SaLIS Vol. 65, No. 2

June 2005

 

The National Spatial Reference System Readjustment of NAD 83

Chris Pearson

 

The National Geodetic Survey (NGS) is responsible for the establishment and maintenance of the National Spatial Reference System (NSRS). As such, our goal is to maintain a network of stations which are of high enough accuracy to serve as control for any project undertaken by local surveyors. As the Global Positioning System (GPS) became operational in the early 1990s, NGS initiated a series of high-accuracy GPS surveys to realize the advances in surveying with GPS. The availability of these high-accuracy data—along with substantial growth in the NSRS due to the addition of local GPS surveys to the network—has resulted in a need for a national simultaneous readjustment which utilizes all these observations. Although NGS has strived to maintain a consistent network, inevitable discrepancies between surveys and between states have taken place. A readjustment utilizing all available GPS data archived in the NGS database will also allow for the computation of local and network accuracies for each mark. This readjustment is scheduled to begin in June 2005 and be completed in February 2007. Some of the key points are: Only GPS will be adjusted. The CORS stations will serve as control. A Helmert blocking strategy will be used for the adjustment. Both NAD 83 and the International Terrestrial Reference Frame (ITRF) coordinates will be produced and published. User densification projects will be included if they are observed with GPS, tied to the HARN network, and submitted prior to the June 1, 2005, deadline.

 

 

A Control Survey and Mapping Project for an Urbanizing Region

(A Study in Persistence)

K.W. Bauer, PE, RLS, AICP

 

In 1964, the Southeastern Wisconsin Regional Planning Commission proposed a large-scale topographic and cadastral mapping program for its 2,689 square-mile seven-county planning area. The integrated maps were to be based upon a then unique system of survey control which combined the U.S. Public Land Survey System with the State Plane Coordinate System, and which could provide a sound basis for the conduct of land and engineering surveys throughout the planning area. The Commission has pursued implementation of the recommended control survey and mapping program for 40 years. Under the program, all 11,753 U.S. Public Land Survey corners within the planning area have been re-monumented and placed upon the State Plane Coordinate System by high-order traverse and global positioning system surveys. Elevations of bench marks accessory to the re-monumented corners have been obtained by high-order differential level circuits, thus placing a monumented control survey station of known position on both the U.S. Public Land Survey and State Plane Coordinate Systems, and of known elevation, at one-half mile intervals throughout the planning area. Large-scale topographic maps have been completed for about 89 percent of the planning area and companion cadastral maps for about 76 percent of the planning area. The mapping and control survey system has served the area well over time, facilitating area-wide and local planning, engineering, and surveying operations. Importantly, the mapping and control survey system has provided a sound basis for the creation of computerized, parcel-based land and public works information systems within the planning area.

 

 

The Effects of Unestimated Parameters

Charles R. Schwarz

 

Constrained adjustments are the means by which new points are brought into an existing geodetic datum. In most of these adjustments the coordinates of existing control points are held fixed. Some fixed control points are necessary to define the coordinate system, but usually more than the minimum number of existing control points are held fixed. When this happens, the coordinates of the extra control points are called “unestimated” parameters, emphasizing that they could have been determined from the observations but were not. The coordinates determined in the least squares adjustment are valid, but the conventional equations for linear error propagation must be extended to take account of the uncertainties of the unestimated parameters. The equations for doing this are known for the case of absolute constraints. Since most adjustment programs use weighted constraints to hold fixed the coordinates of existing control points, the equations for estimating the effects of unestimated parameters are extended to the case of weighted constraints.

 

When Ellipsoidal Heights Will Do the Job, Why Look Elsewhere!

Muneendra Kumar

 

Presently, with GPS we can directly survey ellipsoidal heights (h) with 5-10 cm accuracy for geodetic control points and differential heights (Dh) between bench marks and well defined topographic features with relative accuracy of 1 part per million or better. This type of height information is sufficient to contour the Earth’s real topographic relief on “no-projection” maps drawn on ellipsoidal surface. Instead of using mean sea level or geoid, the ellipsoid depths or heights referenced to a time-invariant ellipsoid, as a zero reference will ensure safe marine and air navigation worldwide. This paper explains and provides important details how the ellipsoidal heights (h) and/or differential heights (Dh) will work in non-engineering applications. It also includes simple algorithm(s) or procedural steps ensuring the best results in using ellipsoidal heights and/or depths.

 

GPS Geodesy and Applications (GPS-GAP)—

An Internet-based Geodesy and GPS Educational Application

Peter Lazio, L.S.

 

Advances in GPS have outpaced the training infrastructure. To fill this void, surveyors are attending seminars or manufacturer-sponsored training courses. The courses do a satisfactory job at presenting fundamental concepts and foster basic proficiency with the particular manufacturer’s receivers and software. Seminars, on the other hand, present no more than a general overview of theory. Sound theoretical principles cannot be taught in the short period of time allocated to either form of instruction. A detailed, mathematically based course of study is not possible via training courses or seminars: mainly because of time limitations, the mathematics powering the black box is either overly simplified or ignored. The University of Maine sponsors an Internet-based program—GPS Geodesy and Application Program (GPS-GAP)—which fills the need for mathematically rigorous courses of instruction for surveyors using GPS and other geodetic applications.

 

 

Assessing Individual Epochs in Airborne GPS

Peter Kuntu-Mensah and Raymond Hintz

 

In airborne GPS photogrammetry, the determination of the camera positions is based on single instantaneous antenna epoch measurements, or the interpolations thereof. This is unlike ground-based GPS applications where positional coordinates are determined from statistical evaluation of numerous epoch measurements. In spite of the technological advancement in GPS measurements, it still remains that exposure station coordinates are based on accuracy of the individual epoch measurements. This paper presents the results of a project that assesses the accuracy of individual epoch airborne measurements using multiple ground base stations.

 

Development of Quality Metrics for Linear Features

Raul Ramirez and Tarig Ali

 

At the Ohio State University we have completed a three-year digital, government project on “Digitalization of Coastal Management and Decision Making Supported by Multi-Dimensional Geo-Spatial Information and Analysis,” funded by the National Science Foundation (NSF). The goal of the three-year project was to investigate and develop technologies to enhance the operational capabilities of federal, state, and local agencies responsible for coastal management and decision-making (Li et al. 2002).

     Coastal features are very important components of our research. As most hydrographic linear features, shorelines are geo-spatial features that are difficult to define and represent in a geo-spatial database because of their dynamic nature. Generally, the graphic representation of a hydrographic feature in a geo-spatial database is only valid for a particular period of time. Also, representations of these features at different levels of detail are not uncommon. Cartographic generalization can be used to generate these more general hydrographic linear features for a particular instant of time from detailed representations. This notwithstanding, a cost-efficient way to estimate the positional and attribute quality of these features is needed to maintain appropriate levels of quality in the production of digital nautical charts and costal databases.

 

 

Location of Boundaries Defined by Simultaneous Conveyances—A Question of Timing

Joseph Curd, PLS, and Andrew C. Kellie, PLS

 

The subject of simultaneous conveyances is a familiar one to surveyors.  Simultaneous conveyances of property can result from conveyance from a platted subdivision, division of heirship property, or the surveys of the United States rectangular survey. The “apportionment rule” is frequently seen as a general method for distributing excess or deficiency in simultaneous conveyances.  This paper reviews this general method and examines boundary cases involving simultaneous conveyances examined by the courts.  While the apportionment rule has a place in boundary retracement, its application is limited. Thorough examination of the evidence on the ground as well as the evidence of record is conducted before the apportionment rule is used.