Handheld GPS dramatically reduces time in field for outfall survey
Frederick Morin, Director of the Department of Public Services, Village of Milford, Michigan
Brian Slizewski, P.E., Municipal Services Group Manager, Wade Trim, Taylor, Michigan
Collecting field data has traditionally been a labor-intensive effort increasing the cost and time required to complete a public works project. Technology is streamlining this effort when the right equipment is matched to the project's needs. The Village of Milford, a community of 6,000, recently found an affordable way to inventory its stormwater outfalls to meet National Pollutant Discharge Elimination System (NPDES) permit requirements. Using a handheld global positioning system (GPS) unit, a two-man crew mapped 57 outfalls in one and a half days—a fraction of the time that would have been required using traditional surveying methods. This project demonstrates how technology can supplement DPW staff knowledge to save time and lay the foundation for future DPW applications.
The Michigan Department of Environmental Quality (MDEQ) requires Phase II communities to submit a map identifying all outfalls as part of their permit requirements. Depending on the number of outfalls, size of the community and amount of developed land, this can be a daunting task. Developing a game plan to efficiently collect and record the data before heading into the field is critical. The Village of Milford developed a plan that:
Hands-on knowledge speeds up efforts
The more you know going into any situation, the better prepared you will be. Milford's upfront planning efforts minimized time in the field. Since several water bodies meander through the Village with numerous discharge points, the definition of an outfall had to be clear. Working with the MDEQ, an outfall was defined as any discharge point where water left the Village's jurisdiction and entered waters of the state, specifically Pettibone Creek or the Huron River.
The DPS Director and Village Engineer reviewed the available storm sewer maps and developed a plan to visit each outfall. While this sounds simple, it took awhile to determine the best access route to hidden outfalls in wooded areas along the river. A numbering system was developed for outfalls that followed the preestablished field survey route from west to east, then south to north. Information identified for field collection included pipe size, pipe material (concrete, PVC or corrugated metal), end treatment (end section, headwall, abutment or bare pipe), location (submerged, at bank or upstream of bank) and other notes. Each outfall was also photographed showing its designated number.
Armed with marked-up copies of the ink and mylar sewer maps, a handheld GPS unit and a digital camera, the DPS Director and engineer headed out in the field once the leaves had fallen. Drawing on over 35 years of experience with the Village, the DPS Director quickly located hard-to-find outfalls because he knew roughly where they should be. He also identified maintenance concerns that needed to be addressed and radioed instructions to DPS staff on what to do. The engineer quickly identified materials, end treatment and any structural deficiencies or concerns worth noting. Data was entered into the GPS unit, coordinate data was recorded and electronic pictures were taken—the only paperwork required was writing the number to include as an outfall label in the photograph.
In this manner, all 57 outfalls in the 2.5-square-mile area were located, photographed and inventoried in less than two days. It became evident that the ink and mylars of the storm system were not as accurate as previously thought—several outfalls were not mapped or found in different locations.
Two-dimensional GPS data integrates smoothly with county GIS
A Trimble GeoXT GPS receiver was used to collect data. Less than an hour of training was required to bring the engineer up-to-speed on how to properly use the device. The receiver was pre-programmed with the appropriate data fields to prompt the user for required information at each outfall. When each outfall was field located, the receiver was activated to receive satellite data while the engineer entered the outfall identification data. In the minute or so it took to manually enter the information, the receiver obtained an accurate fix on the location. There was never downtime waiting for satellites and very few physical obstructions were encountered during GPS data collection. Only X and Y coordinates were collected but this was not an issue since elevation was not needed.
The handheld GPS unit used is accurate within one meter 90% of the time, for each logged position. Seven or more positions were logged and averaged for each outfall to ensure a high degree of location accuracy. A differential correction had to be calculated since a base station was not used to correct data in real time as it was collected. This correction was performed by a geographic information system (GIS) programmer back in the office who downloaded data from fixed base stations that were operational when the original data was collected. Changes from this correction typically range from 1 to 3 meters; Milford's data resulted in a 1.25 meter change on average.
The corrected GPS data was then exported to a GIS data format. While the Village does not have its own GIS, it has free access to Oakland County's base map information. This extremely accurate base map easily accommodated the GPS outfall data. Very little shifting occurred when the data sets were brought together, creating a final outfall map with accuracy down to one meter. A survey crew using more sensitive GPS equipment in conjunction with established ground base stations could have identified outfall locations within one centimeter accuracy and obtained elevation information. However, the one meter accuracy was sufficient for this project and several hours were saved by using the handheld GPS and making simple corrections.
Provisions for the future
The outfall base map can serve as the foundation for stormwater GIS inventory and analysis for the Village in the future. Storm sewers can easily be added to eliminate the need to use outdated mylars. Since data collected on each outfall is stored in the GIS, the information can also be used to develop an outfall maintenance program and as part of the Illicit Discharge Elimination Program (IDEP) that will be performed under Phase II. Future work was also considered during the field survey; appropriate number signs were posted at each outfall to enable easy location. Successful use of the handheld GPS unit has also led the Village to consider other applications where traditional survey or high-accuracy GPS techniques aren't critical such as locating sanitary sewer manholes, fire hydrants, streetlights and many more structures.
Frederick Morin has 35 years experience and has been DPS Director of the Village of Milford since 1987. He can be reached at (248) 685-3055 or firstname.lastname@example.org. Brian Slizewski has over 20 years municipal engineering experience and has worked closely with the Village of Milford since 1998. He can be reached at (734) 947-9700 or email@example.com.