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Home arrow Archives   The American Surveyor     

OPUS-DB and Uncertainty Testing Print E-mail
Written by Dr. Stacey D. Lyle, LS and Lauren Weatherly   
Saturday, 25 September 2010

A 1.640Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

The Conrad Blucher Institute for Surveying and Science at Texas A&M University--Corpus Christi conducted a research project with the National Park Service (NPS) and National Geodetic Survey (NGS) to determine the heights of five peaks in the Guadalupe Mountains. This mountain range is located in West Texas near its border with New Mexico. Students wanted to determine the elevation using Global Navigation Satellite Systems (GNSS) and to publish the results. Work was coordinated with the NGS and submitted to their OPUS-DB tool. NGS-published survey disks were found on two of the five peaks. While it was intended to place survey monuments atop each of the other peaks, the NPS ruled that "No permanent monument may be left behind." In place of monuments, a cairn (a mound of stones) was established at three of the peaks.

The students had to travel to the peaks, locate the highest point using leveling, and make GNSS observations consisting of two six-hour sessions. Another element of the project was the recovery of NGS-published NAVD 88 bench marks in the vicinity. Only one NGS-published bench mark was recovered for this project. The NAVD 88 height of this monument was used as a constraint in an adjustment using the commercial package, Topcon Tools. The determination of ellipsoid heights at points already having NAVD 88 values will be used to validate the GEOID 03 model in the area, which is one of the goals of the Texas Height Modernization program. When comparing the First-order NGS vertical bench mark set in 1977, published elevation to the orthometric elevation computed by OPUS to be -0.050 meters difference. This indicated that the GEOID in this region matched the vertical control. Future monuments will be recovered and surveyed for GEOID 03 verification. Previous surveying of 66 vertical bench marks across Texas showed approximately 0.05 meters standard deviation when comparing NGS vertical bench marks to published elevation to the orthometric elevation. Figure 1 illustrates the distribution of the GPS on the NGS vertical bench mark campaign. The Guadalupe Mountain survey, as a research and education endeavor, had many successes. Information gathered from this survey will be useful to the Texas Geospatial Reference Center. Methods for surveying and publishing vertical monument elevations for general public access were determined.

Monument Survey
It was determined during reconnaissance that the published monuments on the peaks were not set at the highest points. In accordance with project goals, a cairn was installed at the highest points of each peak. Sketches and photographs were made of each cairn to facilitate future use. Students previously made recovery journeys to the peaks to search for the monuments. The time from base camp to peak was determined to best organize the survey.

Field Data Review
Data collected was subsequently uploaded to the NGS Online Positioning User Service Database (OPUS-DB) tool. OPUS-DB was developed by the NGS to allow users to submit points for publication outside its normal processes. Requirements for an OPUS-DB submission include meeting equipment requirements, minimum session lengths, obtaining results meeting NGS-specified accuracies as well as providing additional data including photos of the point observed and the instrument setup.

Students were also required to name one of the locations, "PRATT PEAK," in compliance with the United States Geological Survey feature-naming process. The publication of these peaks and research by undergraduate students would place the peaks in a database for future surveyors to use. This survey will also provide data for researchers planning to monitor the peaks to determine if they are uplifting or subsiding.

Field Observations
The project posed a significant challenge for participants due to the remote location and difficulty of access. Hikes to the peaks (pack times) ranged from five to eight hours. In addition to the survey equipment, observers had to pack camping equipment and a minimum of two gallons of water. The field party consisted of four groups of three students each. Observations were made on five mountain peaks in two days.

ALTUS GNSS receivers were used on the survey­—a dual-frequency 66-channel receiver that tracks GPS L1/L2/L2C and GLO L1/L2. The system has a built-in GSM modem or cellular phone card that will be used for future RTK-GNSS measurements. The integrated satellite antenna made it easy to utilize. Its light weight, ability to operate with just the antenna and batteries, and acquire highly accurate results made it the ideal receiver. The available data collector was not needed. This dual-frequency receiver was equipped with a 1 Gb SD memory card which made it easy to retrieve data from the unit. Power was supplied by two hot swap low-weight Li-ion batteries capable of running for +10 hours, which was useful on long occupations. Observations were made using a one-second sampling rate. Data showed a small number of cycle slips and high quality data result.

Students were deployed from the base camp at 0800 with the goal of starting their recording by 1800. Students were instructed to log data for six hours then end the session. The setup was to be removed, then replaced, the batteries changed, and a new six-hour session started. Data was to be logged until 0600. Two six-hour sessions were to be obtained for each point observed.

Due to the inaccessibility and safety concerns of several peaks, five of eight peaks were occupied and measured (Table 1). Previous measurements came from past surveys or from USGS quad sheet elevation determination.

Data Processing
The data was then brought back and processed for the computation of the position. Post-processing was conducted that showed a free adjust accuracy of the network to be 0.03 meters maximum. This was considered to be well beyond the manufacturer's stated accuracy statements, as some of the vectors were 378,000 meters long. It was then determined that the points would be recorded and published in OPUS Database with the NGS. This is done by completing a similar OPUS solution, but providing the data collected using the standard NGS Field Observation sheets. The data was then processed using local post-processing.

The benefit of publishing the monuments within OPUS-DB is that the monument location and values are now available for other surveyors, engineers, and scientists to verify and/ or use the current values for research. Utilizing OPUS-DB and a high quality GNSS receiver such as the ALTUS APS3 results show good correlation as compared to a traditional post-processing network solution with all vectors computed and adjusted. Table 2 shows the results of the computation of the differences between the peaks in location and height using OPUS (6-hour solutions generated radial solutions from CORS) versus post-processing (2 x 6-hour/full network processing with adjustments, which took into consideration the phase antenna offsets for four different GNSS receivers) methods. The residuals are based on the differences in the distance and difference in height between the peaks and vertical ground bench marks. This was done in order to remove any error that could exist in the coordinates used. A standard deviation of 1.7 cm horizontally and 1.9 cm vertically is present within the network.

Submitting the data into OPUS-DB was simple and straightforward. The standard OPUS connection is utilized, but a selection to add to OPUS-DB is offered. When surveying the point, the user must complete the standard point GPS log files and point description. Data is processed through OPUS and the user can select the antenna type, antenna height and then customize the solution. Users can utilize the options to select the stations to be used in the base station processing. A new geospatial procedure is to check solutions for

"uncertainty". Uncertainties exist because of the imperfection in the system. All GPS solutions have some amount of uncertainty due the complexity of the solution. The options give you a chance to do uncertainty checks by varying the solution to determine if any error exists. Redundancy, solution options, and field/office methodology are our tools for uncertainty. The NGS has provided this tool, but it must be utilized to ensure correctness. Additionally, the user could use the processing option as a means to check uncertainty. It is suggested that the user make changes to the options and force different solutions before publishing a final solution to OPUS-DB.

The results from an OPUS solution are the indicators of the uncertainty. When analyzing an OPUS solution the user should look to several key factors to determine if errors exist. "More data is good data, if the data is good." This means that when looking at the OPUS Solution Report the user should look at the observations used, and the number of fixed ambiguities. The percentage of good data used to total data is an indicator of the uncertainty. If less than 95% of the data is used in the solution then the data might need more uncertainty testing. Overall RMS (root mean square) is important, but the user should look deeper into the residuals in each dimension for each base station. Users need to have a maximum acceptable error.

Once the user has confidence that the solution is correct then a final processing in OPUS using OPUS-DB is possible. Users simply select that they wish to have the solution published in the Options section.

Users have now entered their marks into the OPUS-DB and made the marks available to other surveyors and interested parties for research and discovery. This opens up the opportunity for sharing data and reducing uncertainty. We envision OPUS-DB as another tool to improve the quality of survey control and monumentation. Users can utilize OPUS-DB for GPS elevation marks to check vertical bench marks, boundary control for state and national cadastral GIS, construction and engineering control monumentation, and other surveying and geospatial projects.

While these tools are used by practicing professionals to provide high quality data to clients, they do not replace the professional services offered in surveying. OPUS-DB open data sharing provides an advanced means to propagate freely available surveying monumentation. The vetting processes of accepting monuments into OPUS-DB ensures the quality of the data. Surveyors should embrace this technology and use it as a means of business growth.

Stacey Lyle is an Associate Professor of Geographic Information Science's Geomatics Program and the online Geospatial Surveying Engineering Program at Texas A&M University Corpus Christi, and Research Scientist at the Conrad Blucher Institute for Surveying and Science.

Lauren Weatherly is a research graduate student in the Geospatial Surveying Engineering Program at Texas A&M University Corpus Christi and a Research Assistant at the Conrad Blucher Institute for Surveying and Science.

A 1.640Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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