Airborne—County Reaps Huge Rewards From Airborne Laser Survey
By David Stone

Seven million dollars will get anyone's attention. And, when this amount was identified as a cost reduction, it certainly got the attention of Al Bungard, P.E., county engineer for Chatham County.
    Chatham County, home of Savannah, Georgia, nestles against the Atlantic coast where Georgia and South Carolina share a border. Like so many counties in this famous 'low country,' it is a low-lying arrangement of slow moving rivers, salt marshes, and tidal flats. Hills are nowhere to be found and water drains at a snail's pace, if at all.
    When the county retained the Savannah-based firm Thomas & Hutton Engineering Co. to analyze and design flood control improvements in the Hardin Canal drainage basin, they knew it would be another step in a never-ending challenge. Using USGS quad maps and minimal conventional survey data that was available for the basin, Thomas & Hutton accomplished the initial study arriving at a solution that would cost $31.5 million.
   Although USGS maps are good planning tools, it is difficult to build solid engineering solutions from them. Despite their inaccuracies, they have been the only affordable source for extensive topographic data for large-scale projects like a drainage basin... until now.
    Flash back to 1984 when Chatham County was working from vintage 1970, ink-on-mylar base maps. Each new subdivision plan was reduced on a photocopier and traced onto the county base map. Looking ahead, officials in both public and private sectors saw the beginnings of a strong growth curve that demanded better geographic information. They also realized a cooperative effort would benefit everyone.
    So began the Savannah Area Geographic Information System (SAGIS). In existence since 1985, SAGIS is a public/private consortium of the City of Savannah, Chatham County, the Chatham County-Savannah Metropolitan Planning Commission, and Savannah Electric. The group is mandated to develop and implement a regional economic development GIS.
    Using technology that was just beginning to emerge in the mid-1980s, they determined to jointly fund a digital mapping effort to establish a common GIS database. What began as a four member consortium has now added neighboring Effingham County and Bryan County. Together, and in cooperation with real estate, banking, development, engineering, and survey firms, SAGIS is building a comprehensive GIS database for economic development.
    So far, using both newly created and existing data, SAGIS has added planimetric data, tax parcel mapping (including digital photos of practically every structure on every property), zoning, digital orthophotos, census, soils, roadway centerlines, and address systems. Current efforts will add water, sewer, and storm water system coverages.
    An important missing detail has been county-wide contour information. For years, the community has expressed a need for this data but, even though the County rises just 48-feet from one end to the other, the cost to develop 1- or 2-foot contour intervals using traditional methods was just too high. When SAGIS initially asked for proposals to collect topographic data, they 'stopped talking when the price for conventional topographic survey went over $20 million.'
   County engineer, Al Bungard, a strong technology advocate, and SAGIS director, John Brannen, who is actually an employee of Savannah Electric, a Southern Company, were determined to addtopographic data to the GIS.
    In late 1996, Thomas & Hutton Engineering Co. became aware of airborne laser terrain mapping (ALTM) technology. Anticipating wide applications for this technology, Thomas & Hutton, together with the County engineering department, the City of Savannah, and several land developers investigated further.
    Using funds allocated for topographic mapping from the County's 1% sales tax for transportation and drainage improvements, the City and County cooperated on an RFP to create a 1-foot interval contour map of Chatham County. The schedule and cost restraints in the RFP could only be met using laser technology.
    By teaming with Waggoner Engineering of Jackson, Mississippi, the first engineering firm in the country to obtain ALTM technology, Thomas & Hutton was awarded the contract.
    Using the new ALTM data for the Hardin basin in their hydraulic modeling, the estimated cost of construction for the drainage improvements to the Hardin Canal dropped by $7 million.
    When Bungard, Brannen, and others at SAGIS saw what ALTM could do, they immediately exercised their option to develop contour data for the entire 250,000 acre county. Not only had the price plummeted compared to conventional techniques, the entire mapping could be completed in less than a year. (Recent improvements now allow the same project to be completed in 6 months.) Chatham was to become first in the country to develop a county-wide, digital, 1-foot contour map using ALTM.
    Emad Al-Turk, P.E., chief operating officer of Waggoner Engineering, explains that, "ALTM is a marriage of technologies allowing accurate surveys of large tracts or long corridors to be conducted at dramatically reduced cost and remarkable speed."
    The technology combines developments in Global Positioning System (GPS) locators, precision inertial aircraft guidance, laser range-finding, and high-speed, high-density computer processing. A typical survey, which can collect data at a rate of up to 10,000 acres per day, goes like this:
    Before a flight, a ground-based GPS is set up at a known point in the survey area. Flight planning determines optimal ALTM settings and aircraft parameters. From an aircraft flying a regular pattern over the survey area, a focused, infrared laser, eye-safe at survey altitudes, sends up to 5,000 pulses per second to the ground below.
    A high accuracy scanner sweeps the laser pulses across the flight path and collects the reflected light. A laser range-finder, consisting of high precision discriminators and interval meters, measures the time between sending and receiving each laser pulse to determine the ground elevation below. While the laser signals are collected, a digital color video camera records the area being scanned by the laser and encodes the video image with a time tag.
    As the laser and video data are collected, a differential GPS receiver monitors aircraft position and an inertial reference system reports key aircraft data such as roll, pitch, and heading 50 times per second. An on-board computer manages time stamping and recording of laser, video, GPS, and aircraft position data onto high density, 8 millimeter digital tape. At the same time, an operator in the aircraft controls the status of the data collection through an on-board computer.
    Following the flight, the data tapes are transferred to a ground-based computer where a display of the recorded data is immediately available. The ALTM software processes the data in steps to allow frequent verification of data quality. When processing is complete, data can be displayed and printed using CADD software or exported in standard formats. For large survey areas, an ALTM topographic survey can be delivered in days instead of the weeks or even months normally required using conventional techniques.
    While each of the contributing technologies has been available for several years, this combination has only recently been made possible as computer speed and memory have grown to efficiently handle the enormous amounts of data collected.
    In 1996, Waggoner Engineering was the first engineering and survey firm in the US to utilize this advanced technology. Since that time, they have been working with public and private agencies across the country as more and more uses for this remarkable capability are recognized and demand snowballs.
    The most remarkable aspect of ALTM technology is the accuracy that can be achieved as huge areas are quickly mapped at significantly lower cost than conventional survey. According to Al-Turk, horizontal accuracies are better than 0.3 meters. Vertically, the technology can be accurate within less than 15 centimeters.
    With the ability to survey up to 10,000 acres per day and a cost structure which goes down as the survey area increases, ALTM offers huge benefits to municipalities, counties, government agencies, developers, transportation agencies, and other public and private organizations with large tracts or lengthy corridors to be surveyed.
    Other significant benefits include the ability to survey without ever setting foot on the ground. This allows surveying of dangerous or difficult access areas and properties for which right of entry is unavailable. Additionally, survey crews are safe from traffic hazards, and operations or traffic flows do not have to be interrupted.
    In the spring of 1997, when Thomas & Hutton first learned of ALTM, they assembled a group of six property owners to conduct an ALTM survey of a 12,000 acre parcel of their properties. The results of this pilot project convinced the firm and its clients of the high accuracy and broad applications of this technology.
    Having such accurate, thorough, and low-cost topographic data available made it easier to market the individual projects within the area. Where conventional survey costs were running to $150 per acre, airborne laser was providing surveys in considerably less time for a small fraction of the cost.
    Even with these remarkable results, Thomas & Hutton wanted further assurance about ALTM accuracy. Along with Waggoner Engineering, they compared a 40-acre plat which had been surveyed conventionally with the ALTM results. Steve Roach, secretary and treasurer of Thomas & Hutton says, "When we overlaid the two surveys, the accuracy was incredible. Elevations were within two-tenths of a foot and the ALTM had thousands more shots."
    One area of initial concern was the accuracy of the survey in heavily vegetated areas. Traditional aerial survey techniques require that flights be conducted in the winter season when trees have shed their leaves in order to collect topographic data. Because ALTM's laser has a footprint of only 2"-3" on the ground, it can penetrate most foliage any time of the year. While a certain percentage of the laser shots will bounce back from the foliage, the software can discriminate and remove vegetation as it processes the data.
    Chatham County has numerous pine plantations, concerning Thomas & Hutton about ALTM's ability to penetrate the dense foliage. Roach was convinced after a simple walk in the woods illustrated the effectiveness of the system. As he puts it, "If you can look up and see the sky, accurate data can be collected." In some respects, ALTM does not yet replace conventional surveys-pipelines, manholes and culverts for example, must still be obtained by ground survey.
    Roach says, "ALTM is a wonderful technology for obtaining accurate topographic information over a wide area. However, I would not contract for its use without some ground control and ground verification." With Waggoner Engineering's plane in the air, Thomas & Hutton provides accurate horizontal and vertical control county-wide. They also run random transects, at least one 500-foot transect every 1,500 acres, to verify accuracy of the data.
    Nonetheless, digital data has countless uses and SAGIS has many plans for the laser survey information. The ALTM data is already being used by other consultants for hydraulic analyses of additional drainage basins. Eventually, the contour data will be posted on the SAGIS Web-site and become publicly available. Developers looking at the area can download the topographic information to begin their planning and design work. Of course, if more detailed information and actual data from the laser shots is required, it will be available for a fee. Regardless, the ability to access complete survey information for the entire county will be a huge benefit to public and private interests alike.
    Brannen visualizes the day when anyone can log onto the SAGIS Web-site and zoom to a piece of property. There, you will be able to access topographic, soils, zoning, traffic counts, and census information. You will be able to set down a proposed building pad and see the cutting or filling that will be required in various configurations. You could then simulate a 2-inch rain in a 24-hour period and model where the runoff will go. While that day won't come next week, the GIS database to support scenarios like this is just about in place today.
    In a rare example of intergovernmental partnership, Chatham County and neighboring Effingham County are beginning to share the costs of this data collection. They are working towards full, seamless GIS coverage of the two counties. In this low-lying area of the country where storm water doesn't recognize political boundaries, this inter-municipal partnering could have enormous benefits for engineering and budgetary cooperation.
    Brannen also feels these cooperative attitudes send a strong message to people and businesses looking to locate in the Southeast. "When city, county, and private industry get together to help each other on a massive scale like this, we think that says a lot about what it's like to live and work in Chatham County and Savannah."
    Roach states that, "This is already saving a lot of people lots of money. Other engineers are already using the data for design work and drainage studies." In another example, Bungard points out that the Flood Insurance Rate Maps (FIRMs) for the area are out of date. He has already contacted the Federal Emergency Management Agency (FEMA) about taking advantage of the data and the county-wide drainage improvement program to partner on updating and digitizing the FIRMs. According to Roach, the new ALTM data has shown that the actual flood-prone area is smaller than shown on FEMA's conventional mapping. This may open new areas to development without endangering future buildings and property.
    In a low-lying area like Chatham County, there are certain properties that are flooded in every major storm. Rather than continuously building more costly engineering solutions, Brannen is looking forward to, "using ALTM survey data and the GIS mapping to see the 'big picture' and conduct large scale land-use analysis that may suggest solutions for municipalities and property owners that are mutually beneficial."
    Chatham County would be the first to tell you: Airborne Laser Terrain Mapping brings numerous unique advantages over conventional survey methods:
1) As a remote sensing technology with minimum operating constraints, it is safe. High cloud cover, most vegetation, and difficult or dangerous access are no longer a problem. Additionally, ground activities including operations and traffic flow can continue, uninterrupted.
2) It's fast. Hundreds of acres or miles of roadway can be surveyed in minutes instead of days or weeks. Data can be processed in days instead of weeks or months.
3) It's efficient with a cost structure that decreases as the size of the survey increases.
4) With horizontal accuracies less than a foot, and vertical accuracies as fine as 6-inches, it is nearly as accurate as conventional survey.
    With these kinds of advantages, it won't be long before ALTM is the method of choice for all large tract or corridor surveys. Chatham County is proud to be among the first out of the gate.

More info:
SAGIS John Brannen, Director Tel: 912-659-6900 E-mail: [email protected] Chatham County Al Bungard, County Engineer Tel: 912-652-7800 Waggoner Engineering Emad Al-Turk, Chief Operating Officer Tel: 601-355-9526 E-mail: [email protected] Thomas & Hutton Engineering Co. Steve Roach, Secretary/Treasurer Tel: 912-234-5300

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