NWDTW 2024

Historically, an engineer’s transit was considered the most effective way to determine the geometry of a channel, bridge, or culvert because data collection was simple, rapid, and accurate (Benson and Dalrymple, 1967). After flooding, transits were also used to collect positional data of important features such as high-water marks that correspond to peak flood stage and cross sections of a stream channel along a reach. A “transit-stadia” survey method was used to simultaneously collect horizontal and vertical positioning data. With the advent of modern land-surveying equipment, total station instruments have become the standard for rapid and accurate three-dimensional positioning using terrestrial-based surveying methods.
Common field techniques to obtain quality results include averaging zenith angles and slope distances observed in direct and reverse instrument orientation (F1 and F2, respectively), multiple sets of reciprocal observations, quality meteorological observations to correct for the effects of atmospheric refraction, and electronic distance measurements that generally do not exceed 500 feet. In general, third-order specifications are required for differences between F1 and F2 zenith angles and slope distances; differences between redundant instrument-height measurements; section misclosure determined from reciprocal observations; and closure error for closed traverse. For F1 observations such as backsight check and check shots, the construction-grade specification is required for elevation differences between known and observed values.
Two types of closed traverse surveys have been identified as reliable methods to establish and perpetuate vertical control: the single-run loop traverse and double-run spur traverse. Leveling measurements for a double-run spur traverse are made in the forward direction from the origin to the destination and are then retraced along the same leveling route in the backward direction, from the destination to the origin. Every control point in a double-run spur traverse is occupied twice. Leveling measurements for a single-run loop traverse are made in the forward direction from the origin point to the destination, and then from the destination to the origin point, along a different leveling route. The only point that is redundantly occupied for the single-run loop traverse is the origin. An open traverse method is also considered an acceptable approach to establish and perpetuate vertical control if the foresight prism height is changed between measurement sets to ensure at least two independent observations.
Specifications that were developed by the National Geodetic Survey for geodetic leveling have been adapted by the U.S. Geological Survey (USGS) for the purpose of developing standards for trigonometric leveling, which are identified as USGS Trigonometric Level I (TL I), USGS Trigonometric Level II (TL II), USGS Trigonometric Level III (TL III), and USGS Trigonometric Level IV (TL IV). TL I, TL II, and TL III surveys have a combination of first, second, and third geodetic leveling specifications that have been modified for plane leveling. The TL III category also has specifications that are adapted from construction-grade standards, which are not recognized by the National Geodetic Survey for geodetic leveling. A TL IV survey represents a leveling approach that does not generally meet criteria of a TL I, TL II, or TL III survey.

This discussion will focus on best practices for Real-Time GNSS surveys, using Real-Time Networks and local radio-linked base stations for kinematic observations. Part of this discussion will highlight the process of evaluating uncertainty.

A combined point cloud of about 85.6 million points was collected during 27 scans of a section of the western shoreline along the Shinnecock Peninsula of Suffolk County, New York, to document baseline geospatial conditions during July and October 2022 using a scanning total station. The three-dimensional accuracy of the combined point cloud is assessed to identify potential systematic error sources associated with the surveying equipment and the novel methodology used to collect and field-register (data are oriented and aligned in real time) point cloud data. The accuracy of the combined point cloud was assessed in terms of relative and absolute reference frames. Relative accuracy provides a measure of error within the local coordinate system and is determined by combining the uncertainty associated with the position of the scan station (the point being occupied by the scanning total station during the scan), the uncertainty associated with the position of the network control points, and the uncertainty associated with the laser of the scanning total station. Assessment of the absolute accuracy includes these three potential error sources combined with the uncertainty associated with the geodetic coordinates to which the local control network is referenced. The combined overall relative horizontal and vertical accuracy of the point cloud is 0.0156 and 0.0241 meter, respectively, at the 95 percent confidence level; the combined overall absolute horizontal and vertical accuracy of the point cloud is 0.0374 and 0.0733 meter, respectively, at the 95 percent confidence level.
A second survey was conducted during March 2023 following a substantial erosion event associated with (unnamed) Winter Storm “Elliot” (weather channel assigned this unofficial name). A bare-earth digital elevation model was then created of “pre-storm” (1st survey) and “post-storm” (2nd survey) conditions. The pre-storm, bare-earth DEM, was then compared with the post-storm DEM to detect topographic (and shallow bathymetric) change along the western shoreline and determine areas/features that are most susceptible to erosion during a major coastal storm event. The distribution and magnitude of erosion and deposition, and potential volume changes, will be disseminated in a USGS scientific report.