| St. Louis Maps Impervious Areas
Contractors combine technologies
By Kevin Corbley Paying taxes and other government fees may be one of the few certainties in life, but the citizens of St. Louis can count on at least one more thing-their stormwater useage fees will be assessed fairly and accurately thanks to state-of-the-art GIS and mapping technologies.
The Metropolitan St. Louis Sewer District (MSD) is nearing completion of an impervious surface mapping project undertaken to ensure that a proposed charge to upgrade stormwater infrastructure is enacted equitably. The new fee will be voted upon by citizens in the district's service area, which covers the city of St. Louis and most of St. Louis county.
"The only way to create an equitable [stormwater] fee system is to charge people based on the amount of water that runs off their property," says Bob Puricelli, Manager of GIS and Graphics in MSD's Engineering Department. "That meant we had to accurately map and measure buildings, patios, driveways and any other impervious surface on each property."
MSD hopes the accuracy and consistency of the assessment will help make the new rate proposal acceptable to voters. Currently, the district provides services to collect and treat wastewater and to manage stormwater for a 524 square-mile area. However, MSD only has authority to charge stormwater fees for half of that territory, which needs substantial upgrades to its stormwater infrastructure to avoid flooding and runoff problems.
Prior to initiating the mapping program in early 1997, MSD reviewed similar projects to determine which technologies and procedures should be employed to accomplish the job quickly and accurately. They devised detailed plans that included updating an existing digital basemap, calculating impervious surface area on each property, and linking each computation to the correct property ownership record. Working with Multiple Contractors
After open bidding, MSD contracted several geo- technology service companies to utilize high-tech mapping and GIS techniques to complete specific project phases. MSD worked directly with three contractors and one sub-contractor throughout the project. Each company performed a specific role: ¥Surdex Corporation of St. Louis acquired and scanned new aerial photography; ¥Kucera International, Inc., of Willoughby, Ohio, performed GPS ground control and aerotriangulation, updated planimetrics on the existing GIS basemap, and created digital terrain models, contour maps, and orthophotographs; ¥Barton Aerial Technologies of Columbus, Ohio, hired as a subcontractor to SDS, Inc., digitized the impervious surfaces; ¥SDS, Inc., of Franklin, Tennessee, performed cadastral mapping and calculated impervious surface areas.
Employing multiple contractors is not a typical approach to this type of project, acknowledged MSD's Puricelli, but it proved highly successful. "We were able to manage each phase of the project directly instead of throwing it into one contractor's lap and losing touch with it through a series of sub-contractors."
The contractors agree that working as a team helped them remain aware of the overall project schedule because a missed deadline would have a domino effect on other companies. Project success, they said, hinged on keeping communication lines open and establishing efficient methods of transferring data to each other.
"The multiple contractor arrangement allowed each organization to focus on its phases of the project," says John Antalovich, Sr., CEO and chief project administrator for Kucera International. "Considering its size, we are completing this project in a short period of time, which has meant a considerable financial savings for MSD."
When final products are delivered early this year, impervious surfaces will have been mapped for 505,000 parcels in only 18 months. And the final cost will be about $1.1 million-roughly half what smaller impervious mapping projects elsewhere have cost. Updating Maps
GIS has been an integral part of MSD operations for a decade. Built with Intergraph Corporation's MGE software, the district's GIS includes one of the largest sewer databases in the nation. It contains 500,000 features relating to 8,500 miles of stormwater, sanitary and combination sewers, as well as 13 wastewater treatment facilities in 38 watersheds.
In recent years, St. Louis has experienced significant growth, compelling MSD to order an up-to-date aerial survey before every new mapping project. Photography was especially critical for this project because MSD wanted to map every parking lot, backyard patio, and home addition built within the past year to ensure the accuracy and timeliness of its calculations.
Surdex Corporation, a photogrammetric mapping and GIS firm, was the logical choice to complete this phase because it has been conducting aerial surveys and photogrammetric mapping for various St. Louis city and county agencies since the 1960s. Its inclusion in the project ensured consistency between new and existing digital GIS maps.
"Quality is the most important consideration in the aerial portion of the project," says Surdex Vice President Russ Hoffmann. "The accuracy of data derived from every other phase depends on the quality of the air photos."
A map scale of 1:15,000 was deemed suitable for identification of surface features sought by MSD. To achieve this, Surdex flew at 7,500 feet above ground level and shot black-and-white frames at one inch equals 1,250 feet. Photography was completed in March 1997, a time of year when minimal vegetation offered a maximum view of ground surfaces.
"Consistency was another critical requirement because the new air photos would be overlaid on the existing photographic basemap in the GIS to update changes," says Hoffmann. "We operate a flight management system in our aircraft to match photo centers with the map sheet grid MSD has used in its GIS for years."
The aerial firm uses a Zeiss Jena LMK 2000 camera with computerized flight management. Prior to the flight, planners entered the desired centerpoint coordinates for each photo frame. During the flight, the GPS-guided system helped the pilot navigate the aircraft to the correct acquisition locations. The system automatically fired the camera shutter.
Surdex scanned the hardcopy prints to one-foot pixel resolution with a Vexcel 3000 scanner. As images were scanned, files were sent in groups via E-mail to MSD. The contractors experimented with several digital compression and transfer methods until a suitable one was found.
"WinZip 6.3 was the best compression software available to E-mail the huge digital files between contractors," says Evan Montgomery, the MSD GIS supervisor. "If compressed files were still too large for E-mail, we saved them on CD and shipped them to the various vendors." Updating Planimetrics
MSD contracted Kucera International, a full-service mapping and GIS company, to generate orthophotographs and update planimetric features on the existing GIS basemap for the impervious surface mapping project. The contract also called for Kucera to produce digital terrain models and contour maps for later incorporation into hydraulic modelling applications.
Kucera received the new scanned air photos and copies of the existing digital orthophoto basemap, part of which it had created during an earlier project for MSD. The firm sent a field crew to St. Louis to gather GPS control points for aerotriangulation and orthorectification of the air photos.
The firm utilized softcopy digital photogrammetry technology-Zeiss Phodis stereoplotters-to produce most project deliverables. In the past two years, it has been shifting mapping work away from the manual world of analytical stereoplotters in favor of the speed and accuracy of semi-automated digital techniques.
"We have made the transition to softcopy because mapping is predominantly a digital discipline today," says Kucera President John Antalovich, Jr. "Analytical techniques have reached their limits, while softcopy has just begun to show its potential."
Working with the new air photos, Kucera technicians first entered the GPS control points into the softcopy system, which automatically matched ground points with photo points to set up stereo pairs. Although the softcopy plotter can choose its own points to autocorrelate images, the technicians selected tie points manually for most frames.
In previous projects, they had learned that accuracy of generated DTMs is higher if autocorrelation is applied only to areas of open ground while tie points are selected manually in forested or developed areas. Trees and rooftops can throw off the precision of elevation measurements in autocorrelation.
Once the DTMs were completed, technicians examined the images to correct misplaced points and add key breaklines such as the edges of road surfaces. The Phodis system then automatically generated the orthophotos from the corrected DTM data.
"Softcopy technology is much easier to use than analytical methods in projects like this involving updating of planimetric features," says Antalovich. "We superimposed the old [basemap] vectors on top of the new orthophotos in the digital stereoplotter."
Kucera technicians then visually examined the superimposed images and updated planimetric vectors directly on the GIS basemap. They paid special attention to recently built house additions, pools, patios, or driveways on private properties, as well as new streets constructed in housing developments. Any non-grass area was identified and added to the basemap. Digitizing Impervious Surfaces
Updated basemap sections were transferred to Barton Aerial Technologies, an aerial surveying and topographic mapping company, to capture impervious surfaces in vector files. Digitization was conducted manually on a Leica SD 2000 heads-up analytical stereoplotter.
Using roadway pavement lines as references, technicians digitized the edges of building rooftops, driveways, patios, and other impenetrable surfaces. They created separate MicroStation .dgn files for each type of impervious feature polygon. This manually intensive phase of the project took seven months to complete.
"For a square house on a small property, each parcel takes about 30 seconds to complete," says Duane Barton, the firm's owner and president. "But larger properties with pools and outbuildings took as long as 15 minutes to digitize."
Closing each polygon was a major concern because so many features had common boundaries and the correct association between them had to be represented correctly. For instance, a driveway often abuts a house, sharing one edge with the building and another with the street. Likewise, a walkway may be bordered by the building, street and driveway.
"There was a hierarchy of surface elements that we followed to make sure each feature was digitized as a complete polygon. We digitized buildings first, driveways second, sidewalks third, and other features in descending order," says Barton. "This ensured that each feature was identified correctly and input in the right file."
Additional manual editing and tagging was required on many properties to locate and eliminate false polygons„grass surfaces surrounded by impervious surfaces which could be mistaken by software as impervious features. On such panels, the technicians input nodes to identify walkways, patios, and other elements, which made non-impervious surfaces stand out. Calculating Impervious Areas
MSD chose SDS, Inc., to perform the cadastral mapping because it specializes in assessment-based GIS mapping services and has expertise with large data sets. SDS created a GIS from the CAD data provided by other contractors. Its assignment was to close the impervious surface polygons, reference the existing parcel basemap, and establish relationships between features and owners. SDS was also tasked with computing the surface area for each feature.
The digital parcel base, compiled by the county assessor in MGE format, contained parcel and right-of-way lines as well as parcel centroids. This number tagged each parcel to the database of owner names and addresses. SDS also received the updated GIS planimetric basemap and impervious surface files.
"The parcel map had never been spatially oriented to the same NAD83 coordinates as the GIS basemap," says Susan Marlow, SDS owner and president. "We had to construct exact rights-of-way aligned with the digital planimetrics in order to match the existing digital parcel data correctly."
Working in Bentley MicroStation software, SDS constructed exact right-of-way lines using the parcel files with road edges on the basemap as reference. Next, they wrote CAD routines in MicroStation to move each parcel so it overlaid the building footprint on its correct property. Automated rubbersheeting routines were applied to match parcel lines with correct properties on the basemap.
"The match didn't have to be exact," says Marlow. "We corrected each parcel enough so that all of the impervious surfaces were enclosed in the parcel polygon."
Next, SDS transferred all data into ARC/INFO format. Technicians ran an ARC process to attach impervious surface areas with correct parcel centroid numbers. They reviewed the final data on screen in ArcView to ensure that impervious surfaces and parcels had been linked correctly.
SDS then calculated total square footage for each feature type. The resulting data provided a summary of impervious areas by feature type in each parcel. Matching Data with Owners
"At that point, SDS saved the output files of impervious surfaces and area calculation in MGE format and sent them back to us," says Tim Cox, MSD's GIS specialist. "Our job was to link them to our Oracle database."
MSD created two Oracle tables to complete the project. The first contains the impervious surface polygons and associated area calculations. In this format, MSD can employ simple SQL commands to calculate impervious areas within any or all parcels.
The second table holds parcel ownership records. The parcel number links this table with the other. MSD can cross reference Oracle data files to determine how much each property should be billed based on its total impervious surface area and where the bill should be mailed.
"The new impervious fee system will be highly automated and very accurate," says MSD's Puricelli. "Everyone is very pleased with the mapping project. The combination of new technology and the multi-contractor approach enabled us to get the best product for our rate-payers' money." About the Author:
Kevin Corbley is a freelance writer and consultant specializing in remote sensing, digital mapping, GIS, and GPS. He is located in Denver and may be reached at 303-722-0312 or by E-mail [email protected] Back |
