naval Research Lab (NRL), and
cooperative gravity collection projects, several of which were undertaken in conjunction
with the University of Leeds. These collection efforts have improved and densified data
holdings over many of the world's land areas. Some of the notable geographic regions
include Alaska, Canada, parts of South America and Africa, Southeast Asia, Eastern
Europe and the former Soviet Union. In addition to the above gravity collections there
have been major efforts to improve DMA's existing 30' mean anomaly database by mean
anomaly contributions over various countries in Asia.
The details of two major DMA data acquisitions illustrate the significance of the
improved gravity coverage over previously void areas. The former Soviet Union is now
covered by a set of 5'x7.5' refined Bouguer anomalies and the Greenland Aerogravity
Project resulted in complete aerial gravity coverage (200000 km of flight lines) at 4
kilometers elevation with ground densification along many of the coastal regions for
downward continuation and evaluations of the aerogravity.
Important steps in DMA's gravity anomaly pre-processing algorithm include:
1. Gravity anomalies adjusted to IGSN 71 system.
2. Major effort to reference all point gravity anomalies to the WGS-84 horizontal datum.
3. Molodensky free-air gravity anomalies defined on the Earth's surface. The formula
used to compute these anomalies is given by (Heiskanen and Moritz, 1967, eq. 8-9):
a g =g-r.{-2(1+ f+m-2fSin21/J)~· +{ ~·rJ
(1)
where g is the observed value of gravity on the Earth's surface and Yell is the value of
normal gravity on the surface of the reference ellipsoid. The normal height H* of the
gravity station is generally unavailable, so the orthometric height H is used instead. For
the definitions of the quantities appearing in (1) see (Heiskanen and Moritz, 1967).
4. The geometry and the gravitational potential of the reference ellipsoid adopted for this
project were defined as follows:
We adopt the second degree zonal coefficient of the JGM-2 model as:
(C2,o)~~ __ fee = --484.1654767 x 10-6
(2a)
and the transformation:
(J2)ffI~A/DMA = -(C2,O)~g:,-=tee . ..J5 - (-3.11080 x 10- 8 .0.3)
(2b)
yields the 'zero' (permanent tide) 12 value adopted for this project. This value, along
with:
83