Mapping America's Oldest City
As the oldest continuously occupied settlement in the continental United States, St. Augustine, Fla., has had to confront land planning and facilities management issues for a long time. How long? Information from the National Park Service indicates that subsequent to Ponce de Leon's 1513 claim to Florida, a Spanish fort was established there in 1565.
The mapping problem for the city of St. Augustine Department of Public Works actually had several components. The city wanted to determine the accurate location of facilities and cultural features for engineering and site studies, as well as develop a stormwater management plan. In order to tackle this problem, the city joined with the Jacksonville Regional Office of the Corps of Engineers to obtain this information. The Corps was interested in supporting its hydrology and hydraulics modeling efforts for local beaches and the Intracoastal Waterway. These activities required development of an accurate image base and a comprehensive contour base for 21 square miles of the City Service Area.
Traditionally, aerial photography and either existing or created digital elevation models (DEMs) are acquired for both producing current digital orthophotography and generating 2' topographic contours. This information is supplemented with stereo-compiled features, which can be used as breaklines to increase the accuracy of contour interpolation. The lack of DEM data with resolution suitable for generation of 2' contours threatened to push the project over budget, as stereo-compilation can be time-consuming and expensive.
The Aeroscan Laser Mapping SystemHere is where the new technology comes into play. The Corps' Jacksonville Office was aware of DEM production being undertaken by one of its contractors, 3001 Inc. (Gainesville, Fla.). This production involved the use of an airborne scanning laser, the Aeroscan Laser Mapping System (ALMS). The system uses pulses of light at specified wavelengths to measure variations in surface features. Operating in the range finding mode, the elapsed time between generation and return of a laser pulse is measured. The distance to the feature causing the return signal is then determined through consideration of the aircraft altitude, the elapsed time and the speed of light, which is adjusted based on local atmospheric conditions. Return signals are digitized and stored on magnetic media. When combined with post-processed GPS and inertial navigation data, a digital three-dimensional representation of the land surface can be generated. 3001's LIDAR (Light Detection and Ranging) unit focuses on topographic mapping and DEM production from much higher altitudes than existing commercial devices have currently achieved. This device is capable of developing contours in a fraction of the time and cost of traditional stereo-compilation techniques.
Flying the MissionsTwo missions were flown to complete the project, one to capture aerial photography and one to capture LIDAR data. The aircraft is equipped with dual ports capable of capturing photography and LIDAR data simultaneously; however, atmospheric conditions for LIDAR capture were better the following night. Operating in the near infrared portion of the electromagnetic spectrum, the ALMS is subject to attenuation by heavy cloud cover and high levels of atmospheric moisture.
A workflow similar to one used in aerial photography was designed and used for LIDAR data capture. A base map of the study area was created. Eight flight lines were laid out based on an altitude of 8,000' above ground level, a 50-degree swath width and a pulse rate of 15 kHz. A single swath covered 1.5 Km on the ground, with 385 points per acre. The entire 21-square-mile public works service area was flown in one hour, generating 670 megabytes of raw scanner, GPS and inertial measurement unit (IMU) data. The length of the flight lines varied between 12 Km and 45 Km.
Post-processing of the raw LIDAR, GPS and IMU data occurs at the conclusion of each flight. Coefficients for translation, rotation and scaling are calculated, along with several device-specific parameters to generate an ASCII x, y, z file of points with UTM coordinates and ellipsoid heights. The ellipsoid height values are then converted to orthometric heights using local geoid parameters. The operator generates a grayscale image of the data to perform a preliminary quality assurance check on the unedited data.
The ASCII file is now ready for feature extraction and editing. 3001 uses proprietary software to identify, delineate and filter vegetation and cultural features. This is an iterative process, which incorporates existing map and digital data, as well as ground control and kinematic information. The complexity of the ground surface dictates the amount of time and effort required to edit the data. Editing time for the St. Augustine data averaged approximately six hours per square mile.
Deliverables included seamless digital 2' contour maps for the service area and digital elevation models. All data products are quality assured and statistically tested to ensure that they meet the National Standard for Spatial Data Accuracy. A metadata quality report is produced that documents all collection and processing parameters as well as the accuracy testing procedures and results.
Sizing Up the New TechnologyLIDAR technology has proven to be a cost-effective and efficient means to acquire high-resolution elevation data over large areas in a short period of time. Problems and issues to be addressed in the widespread use of LIDAR technology include technical considerations in aircraft capacity, flight operation, post-processing of raw data and continual enhancement of editing software capability. Typical aircraft considerations such as payload capacity, power supply and number of viewing ports are dealt with by using a company plane that has been modified to accommodate the ALMS. Flight operation issues are very similar to those involved in designing an aerial photography mission, with some deviation. Study-area size and orientation, airspeed, flying height, pulse rate, scan rate, scan angle and scan overlap are the primary considerations. Post-processing of raw data requires that specific information about the aircraft altitude and orientation be collected and applied. In addition, both the airborne and ground-based GPS data needs to be processed and quality assured. 3001 is undergoing a continual process of fine-tuning and enhancing our various editing and display modules. The primary objective is to increase automation and decrease operator interaction while preserving acceptable levels of accuracy.
Although still an emerging technology, LIDAR has proven its reliability and applicability for generation of high-resolution DEMs and contours over a wide range of surface types. As with any service, it is important to query several contractors as to their LIDAR project experience, capabilities and references. Be especially careful of those making claims of resolution and accuracy far above the norm.
It's only fitting that St. Augustine, the United States point of beginning, should use the latest technology to develop accurate spatial data for the preservation of its past and the security of its future.