Bathmaster
An inexpensive system for bathymetric mapping.

Roger Windhorn

• Technical Note
• Design notes for battery/solar power source
• Erosion and sedimentation inventory procedure

Background

Reservoirs for public water supplies, flood control and recreation are common across the country. In Illinois there are 1,222 reservoirs, 162 of which have public water supply as a primary purpose (U.S. Army Corps of Engineers, 1996). Sedimentation of these structures is a natural phenomenon, the rate of which is dependent on landuse, landcover, soil, slope and climatic factors. Maximizing the lifespan and economic investment of these structures is dependent on controlling erosion and sediment delivery within the watershed.

Watershed planning is a common activity of the Natural Resources Conservation Service (NRCS). NRCS typically works with local communities and planning committees to identify concerns and develop solutions to address problems within the watershed. When the watershed contains a reservoir, mapping water depths and measuring sediment accumulation are common inventories performed in support of the planning process. The goal is to estimate the current normal water volume and rate of sedimentation.

How it was done

The traditional method for sampling water depth required surveying and creating permanent concrete monuments on the ground along several perpendicular lines (cross-sections) across the reservoir. This could typically require several weeks for a survey crew to finish. Cable or rope was then suspended across the reservoir from adjacent stations. Water and sediment depths were measured at predetermined intervals along the line using a hand held probing device (Figure 1). Due to the time and labor involved, a minimum of cross-sections were usually surveyed and sampled. For example, a 100 acre reservoir might have had 8 cross-sections, sampled every 2.5 feet along the section. The maximum depth one could sample was limited to 30 feet, the maximum length of a probe one could manipulate. A manual procedure was then employed to estimate volume based on the cross-sections (United States Department of Agriculture, 1975). These surveys were limited to small and moderate sized lakes, due to the physical limitations of stretching cable or rope an extended distance. These surveys were also labor and time intensive and precluded water skiing, fishing and other recreational opportunities of others for the duration of the sampling period.

New needs, new tools

In Illinois, there is now an increased demand for sediment information on all sizes and types of lakes. With the increased boat traffic on many lakes, a method was necessary that was easier, quicker, safer, less personnel demanding and less costly than traditional methods. The Global Positioning System (GPS) provided the tool to streamline much of the effort. Several surveys were performed during the summer of 1995 by georeferencing depth and sediment sample sites using GPS receivers obtaining Precise Positioning System (PPS) signals. This data was then input into a GIS for generation of surfaces using various interpolation methods, and eventual volumetric determinations. This method was an improvement over traditional methods, but still required manual determination of depths, and interactive use of the GPS receiver by an operator. These limitations also restricted the number of samples that could be obtained and the maximum depths that could be observed. A more streamlined method was needed.

Bathmaster

As our GPS assisted sampling was proceeding, we began investigating the possibility of automatically capturing sensor data along with position, eliminating manual observations and interactions. We use the real-time mapping package Geo-Link1 for building and updating GIS data and have realized the benefits of collecting data this way. The Geo-Link package has an additional version, XDS, specifically designed for capturing data from external sensors, such as sonar, data loggers, and laser range finders. Many of those involved in bathymetric mapping are engaged in underwater exploration, drilling or mapping and maintaining navigational waters. Sonar devices and software used by these groups were beyond the budget and requirements of our needs. XDS can read the Naval Marine Electronics Association (NMEA) 0183 sentence (Naval Marine Electronics Association, 1997). We needed to find a sonar within our budget that output the NMEA 0183 data stream. The Lowrance LMS 350A fit the bill. Our finished system included a laptop with a serial port PCMCIA card, Geo-Link XDS, Lowrance sonar, and a Rockwell Precision Light Weight Global Positioning Receiver GPS receiver capable of receiving PPS. All components of the system were previously in place except the sonar and XDS module, so our monetary investment was modest.

Using Bathmaster

A number of trial runs at a local lake were required to become familiar with the system and develop operational procedures. During these tests, a case was built to easily transport and contain the unit during operation (Figure 2). This was important, as all of our sampling is dependent on boats loaned or operated by other agencies or individuals.

Since Geo-Link XDS displays background maps, our first step includes digitization of the project area. United States Geological Survey 7.5' topographic quadrangle maps, (Figure 3) augmented with current aerial photography, (Figure 4) serve as the base for developing this layer. These two sources usually are adequate for providing current and historical surface area estimates. The most current boundary is used as the background map. The definition of the sensor type and sample interval is required before mapping. Samples can be collected at any fixed rate between 1 and 255 seconds. We select an interval that will provide a posting every 30-40 feet along our sample route, typically 2-4 seconds for the rates commonly traveled (Figure 5). The sonar must be set to output depth in meters to conform to software requirements. Once XDS and the sonar are set and the GPS receiver has established a lock on satellites, the current position is displayed in relation to the base map.

At this point, sampling is a matter of driving the boat along random cross-sections of all navigable portions of the project area. The screen display is a good reference for insuring adequate sample distribution. Two problems we have encountered include the GPS receiver losing lock on satellites and operational length of batteries. The only recourse when satellites are the problem is to stop and wait until a lock is reestablished. It is important to verify the satellite constellation for the intended sample period before sampling and plan fieldwork accordingly. In our region, periods lacking visible satellites are usually less than 30 minutes during a working day. Prior knowledge of this helps insure efficient use of time. Battery problems are a larger issue as all components are dependent on them. We use several rechargeable 12-volt batteries to power everything and often drain the power after 3 to 4 hours of continuous operation. This is often enough time to map areas 300-400 acres in size. For larger lakes, the only proven solution is more batteries. The ideal solution would be solar power, but we have yet to pursue this option.

Post Processing

When sampling is completed, data is input into a GIS package back at the office for processing. The depth data are supplemented with points of zero depth, derived from the current lake boundary. (Figure 6) At this point, any number of interpolation techniques are employed to develop a surface including, triangulated irregular networks, inverse distance weighting, spline and kriging. Usually several interpolations with several methods are performed and evaluated before selecting the surface that appears to be the best fit.

When it comes to interpolation, there is no one correct way. It is an iterative procedure with results evaluated visually, and quantitatively with cross-validation (Berry, 1997). When local experts are available, it is beneficial to have them review the map to insure the accuracy of the interpolation. When an acceptable surface has been developed (Figure 7), determination of volume is possible with the following formula:
SUM(resolution)2 x depth x (number of pixels of given depth).

Closing thoughts