A potential client recently approached raSmith’s survey division for the completion of a hydrographic survey, also known as a bathymetric survey, for a 70-acre lake in southeastern Wisconsin. The individual was looking for an explanation of a hydrographic survey and a price estimate to complete the work.
As the assistant director of survey services at raSmith, I often encounter these types of survey questions. I enjoy sharing my expertise related to how a particular type of survey is performed and the level of effort needed to accomplish the desired result. (Have a question? Ask now.)
There are typically two ways to gather data for the bottom of a lake. One method is gathering data points with sonar, the other with a rod. Sonar can be collected by a remote control boat or a manned boat with an electric trolling motor. The process that uses sonar enables the surveyor to collect the most data in the least amount of time, thus reducing the cost per acre. However, this process is limited to water depths of two feet and greater, which means that some data must be collected with a survey rod along the shallow edges of the lake. The sonar collects data points (spot elevations) on the bottom of the lake at a surface of return. Vegetation, logs, rocks, debris, cars and anything else at the bottom of the lake will return a signal that is interpreted as the bottom of the lake and is not differentiated from the true lake bottom. Additional efforts are required to reach the true bottom.
A surveyor using the rod method can obtain sample sediment measurements in a smaller frequency and then map them. The key difference is that sonar mapping can be performed at a rate of approximately 10 to 15 acres per eight-hour day versus mapping the bottom of the lake by rod in the range of approximately five to eight acres per eight-hour day.
Our survey staff works with the client to determine the method(s) that will be the most cost-effective depending on the needs of the client and the project. A couple of considerations that can impact the cost of the survey are limiting the survey to particular areas of concern (for instance, potential dredge areas) as well as minimizing the data collection effort in other areas depending on the particular purpose for the survey and how much information is needed.
raSmith has a long history of practicing surveying in the state of Wisconsin (since 1929). Our growing team of water resources engineers complements our hydrographic survey capabilities based on their vast knowledge of the permitting process and lake management, which includes dredging, invasive species control, dam maintenance and other related services.
About the Author
Eric Sturm is the assistant director of survey services at raSmith. Eric has more than 29 years of experience in a wide range of land development surveying projects. He is responsible for project research and setup, calculations, plan review and quality control. He manages multiple projects including coordinating with the design engineers, field crews and drafting technicians and handles client communications. Eric plans and supervises survey tasks for the design and development of projects.