Towards Optimizing the determination of accurate heights with GNSS

1. Towards Optimizing the determination of accurate heights with GNSS • Dan Gillins, Ph.D., P.L.S. October 9, 2014

2. Current Research Efforts • Optimizing the determination of accurate heights with GNSS (NGS) • NGS 58/59 guidelines • OPUS-RS, OPUS-S, OPUS Projects • GPS+GLONASS vs. GPS-only • Real-time networks • UAV remote sensing • Evaluating accuracy of current practice • Improving data collection and processing steps • Earthquake hazard mapping • Megaquake-induced liquefaction (USGS) • Liquefaction & lateral spreading hazard maps (USGS) • O-HELP: a web-based GIS tool for assessing earthquake hazards in Oregon (CLiP) • GNSS surveying in forested environments

3. Importance of Research • Accurate heights are crucial for a multitude of scientific studies and engineering projects • monitoring deformations, engineering layout, flood mapping, sea level rise, development of nautical charts, topographic mapping, crustal movement, subsidence studies • Geodetic leveling remains the most accurate form of obtaining heights • Requires line-of-sight, slow (expensive), prone to errors • GPS has revolutionized the surveying of geodetic networks • Does not require line-of-sight, easy to use, quite accurate • It is desirable to take advantage of the economics of GPS to determine ellipsoidal and orthometric heights

4. Testing and Improving NGS 58/59 Height Modernization Guidelines Objectives: • Evaluate NOS NGS 58 and 59 guidelines • Follow guidelines to establish a control network from Salem to Corvallis • 20 varying benchmarks • B versus C stability under varying canopies • Recommend new guidance based on current technology • Use of GLONASS? • Improved hybrid geiod models (GEOID12A, GEOID14) • Use of modern GNSS antennas+receivers • Improved accuracy and availability of GNSS orbits • Real-time networks • Various processing tools • OPUS, OPUS Projects • Star*Net

5. Summer 2014 Height Mod. Survey • Collect static GPS+GLONASS data • 10 total days of surveying • 3 days of 5 hour sessions (primary network), 7 days of 1 hour sessions (secondary network) • 5 receivers (6 for 3 days) • 20 marks covering 350 square miles • 28 total unique sessions • 264 total baselines observed • 103 independent baselines obtained after removing outliers (avg. length 10.79 km)

6. Phase 2: Static Survey

7. Phase 3: Find ellipsoidal heights following NGS 58 • Use only GPS data • Partially constrained the HARN stations according to their reported NGS network accuracies • Average 95% confidence on ellipsoidal height =1.23 cm • Only 1 mark exceeded 2 cm from the published NGS ellipsoidal height (N99RESET) • J54 appears to have been disturbed

8. Next Steps • Determine which benchmarks have "valid" orthometric heights • Identify any needs to conduct geodetic leveling as a redundancy check • Repeat study using other techniques • GPS+GLONASS (compare with GPS-only results) • Rapid ephemerides instead of precise ephemerides • Use of OPUS-S, OPUS-RS, and OPUS Projects • Use of Oregon Real Time Network (ORGN) • Single base versus real-time networks • GPS only versus GLONASS • Use of Star*NET versus ADJUST versus OPUS-Projects • Give recommendations for optimizing the determination of accurate heights