Field Integration of Lidar and Sonar Hydrographic Surveys

Transcription

1 Field Integration of Lidar and Sonar Hydrographic Surveys Nicholas Gianoutsos, LTJG Samuel Greenaway, LT Benjamin K. Evans, CDR Guy T. Noll National Oceanic & Atmospheric Administration, National Ocean Service, NOAA Ship RAINIER Abstract An increasing trend in NOAA hydrographic survey operations is the use of lidar in difficult near-shore areas for both bathymetric coverage and feature detection. Although this new approach offers many advantages, establishing a junction between lidar surveys and traditional sonar hydrography poses many new challenges. Since 1969, the NOAA Ship RAINIER has conducted survey operations in Alaskan waters for the production of nautical charts. Over the past several seasons RAINIER has worked in project areas that adjoin contracted lidar surveys. Over this period, the methods of integrating lidar data into the survey process have continuously evolved. In recent years, a variety of authors have analyzed the validity of lidar in hydrographic surveys. This paper, however, explores the operational challenges of combining traditional hydrographic survey operations with lidar surveys and highlights the specific methods developed that draw on the complementary strengths of both techniques. Case studies in near-shore regions of Alaska, including the outer Baranof Islands, Mitrofania Island and the Shumagin Islands will be used to explore issues of establishing a junction between the two bathymetric data sets as well as issues of cartographic feature management. We compare both the scope and density of lidar soundings and those of the traditional sonar systems, including single beam and high-resolution multibeam, and discuss the resultant impact on feature detection, coverage requirements, and completeness of a survey. This paper outlines operational methods of establishing a robust junction between lidar and traditional sonar survey hydrography and will be of benefit to those who are considering integrating lidar into their survey process. Background The National Oceanic and Atmospheric Administration (NOAA) Ship RAINIER has spent much of her career surveying in Alaskan waters since being commissioned in Outfitted with six survey launches and a ship s multibeam system, RAINIER and her launches provide shallow water multibeam coverage, single beam systems for near shore work and the use of side scan sonar for object detection in shallow areas. With the current backlog of priority areas as specified by the NOAA Hydrographic Survey Priorities and NOAA s ongoing commitment to safety, lidar has emerged within NOAA as a tool to maximize hydrographic production while avoiding potential hazards 1

2 in the field. A growing trend in NOAA hydrographic operations is the use of lidar data in combination with near shore hydrographic surveys. A principal challenge facing NOAA and the hydrographic community is the integration of lidar surveys with traditional hydrographic surveys. This combination has provided NOAA with additional tools and methods to increase quality, efficiency and functionality despite the finding of many new challenges in the field. Light Detection and Ranging (LIDAR) systems were first introduced in the mid 1960 s for the purpose of topographic mapping. Early on, bathymetric capability became evident when double returns were observed while flying over lakes. Work continued in the 1970 s and prototypes were established by the 1980 s. The leading systems were developed in the US, Canada, Sweden and Australia. In a lidar system, two laser signals are sent from the plane to the earth consisting of an infra-red beam and a green beam. The infra-red light bounces off the water s surface to determine the height of the airplane above the water and the green light penetrates the surface recording the height of the airplane relative to the seafloor. Subtracting the distance of the red beam from the green beam determines the water depth after correctors have been applied. Environmental factors such as wind, cloud cover, topography and water turbidity influence data acquisition on a daily basis. Lidar systems can operate by day or night and operations may be improved at night by removal of the daylight filter. Real-time positioning of acquired data is obtained by a GPS receiver mounted on the airplane providing autonomous GPS. Lidar s ability to detect bathymetry has been well documented (Guenther, 2007) while feature management has to some extent been a challenge. During the 2005 and 2006 field seasons, RAINIER successfully joined traditional hydrographic surveys with contracted lidar surveys. These surveys were conducted by the Tenix Corporation s LADS (Laser Airborne Depth Sounder) Mark II Airborne System in Mitrofania and the Shumagin Islands on the Alaska Peninsula in addition to the outer Baranof Islands in Southeast Alaska. The remainder of this paper outlines the evolution of the process taken to combine lidar surveys with traditional hydrographic surveys during RAINIER s recent field seasons. Theory of Adjoining Lidar Surveys An increasing trend in NOAA hydrographic survey operations is the use of lidar in nearshore areas for both bathymetric coverage and feature detection. Over many years, RAINIER has developed survey procedures that integrate different sonar methods and techniques with a number of outside sources including prior surveys, NOAA charts, Synthetic Aperture Radar (SAR) and digitized aerial photography in addition to satellite photography. Historically, RAINIER developed a processing pipeline where all of these outside sources were field verified and included with the bathymetric data acquired to produce a final product ready for submission. Throughout the past several seasons, RAINIER has worked in project areas that have been previously surveyed by contracted lidar. The fundamental shift in our approach has been to move away from treating lidar surveys as just another source to be referenced and verified with our ship based 2

3 acquisition process, to recognizing the lidar survey as a complete (or nearly complete) stand alone survey to which we establish a junction. The determination of where this junction occurs, and what is complete (or complete enough) is the key challenge of integrating lidar data within the ship based acquisition process. Advantages of Lidar The integration of lidar data with traditional sonar surveys offers four main advantages: shallow water efficiency, safety, reconnaissance, and improved shoreline mapping. Shallow Water Efficiency Shallow, near shore areas are often the most difficult to survey with multibeam sonar systems. Other studies have found that RAINIER spends approximately 50% of its operational effort working around the complex shoreline in shallow waters obtaining only 10% of its total hydrographic production output (Longenecker, 2002). The coverage achieved by a multibeam system is limited to a fixed angle on either side of the launch. This requires survey lines to be spaced closer together in shallower water to achieve 100% coverage. Additionally, as the near shore area often requires complex maneuvers of the survey vessel, a smaller portion of the time is actually spent on line, acquiring data. In addition to increased lines in shallow water, time spent surveying near shore is at slower speeds due to possible hazards in the area. Survey vessels are also prone to losing their GPS position close to shore due to cliff walls and overhanging trees blocking a significant section of the sky. As a result of these four factors, a large amount of acquisition time for each survey is spent working in relatively small, near-shore areas Linear Nautical Miles of Multi-Beam Required for 100% Coverage of 1 Square Nautical Mile lnm/nm Water Depth (m) Figure 1 Multibeam survey effort is weighted toward shallow water. Theoretical linear nautical miles required for 100% coverage in this graph, assumes 120 swath. This plot does not account for time loss due to increased maneuvering and decreased speed in near-shore areas. With present technology, lidar is generally limited to water shallower than 15 to 20 meters in Alaskan waters; although sporadic soundings may be found in depths up to 35 meters if the survey was conducted under optimal conditions. In a typical Alaskan region 3

4 with steep shoreline, the area of lidar coverage is often limited to a narrow strip along shore. However, as discussed above, this shallow, near shore area is particularly time consuming and potentially dangerous to survey by traditional launch-based methods. Figure 2 Track lines from East Bight and Larsen Bay in the vicinity of Nagai Island overlaid on chart The blue lines represent lines in depths less than 15 meters that could have been surveyed with lidar. Figure 3 A detailed view of Larsen Bay showing the increase in the amount of track lines needed to survey an area less than 15 meters of water. As an illustration of the time required performing near shore data acquisition, we have analyzed the survey effort for a project that did not have lidar coverage, though lidar was flown for adjoining areas. This Alaskan project is on the east side of Nagai Island in the Shumagin group. In nearby areas, reliable lidar coverage was obtained to approximately 15 meter depths. The inshore limit of vessel hydrography for this project was 4 meters. We totaled the linear nautical miles of vessel survey lines that were run in water depths shallower than 15 meters. This near shore section accounted for 7% of the total survey area, but 27% of the total linear nautical miles for the sheet. The total survey effort expended in this near-shore section was certainly higher due to extensive line planning, increased maneuvering during acquisition, reduced vessel speed and additional data processing. Safety of Operations The near shore area is often the most hazardous for survey operations. In this near shore area dangerous submerged objects, hazardous surge conditions, and breaking waves pose a serious danger to survey vessels. In recent history, survey operations in treacherous 4

5 inshore areas have damaged transducers and even destroyed NOAA survey vessels. The material costs of these events do not compare with the loss of a life. Even a narrow strip of lidar coverage greatly reduces the risks of fatality and property damage in near shore regions by allowing survey vessels to work farther offshore from potential hazards. Reconnaissance Somewhat obviously, hydrographic survey operations are often carried out in poorly charted waters. Provided lidar is flown sufficiently in advance of vessel operations, the data serves as a valuable reconnaissance tool. This reconnaissance aspect of lidar aids both safe operations and acquisition planning. Even if lidar were only able to reliably identify areas shallower than six meters, all areas of grounding danger for RAINIER and her launches could be identified. For reasons similar to the acquisition effort, most of the planning effort is concentrated in near shore areas. Any information, even spotty and incomplete soundings, in the near shore area greatly improves the accuracy and effectiveness of pre-acquisition planning by showing the general bathymetry of the environment. Even in areas where the bottom is not detected, lidar data can be advantageous. Similar to historical wire drag surveys, a lidar surveyed area with no detected soundings can provide known clearance to varying depths. Shoreline Mapping and Topography Lidar offers a unique advantage over other surveying methods since lidar is capable of mapping and charting areas that hydrographic vessels can not explore. This includes surveying shallow or hazardous water depths, charting difficult shoreline and mapping geographic topography, all of which, provide seamless coverage from moderate heights to depths. Lidar surveys can be used as a stand alone solution for shoreline acquisition or may be used in conjunction with other shoreline sources such as aerial or satellite photography. Challenges of Lidar The two main operational limitations of bathymetric lidar are depth and resolution. Though depth is typically limited to meters, we have already discussed that these near shore areas require a disproportionate survey effort compared with deeper survey areas. The horizontal resolution limitations of lidar are more troublesome. Due to the limited spot spacing of lidar surveys, horizontal resolution is significantly lower than that of multibeam sonar survey operations. For example, a lidar system performing 4x4 meter spot spacing acquires 1 sounding in a 16 square meter area. In comparison, a Reson 8125 sonar operating in 15m of water, could obtain approximately 350 soundings in the same 16 square meter area. This difference of two orders of magnitude has significant impacts on what lidar can detect and how well lidar data can capture seafloor detail. Lidar spot spacing can be increased for higher resolution, but this substantially decreases lidar survey efficiency. Object detection has long been the Achilles heel of modern day lidar systems. Despite good performance in bathymetric acquisition, other authors have documented that lidar systems have difficulty detecting some features on the ocean floor (Smith, 2006). 5

6 Conversely, sonar systems are significantly more capable of detecting features due to the vastly increased sounding density. A major obstacle during lidar acquisition is the presence of kelp patches which absorb returns and give sparse soundings. Rock soundings in kelp patches are difficult to determine and additional boat work is often required to investigate these soundings. It is important to note that at 4x4 meter laser spot spacing; there is a gap of 1 to 1.5 meters between the illuminated areas of adjacent soundings at the sea surface. This allows for the possibility that small objects in shallow water may fall between consecutive 4x4 meter soundings and not be detected. The problem becomes further compounded when particularly narrow objects are located in areas where water clarity is poor and kelp is present Several examples of this occurrence were found during vessel surveys in the vicinity of Nagai Island in the Shumagin Islands. During the survey, a cursory shoreline inspection discovered separate instances of kelp covered rock pinnacles measuring approximately one meter across. Due to the narrowness of these features and the large amount of kelp in the area, the objects were not detected by lidar at 4x4 meter spot spacing and were not discernable in the accompanying photo mosaic. A higher resolution lidar survey of the area, either using 3x3 or 2x2 meter spot spacing, might have detected these pinnacles even with the large patches of kelp in the area. Figure 4 A pinnacle rock in Eagle Harbor that was not detected by lidar 4x4 meter spot spacing. Figure 5 The blue star displays the location of the new rock pinnacle found in an area with spotty lidar coverage. Integration of Lidar Data In the previous sections we have discussed the theoretical aspects of combining lidar surveys with traditional hydrographic surveys. In this section we will outline the process of physically performing survey operations in the field in conjunction with lidar surveyed areas. 6

7 Lidar Survey Specifics In the survey areas of Mitrofania and the Shumagin Islands on the Alaska Peninsula as well as the outer Baranof Islands in Southeast Alaska, Tenix LADS Mark II Airborne System performed lidar surveys with 4x4 meter laser spot spacing at 200% coverage. Contingent upon water clarity, the lidar s laser was able to detect the bottom up to 10 to 30 meter depths. The LADS Mk II survey system also recorded topographic information up to 20 meters during surveys. Establishing a Junction between Lidar and Hydrographic Surveys The greatest challenge of working with lidar surveys is the establishment of a sound junction between hydrographic and lidar data. NOAA standards describe a sound junction as an overlap of at least one sounding line with an adjoining lidar survey. If the depths in these common overlap areas do not agree, the launch-based hydrographic survey is to be extended further into the lidar survey area until agreement is achieved. The NOAA inshore limit of launch hydrography is reached when this agreement is established or when the survey vessel has reached the inshore limit of safety for a given region. Assuming the lidar survey has coverage extending well outside of navigationally dangerous areas, the hydrographer must decide where the lidar survey has sufficiently dense soundings to establish a sound junction. Naturally, the definition of a sound junction becomes relevant upon the situation as there are a few factors to take into consideration including region, environmental conditions and quality of data. During this combination of hydrographic surveys with lidar data, there are two key types of junctions to consider: bathymetric junctions and feature investigations. Bathymetric Junction On a gently sloping flat bottom, junction comparison is simple. In areas where rocky coastline is mixed with sporadic lidar soundings, junction comparison becomes increasingly difficult. Unlike soundings in sloping flat bottoms which portray the general trend of the bathymetry, sparse soundings acquired over rocky outcroppings do not provide high enough resolution to adequately capture the sea floor. This poses a theoretical problem for establishing a junction. The farther inshore the hydrographer must survey to establish a junction with lidar data, the less the lidar data is utilized since lidar surveys lose value when their coverage areas are re-surveyed by sonar. Evaluating the bathymetric junction with lidar data in generally sloping areas normally was not difficult. When attempting to establish a sound junction, the hydrographer first considered whether the lidar data was good or spotty. Sonar coverage was nearly always good due to the increased density of sonar soundings in an area. If the lidar coverage was also good, it normally agreed very well with the sonar data. When combining sonar data with sporadic lidar data, it is important to note whether the general slope of the lidar soundings matches the general slope of the sonar bathymetry in addition to consistent overlapping soundings between surveys. In areas where the general slopes of the surveys were in agreement and the overlapping soundings were 7

8 consistent, RAINIER personnel considered the junction to be sufficient. In areas where the general slope of the surveys did not match or soundings were inconsistent, hydrographic coverage was further pushed in to attempt a better junction. Figure 6 A sound junction adjoining sonar soundings (yellow) and lidar soundings (purple) displayed with a vertical exaggeration of 6. Figure 7 An example where lidar soundings (purple) are consistent with the sonar soundings (yellow) and the general slope of the bank continues into the lidar surveyed area. The image is displayed with a vertical exaggeration of 6. 8

9 Feature Investigation The integration of lidar into the survey process would be much simpler if all that was required was the establishment of a bathymetric junction. Or simpler yet, if there was a well defined boundary where lidar ended and sonar coverage needed to begin. However, because of the difficulties with lidar object detection, lidar surveys can not be considered complete within the limits of their bathymetric coverage. There can be many features in the lidar survey area that require further investigation. Many of these features have been recognized by the lidar operator and flagged for investigation. There can also be a few items that the lidar survey has missed completely. The critical issue then becomes the amount of time the hydrographer should spend completing the lidar survey by boat. Confirming all features inside the lidar area would require a full resurvey by traditional means, yet any time spent by the hydrographer inside the bathymetric coverage of the lidar survey detracts from the realization of lidar s promised efficiency and safety dividends. Included with the bathymetric lidar data, Tenix LADS provided NOAA with a number of distinct cartographic features: features which they had high confidence and required no further investigation; features for which they had partial or suspect information and recommended further investigation ( Lidar Investigations ); and charted features that were disproved with a high degree of confidence ( Lidar Disproval s ). Lidar Investigations consisted of suspected features in poorly resolved areas of lidar coverage. For example, some isolated rocks in kelp were detected that were difficult to correctly classify as either rock or kelp and required further examination in the field. Lidar Disproval s consisted of charted features that were not detected in the lidar survey and recommended for removal from the chart. Due to the known object detection limitations of lidar, NOAA guidelines do not permit the removal of features from the chart based on lidar coverage alone and require the list of disproval s to be verified by boat before eliminating the objects from the chart. Once aboard RAINIER, these lists of features were evaluated by the Commanding and Field Operations Officers to determine their navigational significance. Features deemed navigationally insignificant or dangerous to approach were not investigated. A shoreline boat equipped with a single beam echosounder first investigated navigationally significant items from the feature investigation reports. The hydrographer then decided if the features were appropriate for multibeam coverage. If the features were determined to be too shoal for multibeam coverage or deemed unsafe, then single beam coverage over the object or a detached position was considered sufficient. In the event that the feature marked for investigation was declared unsafe for examination, the item was noted as such in the feature investigation report. 9

10 Figure 8 Feature Investigation items (red stars) and lidar disproval items (blue stars) in the vicinity of Nagai Island were flagged during the lidar survey for further investigation by boat work. The bathymetry displayed in the image was acquired by survey launches using single beam and multibeam sonar systems. The green line represents the recommended area for junction with the lidar survey. When sonar soundings are acquired over a feature investigation item, the first step is to make sure that significant coverage has been acquired over the object and the least depth has been determined. The next step is to verify that the sonar soundings generally agree with the lidar soundings in the area. As long as coverage was obtained over the feature, and the lidar soundings and sonar soundings were not inconsistent, the feature junction was considered sufficient. In each of the three survey areas, completeness of survey and feature management were handled uniquely different depending on the survey area and are discussed in more detail in the following case studies. 10

11 Figure 9 Single and multibeam sonar soundings (yellow) over a lidar feature investigation item generally agree with lidar soundings (purple) in the area. Case Studies: Evolution of the Process The process of combining traditional hydrographic data with lidar survey data has evolved during the last few field seasons in each of the three distinctively different areas RAINIER surveyed. Some parts of the process were the same in all three areas, while others were very different. In the following case studies, we outline the evolution of RAINIER s approach to combining traditional bathymetric sonar surveys with lidar surveys as well as the handling of lidar feature investigations and completeness of the lidar survey. Case Study 1: Mitrofania Island, Alaska Mitrofania is a small, crescent shaped island about 5 miles by 6.5 miles wide located approximately 250 nautical miles west of Kodiak, Alaska. The island features steep jagged peaks and rock cliff shoreline. In figure 10, chart contains the only previously charted survey data for this area. The waters surrounding Mitrofania Island had never been properly surveyed. 11

12 Figure 10 Chart contains the only previously charted survey data for Mitrofania Island. Figure 11 Bathymetry acquired during RAINIER s 2005 Mitrofania Island survey. With few soundings surrounding the island, lidar data proved extremely useful for reconnaissance in the Mitrofania area. Lidar data did detect shoaling around Brother Island, located North of Mitrofania Island which posed a significant danger to navigation for the ship as well as survey launches. The lidar smooth sheets allowed for efficient line planning and data acquisition as well as determining anchorage locations. Reconnaissance was the leading benefit of using lidar surveys in Mitrofania. Being new to the use of near complete lidar surveys in the field, Mitrofania was the start of the evolution process of combining lidar with bathymetric data. Throughout most of the survey area, lidar was unable to obtain extensive bathymetric coverage because of the steep shoreline and deep water depths around the island. A boat crew verified all features within the navigable area during a traditional low water shoreline investigation. Lidar features were treated as another source, compared with the data from aerial photogrammetry, and verified with single beam sonar, GPS positioning, or visual confirmation. In addition, lidar investigation items were investigated as completely as possible, including dive investigations. The net result was an increase amount of effort to survey the near shore area. This was largely due to a late delivery of the lidar product to the field unit, a poor understanding of what the lidar data meant, and a lack of an established way to integrate the lidar data into the survey process. 12

13 Case Study 2: The Outer Baranof Islands, Sitka, Alaska During the 2006 field season, RAINIER surveyed the area around Biorka Island, located southeast of Sitka, Alaska in the outer Baranof Islands. This area was extremely hazardous to navigate with rocky shoreline and open exposure to the Gulf of Alaska. Figure 12 Exposure to the Gulf of Alaska and rocky shoreline made lidar a valuable tool for safety in the outer Baranof Islands. Lidar saved a significant amount of time by reducing data acquisition in near-shore areas as well as minimizing the need for shoreline verification. Safety was by far the biggest advantage of using lidar in the outer Baranof Islands due to the exposed shoreline and surge conditions. Lidar proved very useful in the approaches to Sitka in terms of safety and efficiency although reconnaissance did not play a major role. After gaining knowledge and confidence about the integration of lidar data from the Mitrofania Island survey the previous year, it was decided that RAINIER would utilize the lidar data as almost a stand alone survey. Junctions with the data were established by one overlapping swath width where conditions allowed. Where the required overlap could not be safely met, a gap was left in coverage. The majority of the area was very rocky and most of the survey junctions were made in areas with many features. Shoreline verification in the Baranof Islands was kept to a minimum due to the dangerous shoreline conditions. RAINIER investigated only a limited number of features flagged in the lidar feature reports that were deemed navigationally significant. 13

14 Case Study 3: Nagai Island, Shumagin Islands, Alaska Located in the Shumagin Islands, Nagai Island is situated approximately 300 nautical miles west of Kodiak, Alaska. The area was last surveyed with lead line in 1915 by the Steamer PATTERSON. RAINIER surveyed the area during the summer months of the 2006 field season. Figure 13 Nagai Island was previously surveyed by lead line nearly 100 years ago. Lidar data assisted with the planning of survey operations which reduced the amount of time spent surveying in shallow, near shore areas. This ultimately led to an increase in the number of square nautical miles surveyed. RAINIER finished work in these survey areas three weeks in advance and was assigned two more surveys on the southeast side of the island in Northeast Bight, East Bight and Larsen Bay. Overall, the greatest benefit of using lidar around Nagai Island was efficiency during shallow water data acquisition. During survey planning, RAINIER utilized smooth sheets, sounding plots, bathymetric data and feature reports from the lidar survey. Also included in the lidar survey was photo mosaic imagery comprised from downward looking video. Photo imagery is primarily used as a tool by the lidar contractor to identify potential features. These features are then included in lidar smooth sheet and investigation feature reports. Photo mosaic images are not intended for survey planning but may serve a useful purpose for boat work in the future. 14

15 Figure 14 Base surface created from the lidar survey of Eagle Harbor. Figure 15 Photo mosaic image from a survey of Nagai Island. In the area around Nagai Island, the lidar survey was treated as a near complete, stand alone survey with the exception of lidar investigation items. The majority of these investigation items were examined in the field and a cursory shoreline inspection revealed a few rocks that were not detected by the lidar survey. These rocks were found in areas of spotty lidar coverage surrounded by heavy kelp. After examining lidar feature investigations and performing a cursory shoreline comparison, the lidar survey was considered to be complete. 15

16 Conclusion The use of lidar in conjunction with traditional hydrographic surveys can be a powerful tool. While weighing the benefits of using a lidar survey, the hydrographer needs to remember the challenges presented. Although a lidar survey aids in shallow water efficiency, safety, reconnaissance and shoreline mapping, it is important to remember the challenges of lidar s low density soundings and limited object detection. Lidar data is most beneficial in shallow, near shore areas and traditional hydrographic surveys should be planned to take advantage of lidar s strengths. Prior knowledge of the survey area is critical to effectively develop a process for feature management and establishing junctions between surveys. Following these steps will maximize the effectiveness of integrating lidar data into the survey process. Recommendations Bathymetric lidar has been proven to provide adequate depth information for near-coastal areas. Feature management continues to be a challenge, especially in areas of erratic bathymetry such as the survey projects discussed in this paper. The additional information present in digital orthophotos and hyperspectral imagery may prove to be a critical component of a robust near-shore feature management processing solution. In the interim period before these tools are ready for production work, NOAA Ship RAINIER experience suggests that the most efficient approach to surveying areas that contain a large number of bathymetric features, especially of small areal extent, is to reduce the effort of lidar coverage in these regions and to use alternate technologies, such as tidecoordinated non-bathymetric lidar or sonar, to complete the survey. The reduction in acquisition and processing of bathymetric lidar in these regions will enable the surveyors using this technology to concentrate on the areas in which the tools are most effective. Mention of a commercial company or product does not constitute an endorsement by NOAA s National Ocean Service, Office of Coast Survey. Use of information from this publication concerning proprietary products or the tests of such products for publicity or advertising purposes is prohibited. 16

17 References Guenther, Gary C., 2007, Airborne Lidar Bathymetry, In Digital Elevation Model Technologies and Applications: The DEM Users Manual, 2 nd Edition, edited by David F. Maune. American Society for Photogrammetry and Remote Sensing Longenecker, J.K. and Van Den Ameele, E.J., 2002, Maximizing NOAA s Ship Productivity Through the Use of Airborne Laser Hydrography, TS4.4 Hydrographic Surveying II, FIG XXII International Congress NOAA, Office of Coast Survey, NOAA Hydrographic Survey Priorities, 2006, Smith, S.M., 2006, Empirical Object Detection Performance of LIDAR and Multibeam Sonar Systems in Long Island Sound, International Hydrographic Review Vol. 7 No. 2, July