Skip to main content

Legacy observations

Key research question: How can we leverage legacy observations for current NOAA applications?

National Geodetic Survey (NGS), is the Nation’s first civilian scientific agency, established by President Thomas Jefferson in 1807 as the Survey of the Coast. Its mission was, and still is, to survey the U.S. coastline and create nautical charts of the coast to help increase maritime safety. In 1970, the agency was reorganized as the National Oceanic and Atmospheric Administration (NOAA) and the geodetic portion of the NOAA mission was named the National Geodetic Survey.

The legacy survey observations and records collected by NGS for more than two centuries can be leveraged by the broader NOAA mission to observe, monitor, and model different Earth systems on land, in the ocean, and above the Earth’s surface. By proper documentation of geodetic observations, surveys, and models, it is possible to associate uncertainty values and develop time series and trends for different Earth system analyses. Such record keeping includes proper documentation on the survey itself and the conditions that the survey was conducted, such as benchmarks being used, number of repeated observations and weather conditions.

These Earth-based systems are constantly in motion. As such, it is also important to account for dynamic Earth system processes in the geospatial infrastructure. NGS has been investigating the spatial change of the Earth’s surface and its seafloor. In the past, location with respect to water surface (tidal or geopotential) was fixed until the next version of a reference system was developed. By understanding the accuracy of measurement, it is possible to model spatial changes and provide a time correction to the reference system accounting for geologic and oceanographic changes over climate-scale periods on the order of years to decades. In addition to updates to the geodetic network, the geodetic climate-scale products can be used for non-geodetic applications within NOAA and other federal agencies, such as the inter-agency geohazard monitoring programs (such as Earthquakes and Tsunamis), ground water activities, and sea level rise monitoring.

The traditional highest standard in surveying was a 1st Order survey standard that was accurate to 1 part in 100,000 or better and was used to define the “Basic Network.” The two early geodetic survey techniques used for establishing the high-accuracy network of geodetic horizontal control in the U.S. were:

  • Triangulation: - Survey technique that involves plotting two or more lines from known azimuths. Using the intersection point it is possible to derive back azimuths and find a location (e.g., benchmark). The technique uses basic trigonometric concepts, starting with a baseline and azimuth. The key observations are angles measured using a theodolite, which is an instrument with a telescope connected to two rotating circles (one horizontal and one vertical) to measure the horizontal and vertical angles. A good quality theodolite used for geodetic surveys would be graduated to 0.1 second of an arc and an angle resulting from repeated measurements would typically have an accuracy of about 1 second of arc, which is equivalent to about 5 cm over a distance of 10 kilometers, with respect to the baseline. The mathematical principle in Triangulation surveying is the Law of the sines.

  • Trilateration: - Trilateration determines a position by knowing your distance from at least 3 known points. This technique uses only distances, typically using an Electromagnetic Distance Measurement (EDM) that uses the known speed of light and the timed reflection of a microwave or light wave along the measured line. The early EDM instruments (in the 1950s) could measure long distances with an accuracy of about 5 parts per million (i.e. 50 mm over a 10 km line), but later versions were more accurate, able to measure with an accuracy of about 1 part per million (1 mm per kilometer or 10 mm over a 10 km line). The mathematical principle in Triangulation surveying is the Law of the cosines.

Mathematical principles for two early geodetic survey techniques: (Left) Triangulation: Law of the sines, and (Right) Trilateration: Law of the cosines

Other survey approaches that are considered for Extensions of Network operations are available and provide 2nd Order accuracy (1 part in 50,000 ). These surveys include:

  • Traverse: - Is a method that involves placing survey stations along a line or path of travel, and then using the previously surveyed points as a base for observing the next point. Angles and distances measured using direct computations. The benefit of using Traverse compared to the Basic Network surveys is that less reconnaissance is needed, and the traverse path is flexible to use any shape with fewer observations needed to be taken at each station.

  • Intersection: - This survey technique determines the coordinates of an unknown point that cannot be occupied (i.e., the observed point is inaccessible during a survey in a location that cannot be reached) and is visible from two points that were previously surveyed. Intersection technique is commonly implemented when the unknown point to be observed is inaccessible during a survey.
Schematic illustration of Traverse (left image) and Intersections (right image)

For vertical control, the most accurate geodetic surveying approach is spirit leveling. This survey approach has been in use since the early survey days of the Coast and Geodetic Survey, where the instrument (called a level) includes a small telescope and a vial filled, incompletely, with a colored spirit or alcohol, leaving a bubble in the tube. The level was used to read values from a set of specially constructed and marked rods while maintaining observations at a fixed horizontal surface . Accuracy of surveyor-grade level instruments is so high that the bubble will move 2 mm when the vial is tilted about 0.005 degree. Based on the survey needs (accuracy), it is possible to define the survey performance key standards (order classes):

  • 1st Order Class 2 Leveling accuracy at 95% confidence: 0.2 cm * sqrt(km)
  • 2nd Order Class 1 Leveling accuracy at 95% confidence: 0.4 cm * sqrt(km)
  • 2nd Order Class 2 Leveling accuracy at 95% confidence: 0.6 cm * sqrt(km)
  • 3rd Order Leveling accuracy at 95% confidence: 1.0 cm * sqrt(km)

Since the 1980’s, NOAA’s National Geodetic Survey has adopted the use of satellite navigation systems in land surveying. These satellite navigation systems, known as Global Navigation Satellite Systems (GNSS), continuously broadcast signals containing information about their position and precise time. Early calculations to calculate a position from satellites (e.g., Global Positioning Systems) used Trilateration calculation. Since then, land surveyors are able to use commercial receivers hardware that collect GNSS signals and are able to determine the receiver’s position in space (e.g., latitude, longitude, and height). Today, NOAA manages a Continuously Operating Reference Stations (CORS) and provides the public free access to the high-accuracy National Spatial Reference System (NSRS) coordinates using an Online Positioning User Service (OPUS). Empirical tests using the OPUS service shows the accuracies of 24-hr GNSS data observations from the CORS network are as follows:

  1. Absolute in the national network provides a 1.4 cm horizontal and 2.0 cm vertical (ellipsoid height) network accuracy at 95% confidence.
  2. Relative accuracy between to neighboring stations (up to 1 km apart): provides a 0.8 cm horizontal and 1.8 cm vertical (ellipsoid height) network accuracy at 95% confidence.

It is important to note that it is possible to reduce these errors further by processing multiple daily static files and doing network adjustments. For CORS time series longer than 2.5 years, we estimate measuring velocities to 0.6 mm/year horizontally and 1.2 mm/year vertically at 95% confidence.

The benefit of redundant observations with good metadata associated with the observations allows the end user to improve climate-scale calculations (e.g., trends) by increasing the number of measurements for determining the position and reducing the random survey errors distributed back into the measurements themselves. This type of calculation in geodesy is defined as a network adjustment. There are different mathematical approaches for network adjustment, where a least square adjustment is the most common mathematical approach used today in geodesy.

Once all available legacy data is properly documented with proper metadata and an uncertainty is associated with each observation; using the 200 years of NGS observations, it is possible to supplement modern-day observations and generate time series that can indicate deformation function and relate it with earth system observation that are associated with:

  • Dynamic observation of the geometric and geopotential surfaces,
  • Climate-scale observations and monitoring, and
  • Change detection.
Schematic illustration of horizontal displacement using legacy observation over two centuries (right) collected between two benchmark with an active geological fault between them (left)

In addition to monitoring climate scale changes through position observation using known benchmarks, NOAA’s National Geodetic Survey is also monitoring shoreline changes through the Coastal Mapping Program and height changes of the geoid using deflection of vertical evaluation:

Shoreline monitoring — Using NOAA and commercial georeferenced satellite/aerial imagery, it is possible to identify changes to the coastal characteristics. The main use for shoreline monitoring is for updating NOAA Electronic Navigation Charts (ENCs), especially along port areas. Shoreline changes can also be used to visualize water-level trends over years and decades and quantify the potential impacts on the coastal communities and coastal ecological systems.

Geoid model evaluation — NOAA’s NGS is validating geoid models by performing independent measurements through three field campaigns called “The Geoid Slope Validation Survey (GSVS)” that evaluated the accuracy of geoid as a function of the sloped of the model ("geoid undulations slopes"). These modern day observations overlap traditional leveling observations that were conducted more that 100 years ago. Using these climate-scale geoid observations, NGS established the Geoid Monitoring Service (GeMS) in 2019. This product is actively investigating all potential physical processes that could modify the geoid over time and how to properly incorporate these changes into the national geospatial infrastructure and other civilian and military applications.

Peer Review Publications and Conference Presentations

Ahlgren, K. 2022. GeMS Validation Survey, NGS Webinar series, May 22, 2022, Silver Spring, MD, USA. https://geodesy.noaa.gov/web/science_edu/webinar_series/gems-validation-survey.shtml

Ahlgren, K., V.Childers, T. Damiani, R. Hardy, J.Kanney, N. Kinsman, J. Krcmaric, D.Roman, D. Smith, D. van Westrum, D. Winester, and M. Youngman. 2019. A Preliminary Investigation of the NGS's Geoid Monitoring Service (GeMS). NOAA Technical Report NOS NGS 69, Silver Spring, MD, USA, pp. 120. https://geodesy.noaa.gov/library/pdfs/NOAA_TR_NOS_NGS_0069.pdf

Dracup, J.F. 1996. Geodetic Surveying 1940 - 1990. NOAA special publication NOS NGS 6, Silver Spring, MD, USA, pp. 17. https://repository.library.noaa.gov/view/noaa/51727

Federal Geodetic Control Committee 1984. Standards and Specifications for Geodetic Control, Rockville, Maryland September, pp. 37. https://www.ngs.noaa.gov/FGCS/tech_pub/1984-stds-specs-geodetic-control-networks.pdf

Pe’eri, S., S. Wdowinski, A. Shtibelman, N. Bechor, Y. Bock, r. Nikolaidis, and M. van Domselaar. 2002. “Current plate motion across the Dead Sea Fault from three years of continuous GPS monitoring,” Geophys. Res. Lett., 29, 42.1-42.4.

Scherneck, H-G. 1991. “A Parameterized Solid Earth Tide Model and Ocean Tide Loading Effects for Global Geodetic Baseline Measurements,” Geophys. J. Int., 106, pp 677– 694.

Schenewerk, M.S., W.H. Dillinger, T.M., vanDam, and O. Francis. 1999. “Ocean-Loading Deformations Derived from GPS Observations”, AGU Meeting, June 2.

Scherneck, H.G., and F.H. Webb. 1999. “Ocean tide loading and diurnal tidal motion of the solid Earth centre.” In: IERS Analysis Campaign to Investigate Motions of the Geocentre”. Ray, JR. (ed.), IERS Technical Note 25, Observatoire de Paris, France, pp. 83–89.

Scherneck, H.G., J.M. Johansson, and F.H. Webb. 2000. “Ocean loading tides in GPS and rapid variations of the frame origin”. In: Schwarz KP (eds) Geodesy beyond 2000—The challenges of the first decade, IAG General Assembly. Springer, New York, pp 32–40

Schenewerk, M.S., J. Marshall, and W. Dillinger. 2001. “Vertical ocean loading deformations derived from a global GPS network,” J. Geod. Soc. Jpn., 47(1), 237–242. https://www.jstage.jst.go.jp/article/sokuchi1954/47/1/47_1_237/_article

Smith, C.L. 2011.Bench Mark Reset Procedures: Guidelines to preserve elevation data for a soon-to-be disturbed or soon-to-be destroyed bench mark. NOAA publication NOS NGS, Silver Spring, MD, USA. 26 pp. https://geodesy.noaa.gov/PUBS_LIB/Benchmark_4_1_2011.pdf