Index

Bathymetry Projects

Contact Information

Portland State University
Center for Lakes and Reservoirs
P.O. Box 751 - ESR
Portland OR 97207-0751
Ph: 503-725-9075
Fax: 503-725-3834

Introduction

As part of the nutrient criteria development project bathymetric data were collected on nine coastal lakes in Oregon. The data are a result of recent technology that makes data collection more efficient, thus allowing the collection of many more data points that previous methods. Previous mapping exercises for these lakes took place in the seventies when technology limited the amount of data that could be efficiently collected. This project used digital echosounder equipment connected to a gps unit allowing very accurate maps to be created from high data density.

Data Collection

Information from two data sources are required for accurate bathymetry. Position on the lake is provided by GPS satellites, and depth is privided by a digital sonar. The data is collected and stored on a computer where it is analyzed for morphometric characteristics.

The GPS receiver used on this project was an Alto G-12 from Corvallis Microtechnology. Included was a Coast Guard beacon receiver for real time correction of the satellite signal, giving a position accuracy of plus or minus 100 cm.

Depth information was provided by a Biosonics DE series digital echosounder. The echosounder integrates the depth information with GPS location to give a three dimensional picture of each data collection point on the lake. This information is displayed and stored on a laptop computer.

As data are collected the computer screen displays a color chart of the lake bottom (Figure 1). The ping number is displayed across the top and depth is displayed between the echogram and oscilloscope. The different colors represent the signal frequency when it is received by the transducer. The frequency is a surrogate for the target density. A harder substrate will return a lower frequency. The macrophytes in figure one are shown as a higher frequency blue while the bottom is a lower frequency green to yellow.

When data are analyzed with VBT software it is possible to fine tune the bottom tracking signal to maximize depth accuracy (Figure 2 window A). If the lake has a dense population of macrophytes the software has the capability of discriminating what is a plant and what is the lake bottom. Window B in figure 2 shows two dimensional view of the lake bottom. Comparing the depth returned by the analysis in window A to the bottom as represented in window B confirms the analysis accuracy. Window C in figure 2 shows columnar data that results from the analysis. This data is exported into a spreadsheet program and prepared for importing into ArcView.

Within Arcview the data are represented by points that contain lat/long and depth information (Figure 3). The points of equal depth are connected, creating the bathymetric map (Figure 4). It is also possible to calculate volume and surface area with Arcview. Calculating the volume and surface area at each depth contour provides the information necessary for developing hypsographic curves. These curves will show how much volume is in a lake at each depth.