Hydrographic Surveying Resources @ ODOT David Moehl, PLS, CH Project Surveyor, ODOT Geometronics What is hydrography? • Hydrography is the measurement of physical features of a water body. • Bathymetry is the foundation of the science of hydrography. • Hydrography includes not only bathymetry, but also the shape and features of the shoreline; the characteristics of tides, currents, and waves; and the physical and chemical properties of the water itself. What is bathymetry? • The name is derived from Greek: • Bathus = Deep • and Meton = Measure • Dictionary: “The measurement of the depths of oceans, seas, or other large bodies of water, also: the data derived from such measurement.” • Wikipedia: “is the underwater equivalent to topography.” • NOAA: “The study of the ‘beds’ or ‘floors’ of water bodies, including the ocean, rivers, streams, and lakes and currently means ‘submarine topography’.” Source: https://www.ngdc.noaa.gov The lost city of Atlantis? Source: Google Earth How do we measure water depths? • Manual How do we measure water depths? • Manual How do we measure water depths? • Manual How do we measure water depths? • Manual • Sound • Echosounders • Determines the depth by measuring the two way travel time of sound through the water • Different types of echosounders Singlebeam echosounders • This is the tool that ODOT utilizes • Benefits of this system: • Most affordable and widely used • Less ongoing training, software and maintenance costs • Useful for: • Cross section data acquisition for hydraulic modeling • Uniform topography • Shallow water • Singlebeams come with different grades Singlebeam echosounders • Expandable options • Dual Frequency – Two pings from different frequencies e.g. 200 & 30 kHz • Lower frequency may penetrate sediment layers and underwater plants but is also subject to noisier data (more false returns) • Top of the line survey systems can operate at a faster ping rate (20 Hz) and record a digital echogram • More statistical mapping options due to higher acquisition rate • Echograms can help the surveyor determine if particular sounding is noise or real, also aids in determination of bottom characteristics such as the presence of sea grass • Very useful for habitat and environmental studies What other systems exist? • Singlebeam • Multibeam • Easily visualized as a number of singlebeams arranged in a fan pattern • Typically orientated to project towards the floor from a vessel • Great for coastal and ocean areas when full coverage is warranted for safety • Good in rivers when detailed hydraulics are needed • Coverage dependent on depth What other systems exist? • Singlebeam • Multibeam, cont. • Scanning modification • Multibeam configured to acquire similar to stationary lidar systems • Great for infrastructure inspection / monitoring Source: Minnesota DOT What other systems exist? • Singlebeam • Multibeam • Sidescan sonar • 2D for pictorial representation of the bottom • No depths, although heights of objects can be calculated • Great for flat and shallow areas and searching Source: www.jwfishers.com/ large areas • Often used in conjunction with singlebeam or multibeam to get accurate depths What other systems exist? • Singlebeam • Multibeam • Sidescan sonar • Bathymetric lidar • Can be acquired from airplane or sUAS • Uses a green laser as opposed to terrestrial red lasers; green wavelength can penetrate the water further • Limited to shallow areas, typically less than 30 feet where water clarity is good • If you can’t see the bottom, lidar likely not able to as well. Hydrographic Bathymetric Surveying Resources @ ODOT Summary • There is a tool for the job • Biggest hurdle is understanding what the specifications of the job really need to be • The current ODOT tool can probably handle over 90% of the projects we typically conduct Trying to make things easier • In constant development: ODOT Hydro Tool Kit.