Vol. 60, No. 2 (June 2000)
Estimation of Local Subsidence Using GPS and
Leveling Data
Chia-Chyang Chang
A GPS monitoring network consisting of 52 GPS stations
was set up in Yunlin County, southwest
Taiwan, in order to obtain the first-epoch data set and
investigate the feasibility of monitoring local
ground subsidence with the use of GPS.
Leveling-derived heights were still required, however, as
long-term GPS-derived heights were not available. The
results obtained suggest that geoid height
significantly influences the reliability of subsidence
estimated using an integrated GPS/leveling data
model. A RMS difference of up to 13 cm was found
between annual subsidence rates estimated using
“absolute” and “relative” modes of the integrated
model. However, the “relative” estimates of
subsidence at two neighboring sites were consistent
with those obtained by GPS solutions, with a RMS
difference of around 5 cm.
Aseismic Growth of Ventura Avenue Anticline, Southern
California, 1978-1997:
Evidence from Precise Leveling
Arthur Gibbs Sylvester
Comparisons of three first-order leveling surveys
across Ventura Avenue anticline indicate that, over a
period of 19 years, the heights of five benchmarks at
the anticline crest increased from 10 to 40 mm,
despite continued pumping of oil and water from the
anticline and injection of fluids into it. The growth
of the anticline was aseismic. Cumulative seismic
moment of all earthquakes in and near the anticline
during the19-year study period scarcely exceeded the
equivalent of a M4 earthquake, which, if it had
occurred at the 12-16 km depth where the hypocenters
are located, would be sufficient to cause only
10 mm of surface uplift. Between 1980 and 1987, the
volume of injected water greatly exceeded the
volume of produced oil and water. After 1990, the
proportion of injected water decreased, but the
height of the anticline continued to increase at the
rate of 2 mm/yr, the same rate as it had been
increasing during the past 80,000 years. The location
of the height changes at the crest of the anticline,
their observed rate of change, and their positive
sign, together with the general observation that
subsidence, not uplift, typically happens above active
oil fields, are evidence that the observed height
changes are tectonic. Given the contemporary 6-8 mm/yr
horizontal shortening across the Ventura
basin, the 1-2 mm/yr height change across the Ventura
Avenue anticline suggests that 10 to 20 percent
of the regional crustal shortening is being released
locally in the anticline as aseismic uplift. It is,
however, simplistic to conclude that such partitioning
of strain decreases the size or frequency of future
earthquakes in the region by the same percentage,
because the 19-year sample of aseismic uplift is too
short both temporally and in space to characterize the
behavior of all of the tectonically active western
Transverse Ranges.
Preliminary Investigations Toward an Improved
Geoid Model in Florida
Dru A. Smith, Michael Sartori, and Dennis G. Milbert
The National Geodetic Survey and the University of
Florida jointly investigated the possibility of
improving the geoid model in Florida. Since the
computation of G96SSS and GEOID96, new gravity
data became available to NGS. These included airborne
surveys over Florida and the Bahamas, as well
as updated satellite altimetry-derived gravity
anomalies. Additionally, offshore gravity data found to be
erroneous in the G96SSS and GEOID96 computations have
been identified and corrected. The joint
study attempted to identify how such changes might
improve the knowledge of the geoid in Florida.
Two experimental geoids, FLA98A and FLA98B, were
computed, incorporating the changes in
different ways. The geoids were computed on a 1' x 1'
grid, covering an area of 24° to 33° N, 269° to
282° E. The initial results indicate that the Florida
airborne data does not improve, and may actually
degrade, the geoid solution. Introduction of the new
altimetry, correction of the erroneous offshore
data, adding the Bahamas airborne data, and removal of
gravity value outliers all result in changes in the
geoid model of a few to several centimeters in local
areas. Computation of geoid models in small areas
(such as the state of Florida) is more prone to biases
and tilts, so a state-wide comparison of the
FLA98A and FLA98B models with GEOID96 is difficult.
However, in many localized areas the
FLA98A and FLA98B models showed some improvement over
GEOID96, particularly in correcting
errors in the local gravity measurements. There was,
however, some local degradation in these models
as well, mostly as a result of adding questionable
airborne gravity data.
Detection of Corresponding Objects in
Linear-based Map Conflation
Sagi Filin and Yerahmiel Doytsher
The identification of counterpart elements between two
datasets is a fundamental stage in the map
conflation process. In a linear map conflation
framework, these counterpart elements are themselves
linear entities. The large amount of data contained in
topographic maps and the geographical as well as
topology and content inconsistencies between sets
makes manual identification impractical. In this paper
we present an approach to overcome these difficulties
by using node objects rather than linear features
as the candidate counterparts. Counterpart nodes were
detected by geometric criteria as well as by
using topology. Furthermore, the candidate
counterparts were validated by evaluating path consistency,
rather than working with arcs only. Finally, human
intervention in the process was minimized through a
strategy developed for the classification of the
candidate counterparts into categories, depending on the
uncertainty in the matching.
Assessment of the Need for New Land
Development in Egypt: A Case Study and Prototype for Site Selection and Land
Registration Procedure in the Sinai
Scot E. Smith, Hesham Abd-El Monsef, and
Hossam Fawzy
This paper outlines the problem of acute overcrowding
of the Egyptian Delta and Nile Valley and
considers the pros and cons of alternative sites
within Egypt that could be developed for resettlement.
Potential locations include areas adjacent to the
Aswan High Dam Reservoir in Upper Egypt and sites
in southern and northern Sinai. The paper concludes
that Upper Egypt probably has very limited
potential for accommodating significant numbers of new
settlers due to its remoteness, extreme climate,
and lack of reliable water supplies. Parts of the
Sinai Peninsula, on the other hand, could provide a
suitable environment for housing, agriculture, mining,
and tourism, and the government of Egypt plans to
resettle three million people there by the year 2017.
The Egyptian government Ministries of Defense,
Urbanization, Culture and Tourism have nearly
exclusive control over land in the Sinai, and the
government plans to turn over most of this land to
private entities for development by the year 2017.
Bedouin tribes, however, claim rights to most of the
territory, which further complicates the process of
privatizing land holdings. This paper describes the
official process of land transfer in Egypt and as it is
actually practiced. The essential problem with the
current land transfer system is that it is so
cumbersome that most settlers to the Sinai do not
bother to properly register their claims. This results in
the government losing out on property tax revenue.
Also, due to the fact that the land records system is
incomplete, cultivation of illicit crops may be
ongoing on unregistered parcels. Described in this paper is
a method that would make land transfer in the Sinai
more efficient and so encourage land development.
Also described is a multi-purpose land information
system for the region that could be used by land
records offices, planning agencies, and law
enforcement agencies.
“All you have to do...”
Lawyer and Surveyor—What a Team!
Not a satire
Gunther Greulich
“I hereby certify” is an ominous introduction to many
a surveyor's death wish. Before committing
millions of dollars to a commercial building project,
buyers and sellers, a.k.a. grantees and grantors,
lenders, banks, title insurers and their army of
well-paid attorneys love to pass the proverbial buck to
their eager, and often gullible, land surveyor.
No matter how complex the legal and physical
attributes of a parcel of land may be, attorneys manage
to make it look simple. Despite rigid requirements of
an ALTA/ACSM title insurance survey, the
lawyer’s introduction of legalese into surveyor’s
reports and certifications has duped surveyors into
believing that all they have to do is put their name
and registration number on a preprinted boiler plate.
Lawyers probably have good reason to make it look
simple: it speeds up the process. Borrower's
lawyers need the money in a hurry. Lender's lawyers want to have their money
make more money
quickly and safely. Title insurance lawyers want to
keep their money-risk free. All of them would like to
save money by minimizing the survey effort, and, yet,
all the lawyers want to feel safe and comfortable
by putting the burden of property boundaries, of
easements, of rights-of-way, of encroachments, of
takings, of zoning issues, and even of utility
services on the unsuspecting surveyor.
The International Federation of Surveyors Commission Six:
Positioning and Measurement
Robert W. Foster
ABSTRACT: This paper describes Commission Six
(Engineering Surveys) of the International
Federation of Surveyors (FIG). The Commission is
defined and its primary areas of interest are
described. This is the seventh in a series of articles
on FIG commissions. The intention behind this series
is to provide a comprehensive overview of the FIG, its
structure and its goals, and describe how these
goals are being pursued by members and their
representatives on the Federation’s various
commissions.