| Airborne/GIS: Down the Drain: Tracking Storm Water with GIS
Greensboro, N.C. merges new information with its GIS to obtain storm sewer
permits from the EPA.
By Ruth Carapella In 1987, Congress enacted a two-phase storm water permit program under section 402(p) of the Clean Water Act. In the first phase of this program, municipalities serving large or medium sized populations were required to obtain storm sewer permits from the EPA. With a population of nearly 200,000, Greensboro, N.C. needed a permit and a plan.
Greensboro could have paid for its federally mandated storm water permit through general tax revenue. Instead, the city chose to develop a user fee approach designed to ensure that the landowners who generated the largest amount of runoff would bear the largest burden of the program's expense. Additionally, the city wanted to develop a user fee approach that would provide incentives to control runoff before it left a landowner's property and entered the city's storm water network. A Puzzle
Greensboro information center manager, Steve Sherman, likens the city's storm water project to a puzzle. Before the final picture could take form, the city would need to gather several pieces of new information and merge that information with their existing databases.
Sherman said, "Like most GIS's you spend several years just building data. For the storm water program, we are still at the building data stage."
There are several pieces to this puzzle. One piece is aerial photography information. Another is field survey work to create cross sections and profiles of the streams into which the rainwater flows. Additionally, the city must map the network of storm water culverts and the condition of those pipes. Finally, city engineers will use models to calculate the impervious surfaces that storm water will run on and the contribution of those surfaces to the storm water network.
Eventually, the city will use engineering models to predict how much water a specific storm event will contribute to each stream channel. This will enable the city to address one of the biggest issues in the storm water management program: how much water can a creek hold before it overflows its banks and starts flooding the adjacent homeowners? The Framework
Obtaining new aerial photography and digital terrain models was the city's first step. Sherman said, "Everything is tightly connected with the aerial photography work. We needed a DTM (Digital Terrain Model) that would allow us to generate 2-foot contour lines. And like many places, the city of Greensboro hasn't had new topos since the '50s when the geologic survey was done."
City managers contracted with Kucera International Inc. Based in Willoughby, Ohio, just north of Cleveland on Lake Erie, Kucera provided Greensboro with the baseline data they needed for the project. There were four deliverables: black and white aerial photographs, 1-inch to 100-foot scale digital planimetrics, 1-inch to 100-foot scale (half foot pixels) digital orthophotos, 1-inch to 200-foot scale (1-foot pixels) digital orthophotos that would mesh with the city's existing database, and DTMs.
Greensboro selected Kucera to work on the project for three reasons. As an experienced user of Genasys GIS technology, Kucera could provide Greensboro with products the city could plug directly into its Genasys-based GIS. Additionally, because Kucera had done work for Greensboro in the past, city managers knew they could count on Kucera for quality work at a competitive cost. But most importantly, Kucera had the equipment needed to accomplish the job at the required accuracy levels.
Greensboro had some exacting specifications for the aerial photo work. Sherman said, "The topo has to be very accurate to figure out how many cubic feet of water a specific creek can move away in a specific amount of time."
Planimetric data also required a high degree of precision. At national map accuracy standards, at the 1-inch to 100-foot scale, data needed to exceed an accuracy of plus or minus 2.5 feet. Greensboro had Kucera resample and recompile its existing 1-inch to 200-foot scale data to the new accuracy levels. This way, old data were retained and reformatted so they could be used with the new data.
Greensboro knew that Kucera had the experience and equipment it needed to meet the city's accuracy requirements. Kucera has over 40 years experience in aerial mapping and remote sensing work. In the early '50s, Bob Kucera fitted surplus military equipment into the company's first plane. Two decades later, after Bob Kucera's untimely death, the company's general manager, John Antalovich, Sr., purchased a controlling interest in the company.
Today, Antalovich serves as the company's CEO. In 1996, his son, John Antalovich, Jr. was named the company's president. Together, the father and son team oversee one of the largest privately controlled mapping companies in the United States. Kucera relies on state-of-the-art Zeiss equipment. Its most recent purchase was a Zeiss RMK Top Aerial Survey Camera System. For this project, which was flown in 1995, the company obtained images with a Zeiss LMK-1000 Aerial Survey Camera System.
The LMK-1000 camera has built in forward motion compensation and is placed in a Zeiss gyro-stabilized suspension mount to minimize image tilt. The camera's high-performance lenses produce high resolution images with a minimum of measurable distortion. With exceptional AWAR (Area Weighted Average Resolution) values, the LMK-1000 provided precise, high-quality images.
John Antalovich Jr. said, "Flying out of the busy Greensboro airport proved to be a challenge. It was difficult to schedule flight times over the city and surrounding county. We ended up flying the area over a weekend, completing separate passes at two different elevations to gather the data we needed in just two days."
The project area encompassed portions of Guilford County as well as the city of Greensboro. Data for the larger Guilford County area were collected at only the smaller of the two scales.
To reach the required accuracy levels, Kucera established about 600 vertical control points throughout Greensboro and Guilford County. These points were measured on the ground with Trimble 4000 SSI receivers. Kucera could use GPS for vertical control points because of the large number of benchmarks in the area. It took the field team several weeks to establish all of the vertical control points.
Antalovich said, "Airborne GPS wasn't a possibility for this project because the project was flown before airborne GPS was a reality. However, even today, I'm not sure that airborne would give you the type of precision that Greensboro needed."
Back at their Willoughby offices, Kucera used a Zeiss Phodis ST digital Stereoplotter with OrthoVista Software to produce high quality photogrammetric orthophoto mosaic images. This system allowed interactive editing of the DTM in the stereo model.
For the 1-inch to 200-foot scale orthophoto, Kucera draped the city's existing database over the new data. This allowed them to merge the existing data-building locations and driveways-with the new data. Scales differed. The old data set had an accuracy of plus or minus 5-feet. The new data had an accuracy of plus or minus 2.5-feet. The Kucera team developed a set of rules for making changes.
Some changes were straight forward; others were not. Antalovich said, "We could safely make the assumption that buildings hadn't moved. However, we couldn't assume that stream channels hadn't moved. Refining the old data to the new accuracy levels was as much an art as it was a science."
Additionally, Kucera had to merge newly collected 3-D point data with the 2-D points of the old data set. To do this, they developed project-specific software that generated the missing third dimensional data.
Antalovich believes the ability to merge older data sets with newer ones will become increasingly more important over the next few years. Many GIS users are beginning their second or third generation of data acquisition. Because advances in technology make it possible to collect more accurate information than was possible years ago, the ability to merge data sets without loosing information or introducing artifacts or errors is a critical skill. The interactive editing abilities of the Zeiss Phodis Softcopy mapping system made this type of data merge process possible on this project.
Kucera used the Zeiss TopoSURF program to generate a large number of elevation points and then to develop a grid-type DTM. Finally, Kucera used Zeiss's OrthoVista software to generate the orthophotos. OrthoVista automatically resolves conflicts between overlapping images during the image fusion process and creates high-quality orthophoto mosaic images.
Kucera provided Greensboro with both digital and hardcopy versions of the orthophotos. Using its Genasys GIS software, Greensboro can drape the ortho on top of the topo maps to get 3-D images of the land.
Sherman joked that because Greensboro is flat "there won't be any impressive fly-through demonstrations." However, the ability to drape the ortho over the terrain model will help managers visualize the lay of the land, and that in turn should help in visualizing the storm water runoff problem. A Piece in Place
The Kucera deliverables provide the base for Greensboro's storm water management program. The city will use the 1-inch to 200-foot scale digital orthophotos for casual day in and day out use. Sherman said that the accuracy of the 1-inch equals 100-foot scale is excellent, but the files are huge. Each file contains between 25 and 30 megabytes of data and there are over 600 such files in the project area.
Sherman believes that having the data at both scales allows the city greater analysis options without slowing down the daily use of the city's GIS network.
With this round of data collection, the city of Greensboro will have enough information to start on the engineering modeling for about one third of the basins within the city limits. Engineers will begin refining the process of assessing impervious surfaces and they will start modeling the impact of specific storm events.
Sherman believes that the newly acquired data will eventually be used for more than the storm water management program. In the past, Greensboro has focused its GIS efforts on solving infrastructure problems. However, use of the GIS is expanding. City managers see the benefits of using Greensboro's GIS to solve complex problems. In addition to traditional zoning and planning GIS applications, Greensboro plans to tackle a variety of GIS projects in the upcoming year.
The city's sanitary sewer engineers have expressed some interest in the storm water project. They are interested in developing a map of the sanitary sewer network. Additionally, the city is currently working on a project that will create links from engineering drawings of specific street segments and bridges directly to the planimetric representations of those segments. Users will be able to click on planimetric map features to see the linked images. Sherman also predicts that the DTM and digital orthophoto data may be used for storm water program in ways the city has not yet anticipated.
That's important. Phase II of the EPA's program, which the EPA will outline in detail in September, will address mitigation measures for pollutants associated with storm water discharge.
Greensboro's goal was to develop a storm water management approach that would carry the city into the next century. With the data collected and mapped by Kucera, Greensboro will be in a position to address whatever challenges the EPA sets before them. About the Author:
Ruth Carapella is a freelance writer and graphic designer specializing in forestry and natural resource management issues. She may be reached in Harrison, Id. by phone or fax at 800-689-9235 or by e-mail at [email protected] Back |
