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GPS/GIS Integration: Bedroom Community Mapped in Record Time
City of Edmond GPS/GIS Project Completed in 90 Days
By Bill Elliot In fast growth cities, building complete and accurate base maps to support GIS applications can be a challenge. Just ask the staff at the city of Edmond, Oklahoma GIS department. This upscale suburban community on the outskirts of Oklahoma City is one of the fastest growing cities west of the Mississippi. As roads, residences and subdivisions expanded rapidly to support population growth, the city's transportation map, revised several times over the years, became suspect. Just how accurate were new and expanded road networks being drawn on the original map? What about address ranges? Are address ranges in the TIGER file data current and correct enough to support Enhanced 9-1-1 applications across the city's 101 square mile area? These are questions that concerned city GIS personnel enough to award a services contract to GeoResearch Inc. to create a new digital road map and address ranges for the city using GPS/GIS technology.
"The city of Edmond project represents our first use of PowerMap on an real customer project," commented Michael Mariano of GeoResearch's Billings, Mont. office. PowerMap was ideally suited for the field mapping work required by the city of Edmond. "The project had to be completed in 90 days, so it gave us the opportunity to see just how fast and efficient new data collection features in PowerMap really worked," remarked Michael.
According to Sara Cobb, the city of Edmond's GIS manager, "a major project goal was to provide accurate centerline road maps and address ranges for analysis applications needed by the fire and police departments." Creation of a quality base map that could support a variety of geocoding and location mapping applications requested by other departments was also a consideration in funding the project. "We wanted a map that was accurate enough to support such applications as computer-aided dispatch that may come on-line in the future," commented Cobb. "We are very happy with the results." Addressing by Grid
Like much of the United States west of the Mississippi, the city of Edmond was originally laid out using a system of rectangular surveys divided into townships and sections with section lines forming one mile squares. "Outside the downtown area, many of the city's major roads were built along section lines. This eased the addressing task somewhat. "We created a grid overlay which was used to automatically calculate address ranges for section line roads. This helped a lot," explained Mariano. Interior roads within each section were addressed manually.
The scope of the project called for creating an accurate and up-to-date road centerline for 460 linear miles of roadway that exists within city limits. City officials asked GeoResearch to collect block-by-block address ranges and basic road characteristic information for each road segment mapped. The positional accuracy specified for the new road map was +/-10 feet. Project data needed to be delivered in ARC/INFO coverages for integration into the city's GIS.
In describing the project, Mariano commented, "Addressing went along smoothly except in a few places where road direction changed drastically. For example, if a road running north later turned east, the house addressing sequence might suddenly switch from 692 North to 3012 East on the same street. These situations usually caught us by surprise, and required additional work to create a break in the road segment where the address sequence changed." A Crew of Two
During the preparation phase of the project, an initial digital base map of the city was created from available maps. This digital map was used as a background map in PowerMap, and was displayed on the computer screen during field work for navigational and reference purposes.
Working with Mariano to complete the field data collection portion of the project was Steve Cutler, also of GeoResearch. Cutler supervised the set-up of the GPS base station which provided differential corrections for the roving GPS units used in the project. "Attribute collection was performed from our vehicle with one person driving while the other person collected data using PowerMap installed on a laptop PC," commented Mariano. The following attributes were collected for each road segment:
¥ Road Prefix Direction
¥ Road Name
¥ Road Type
¥ Type of Road Surface
¥ Left Address From
¥ Left Address To
¥ Right Address From
¥ Right Address To
¥ Number of Lanes
This attribute data was collected and loaded in a TIGER style database, a widely accepted data format that would be more easily shared by other agencies. GPS Road Centerlines
"We mounted the vehicle's GPS receiver on a metal arm that extended 2 feet away from the driver's side door," explained Mariano. "This positioned the GPS receiver over the centerline of the road while we drove in our lane. This worked really well even in oncoming traffic." Mariano added that the extension arm was a hindrance when negotiating drive-thru windows at fast food establishments, however.
To achieve the positional accuracies required for the road centerlines, a differential GPS base station was set-up downtown at the City Administration Building. "The city had previously contracted for placement of 1st and 2nd order survey markers near the administration building," stated Mariano. "This enabled us to obtain very accurate coordinates for the base station GPS receiver." At the end of each day, field and base station data were loaded onto an electronic bulletin board for post processing. ARC/INFO Coverages
During the quality control process, daily collection files were translated from PowerMap and viewed graphically in ArcView. Staff wrote AML scripts to automate many of the QC tasks. Each day's collection file was then translated into ARC/INFO coverages. Digital ortho aerial photographs for one half of the project area were available and used as the basis for editing some GPS road centerlines. "We made minor edits to some line work by aligning road intersections with those on the photographs," explained Mariano. Master ARC/INFO coverages for delivery to the customer were created by appending daily coverages and building new topology. Map files were delivered using the Oklahoma State Plane projection in a NAD 83 datum for integration into the city's GIS.
To ensure correctness and completeness of the data, check plots of each square mile map area along with tabular data was provided to city officials for inspection and approval. Mariano explained, "The city had seven days to edit and suggest corrections for each check plot. Over 90 percent of our field data was correct the first time. We did miss a few addresses and one road segment that was blocked by new construction. Overall, final corrections to the initial data were pretty minimal."
After receiving the new maps, the city's GIS and engineering departments reviewed them for quality. City officials were able to overlay the newly created road centerline maps onto their original transportation map for comparison. "As one moves away from the downtown area on the maps, distortions on the old map were visible," remarked Mariano. "In some areas subdivision shifts were significant. This old map/new map comparison provided visual re-enforcement to city officials that this mapping project was quite necessary." About the Author:
Bill Elliott is director of marketing at GeoResearch, Inc. He may be reached at 301-664-8000.
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