SaLIS vol. 63, no. 2
June 2003
Statistics and Adjustments Explained, Part 1: Basic Concepts
Charles D. Ghilani
Analysis and reduction of observations from such tools as
GPS can only be performed using the least-squares method. However, more and
more software packages now employ least squares as part of their adjustment
tools. As an example, many field-to-finish software products are incorporating
least squares into their packages. One of the latest entries into offering the
least-squares method is ArcGIS™ Survey Analyst. Thus the least-squares method
is becoming readily available for use in the analysis of surveying
measurements. However simply plugging-and-chugging data through a software
package does not constitute a proper analysis and adjustment of the data. This
is the first in a series of papers that will look at some analysis methods and
their underlying statistical concepts. The subject of this paper is the normal
distribution of data and the use of its properties in the analysis of a
least-squares adjustment.
How Long Must We Wait? The Duration of GPS Observations for
Short Baseline Measurements
Joshua S. Greenfeld
The Global Positioning System (GPS) is becoming a dominant
surveying tool. Therefore, it is prudent to evaluate GPS capabilities in terms
of accuracy vs. observation methods, and duration of observation sessions as
they apply to typical surveying projects. This paper presents the findings of
such an evaluation. The durations of observation sessions using Static and
Rapid Static methods with dual- and single-frequency receivers were
investigated. In addition, the accuracy of Real-Time Kinematic (RTK) with
dual-frequency receivers was evaluated. It was found that under normal
conditions and with GDOP of less than 3.5, as few as 2 minute (dual-frequency)
and 10 minute (single-frequency) observation sessions may be adequate to
measure short baselines with an accuracy of ± 3 mm (0.01 ft.)
Cadastral Survey Accuracy Standards
Belle A. Craig and Jerry L. Wahl
The fields of land surveying and mapping have benefited from
technology innovations. New technology has not changed the boundary surveyors’
core responsibility, which is to locate, mark, and document the boundary in a
legally defensible matter, for their clients, the adjoiners, all future owners
of real property, and interests in real property adjoining the boundary.
Nonetheless, the global positioning system and computers have changed the way
land surveyors measure, analyze, and calculate data. Land owners and land
managers have turned to geographic information systems to make complex
ecological and economic decisions. Boundaries are often the first line of
evidence of the extent of an interest held in land. Survey accuracy standards
need to address all types of cadastral spatial data and be consistent with the
Federal Geographic Data Committee standards to facilitate data sharing. Current
cadastral survey accuracy standards are inadequate and need to be changed to
reflect the way modern land surveys are conducted and address the fact that
geospatial data, once incidental to the survey process are now one of its
primary products.
Geometric Constraints in Feature Extraction from Aerial
Imagery
Rongxing Li and Fei Ma
This paper presents an integrated method for extracting
features from airborne images to support subsequent object recognition. A
compound feature detection method that combines first- and second-order
derivatives of gray values and regions detected by multi-level threshold
segmentation was developed and tested. Geometric constraints derived from
camera orientations (calculated from GPS/INS observations), perspective
geometry, DTM, and knowledge of objects were applied to enhance the capability
of eliminating noise and unwanted features. Finally, the remaining feature
candidates were refined by a mathematical morphological process to produce
features that were further modeled by using active contours and back
projections of known 3-D models. The method was successfully used to extract
such features as roads, houses, and trucks from AIMS images. Further research
will be carried out using the method for an extended set of features.
KEYWORDS: Feature extraction, geometric
constraint, road delineation, shadow analysis, mathematical morphology
The Education Challenge
By Gunther Greulich
In the last two years, ACSM publications have concentrated
heavily on education. The incentive may have come from the 2002 FIG Congress in
Washington, D.C., which has produced papers that point to the future of our
surveying and mapping profession. On reading, it becomes clear that our
American problems and challenges are not unique.
Creating a National Standard Record of Survey
R. Lee Hixson, PLS
The information revolution has dramatically altered the way
we collect and process our field data, forever changing the surveying
profession. However, during this period, when most of the emphasis has been on
modern data collection, data manipulation, and computer drafting, the art and
science of boundary surveying and boundary resolution has been increasingly neglected.
There is a growing number of surveyors preparing maps that reflect poor
resolutions, and quality varies widely from surveyor to surveyor and region to
region. It is time to stimulate a revival in what has historically been our
most important calling—the proper and professional determination of a parcel
boundary in its relation to the boundaries of the parcels surrounding it. It is
proposed that the American Congress on Surveying and Mapping (ACSM) take the
lead and, with the cooperation of the American Land Title Association (ALTA),
create a National Standard Record of Survey.
This can be accomplished by revising both the “NSPS Model
Standards of Practice” and the “Minimum Standard Detail Requirements for
ALTA/ACSM Land Title Surveys” to include a section that spells out the main
principles of an adequate boundary survey. Surveyors could then voluntarily
choose to comply with these standards and indicate that they have done so by
changing the title of the map to read: “ALTA/ACSM Record of Survey” and include
a short Boundary Statement reflecting said compliance. This concept could also
be extended to any of the different plats produced by surveyors that involve a
boundary analysis; as with the Basis of Bearings statement, a Boundary
Resolution Statement could be added indicating that the resolution conforms to
the new ALTA/ACSM Boundary Standard.