SaLIS Vol. 63, No. 1

March 2003

 

 

Introducing Two Spatial Reference Frames for Regions of the Pacific Ocean

Richard A. Snay

 

The current realization of the North American Datum of 1983 (NAD 83) is defined in terms of a Helmert transformation from the International Terrestrial Reference Frame of 2000 (ITRF00). The parameters of this transformation were chosen, in part, so that horizontal NAD 83 velocities would be zero on average for points interior to the North American tectonic plate.  Unfortunately, islands located in the Pacific Ocean move horizontally by as much as several centimeters per year relative to the North American plate.  To address this motion, this document introduces two new spatial reference frames in terms of transformations from ITRF00, one frame for regions located on the Pacific tectonic plate (Hawaiian Islands, Marshall Islands, American Samoa) and one for regions located on the Mariana tectonic plate (Guam, Rota, Saipan). For each frame, points located interior to the corresponding tectonic plate are to have little or no horizontal velocity relative to this frame.  The Pacific-plate-fixed frame is to be designated as NAD 83 (PACP00) and the Mariana-plate-fixed frame as NAD 83 (MARP00). These frames are defined so that their positional coordinates are consistent with corresponding positional coordinates of the current NAD 83 realization for an epoch date of 14 AUG 1993.

 

A Study on Precise GPS Height Using WVR and Radiosonde Sounding Data

Chun Sung Chen, Chuan Sheng Wang, and Yu Min Chang

 

There are many factors that drop the accuracy of the GPS relative positioning. The error of the factors can be mostly reduced using mathematical models or calibration, but the error derived from tropospheric delay is hard to eliminate. Currently, mathematical models are generally applied to the tropospheric delay. However, their accuracy in eliminating tropospheric delay in GPS height can only be in the cm or dm range. For precise deformation surveying use, the cm or dm range accuracy of the GPS height has been promoted. In recent years, it was shown that more precise results of GPS heights can be obtained by using WVR data to correct for tropospheric delay. In this paper, we describe the collection and application of WVR and radiosonde sounding data to eliminate tropospheric delay in GPS heights. GPS heights determined mathematically and by using WVR accompanied with radiosonde sounding data to eliminate tropospheric delay were compared. The results of this research show a difference of about 40 cm between the surveying tropospheric data and mathematically computed GPS data.

 

Using Positional Tolerance in Simplifying GPS Control Survey Quality Requirements

Raymond J. Hintz

 

The survey world has always grappled with the concept of defining the quality of the results derived from a series of measurements. Traditionally, the closure in a ratio form served that purpose, based on a traverse. Both the proliferation of GPS into our industry, and the power of the personal computer coupled with least squares analysis, lead a movement towards more of a survey network than a simple closed figure. Additionally, the coordinates produced as a by-product of the measurements are being used for more things than ever before. Defining the quality of those coordinates thus becomes more important (FGDC 1998). This article looks at how the use of positional tolerance in a goal-oriented standard developed in 1998 by the Florida Department of Transportation (FDOT) in collaboration with the author has allowed a simpler way of analyzing GPS network data quality.

 

On the Quality of DCW Hypsographic Data: A Study for Taiwan

Tian-Yuan Shih, Young-Chyi Chiu, Hong-Yang Shih

 

The objective of this study is to investigate the quality of DCW hypsographic data.  During the investigation, grid DEMs were generated for Taiwan. The results were graphically evaluated, and five numerical indices are also presented. These indices provide information about the maximum error, the 90 percent ranking error, the root mean squares error, the mean absolute error, and the mean error. The spot heights from DCW were used as check points. For the 30” x 30” grid DEM generated with the constrained TIN-linear model, the root mean square error was 176.836 m and the 90 percent ranking error was 213.149 m, with the bias removed.

 

 

Development of a Geographic Information System for Sustainable Development Planning in South Sinai, Egypt

Hesham Abd-El Monsef,  Scot E. Smith, and Mohamed El-Ghawaby

 

Described in this paper is a comprehensive, accurate, and reliable GIS database designed to deal with natural and human resources in the Wadi Watir Valley in southeastern Sinai, earmarked by the Egyptian government as a priority for economic development. This report highlights efforts to build the database, develop a suitable GIS design, and provide a description of the functionality of the GIS with respect to the planned economic development of the Wadi Watir Valley.

 

 

Quantifying Coordinate Improvement of Vector GIS Data Using Rubbersheeting Methods

Arthur J. Lembo, Jr., Thomas D. O’Rourke, and Barbara C. Moses

 

The paper focuses on a quantitative measure of compliance to evaluate the quality, or degree of fit, between street centerlines and utility pipelines through rubbersheeting methods.  The weighted average amount of deviation between line segments in one network compared to another—referred to as weighted average deviation (WAD)—is used to measure the average distances (deviations) of transformed line segments representing pipelines, with street centerlines that are positioned with a higher degree of accuracy. Numerous scenarios are tested with respect to an actual gas distribution pipeline system and a local road network. The scenarios include areas with large errors vs. smaller errors and many line segments vs. fewer line segments. Improvements of the datasets, as measured by WAD, are plotted as a function of the ratio of the number of control points (nodes) to the number of line segments within the study areas. As more control points are entered, the rubbersheeting adjustment improves the position of the utility mains with respect to street centerlines.  For the datasets in this study, a point of diminishing returns occurs when the node to line ratio exceeds 0.15 to 0.2. At this level of rubbersheeting, relative improvement of all datasets, regardless of size or original accuracy, is minimal. 

 

 

Use of Source Identification in the Optimization of a Measurement-based Land Information System

Kurt B. Wurm and Raymond J. Hintz

 

A measurement-based cadastral system (Hintz et al. 1996a) such as Windows-based Geographic Measurement Management (WINGMM) (BLM 2001) is based upon the assumption that society will benefit from improvement of the positional quality of cadastral (parcel) information.  In some respects it is no different than the benefit that can be gained as one switches to a higher-accuracy datum, as in evolution from the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83) to a High Accuracy Regional Network (HARN). While the benefits have complimentary characteristics, the source data are actually quite different, along with the intent of the output product (parcels vs. control information).  The Source Identifier (SID) is the global term used in WinGMM that categorizes measurement in ways that optimize their use in producing the Bureau of Land Management’s (BLM’s) Geographic Coordinate Data Base (GCDB).