MERGING DATA SETS OF SEPARATE ORIGIN

1. MERGING DATA SETS OF SEPARATE ORIGIN • DATUMS (Horizontal, Vertical, 3D) • CONVERSION TECHNIQUES • CONTROL DATA vs. MAPPING DATA • ORIGINAL INTENT • DATA ACQUISITION METHODOLOGY (Equipment & Procedure) • CURRENT WORK (CERP, Others)

2. HORIZONAL DATUMSMERGING DATA SETS • Need to understand the datum definition for each data set • 3, 5, and 7 parameter transformations involve xyz shift, rotation about each axis, and scale • Multiple regression techniques – complex • Datum grid – best fit surface (NADCON) • Be cognizant of distortions

3. HORIZONTAL DATUMSNAD 27 • Based on Clark 1866 ellipsoid • Computed from a single survey point as a datum point • Distortions at the 10m level • Design accuracy does not support GPS

4. HORIZONTAL DATUMSNAD 83 (1986) • Incorporates historic geodetic survey data, Doppler, VLBI, and limited GPS • Origin approximates earth’s center of mass • Based on GRS 80 ellipsoid • Distortions at the 1m level • Design accuracy sufficient to support GPS

5. HORIZONTAL DATUMSNAD 83(HARN) • Refinement of NAD 83 (1986) • Specific to each state but being revised to become a seamless system • Controlled by and designed to support GPS • Approximately 0.7m difference between NAD 83/86 and NAD 83/HARN • Distortions at the 0.1m level

6. HORIZONTAL DATUMSCONVERSION TECHNIQUES • Measure control points in both datums and reprocess survey data to desired datum • Measure local biases at common points and apply a rigorous adjustment – small regions • NADCON (nominal accuracy of NAD 27 & NAD 83 conversion is 0.3m or 10-15cm RMS - 1 sigma, with occasional outliers approaching 50cm)

7. COMPARISON OF VERTICAL DATUM ELEMENTS • NGVD 29NAVD 88 • DATUM DEFINITION 26 TIDE GAUGES FATHER’S POINT/RIMOUSKI • IN THE U.S. & CANADA QUEBEC, CANADA • BENCH MARKS 100,000 450,000 • LEVELING (Km) 102,724 1,001,500

10. VERTICAL DATUMSNGVD 29 • NGVD 29 was warped across Florida and held fixed to several ever changing tidal stations across the continent • Other distortions in the NGVD 29 network were caused by crustal motion associated with earthquake activity, postglacial rebound (uplift), and subsidence resulting from the withdrawal of underground liquids

11. VERTICAL DATUMSNAVD 88 • NAVD 88 is a minimum constrained adjustment • As much as 9m distortions were observed when trying to force new leveling to fit the NGVD 29 height values • The datum is based upon the mass or density of the Earth instead of the varying heights of the seas

12. BASIC CONVERSION TECHNIQUES • Estimation of bench mark heights by incorporating the original leveling data into NAVD 88 using least squares adjustment techniques • A rigorous transformation of bench mark heights for a particular project using datum conversion correctors estimated from the project’s original adjustment constraints and their differences between NAVD 88 and NGVD 29 • A simplified transformation of bench mark heights using an average bias shift for the area (VERTCON) – NOT RECOMMENDED BY NGS FOR SOUTH FLORIDA, a new VERTCON grid will be developed from the CERP Geodetic Vertical Control Network

13. CONTROL DATA vs. MAPPING DATA • Mapping data is a set of relational measurements dependent on the survey control • Distortions in datums frequently make closing the gap between two different surveys difficult, even on the same datum • If surveys are properly documented and archived, these discrepancies can be easily overcome by making measurements between the respective survey control points

14. The many definitions of “height” • Orthometric Height (H) • Height from the geoid to the Earth’s surface • Frequently (informally) called “height above mean sea level” • On most topographic maps • e.g. “The Height of Mount Everest is 8848 meters”

16. The many definitions of “height” • Ellipsoidal Height (h) • Height from the ellipsoid to the Earth’s surface • Usually derived from GPS measurements • Easy to obtain through GPS • Has few physical applications • e.g. water does not necessarily run ‘downhill’ in ellipsoid heights

17. The many definitions of “height” • Geoid Height (N) • Height from ellipsoid up to geoid • Also called “geoid undulation” • N = h - H • “N” determined from gravity measurements

20. Saving money with a geoid model • The geographic community needs orthometric heights, H • Old way: H = H0 + DH (spirit leveling) • Time consuming, 4 person team. Expensive! • New way: H = h - N (GPS & geoid) • Fast, accurate, 1 person team (note: this is a gross oversimplification – slide obtained from NGS. However, GPS is far more productive and therefore cheaper). Cheap! • Part of NOAA’s “Height Modernization” program

21. Positioning for the Future Today: Surveyors can do sub-cm spirit leveling over lines up to 10 km long Tomorrow: Surveyors expect to do 100+ km lines at similar accuracy with GPS and geoid (“GPS leveling”)

23. Continuously Operating Reference Stations

24. Airborne Laser Scanning

25. HOW WAS DATA ACQUIRED? • Precision of equipment • Accuracy of methodology • How was the data processed • Confidence margin of data set • Original intent of survey • Can the data be used for current purpose • Which data sets are inherently more accurate and should take priority over others

26. CURRENT WORK • CERP Geodetic Vertical Control Network includes first order leveling, 2cm vertical GPS, gravity measurements, and enhancement of the Height Modernization Program in order to facilitate future use of high tech survey techniques and equipment • NGVD 29 to NAVD 88 conversion at USACE Jacksonville

27. SUMMARY • NAVD 88 heights are better estimates of orthometric heights than NGVD 29 heights and have become critical as surveying techniques become more sophisticated and more accurate • When choosing a coordinate system and datum transformation, consider existing data you have and future data you may receive

28. SUMMARY • METADATA, proper documentation, and archival of data is essential • No matter how good the transformation is, if it does not match with your existing data, it will not be of much use

29. Please join us now for the Q&A Session