| Fisher River 5m LiDAR Grid | |
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Data format: Raster Dataset File or table name: e024_fr Coordinate system: Universal Transverse Mercator Theme keywords: DEM, elevation, LiDAR, surface elevation, topography, hypsography, DSM |
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Abstract:
The Fisher River LiDAR data is a raster layer in ArcInfo grid format. The raster is a 5m grid representing elevation. |
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Metadata elements shown with blue text are defined in the Federal Geographic Data Committee's (FGDC) Content Standard for Digital Geospatial Metadata (CSDGM). Elements shown with green text are defined in the ESRI Profile of the CSDGM. Elements shown with a green asterisk (*) will be automatically updated by ArcCatalog. ArcCatalog adds hints indicating which FGDC elements are mandatory; these are shown with gray text.
The Fisher River LiDAR data is a raster layer in ArcInfo grid format. The raster is a 5m grid representing elevation.
To provide a very accurate DEM of the Fisher River study area.
Data Collection and Methodology Report LiDAR Survey: Fisher River Study Area Manitoba Water Stewardship May 2006 July, 2006 EXECUTIVE SUMMARY LiDAR Services International (LSI), a Calgary based LiDAR mapping company, provided airborne LiDAR data collection services for Manitoba Water Stewardship in May 2006. The project involved the collection of LiDAR data for the purpose of topographic and vegetation profiling for approximately 940 square kilometers in the Interlakes / Fisher River area of Manitoba. LiDAR data was successfully collected and processed to meet the following specifications: o Average laser point spacing equal to 0.90 m, with a flying height of 600 m above ground level (AGL) o Processed LiDAR data horizontally and vertically accurate to RMSE 20cm o LiDAR data X, Y - Horizontal Datum: NAD83, UTM Zone 14 o LiDAR data Z - Vertical Datum: CGVD28 using the Canadian Gravimetric 2000 geoid model o Full feature point clouds (ground, vegetation, building, non-ground, blunder points) delivered in LAS binary format with extraction program o ASCII text files for "Bald Earth" Digital Terrain Model and "Highest Surface" Digital Surface Model The following report summarizes the LiDAR data collection and processing phase of the project. LSI appreciates the opportunity to have provided LiDAR surveying services to Manitoba Water Stewardship and looks forward to providing similar services in the future. TABLE OF CONTENTS EXECUTIVE SUMMARY 1. INTRODUCTION 2. GROUND CONTROL 3. GPS MISSION PLANNING 4. SYSTEM CALIBRATION 4.1 HELIX Installation 4.2 IMU-GPS Antenna Offset Survey 4.3 Calibration Buildings 5. DATA COLLECTION 6. DATA POST PROCESSING 6.1 Airborne GPS 6.2 LiDAR Point Clouds 7. CONCLUSION 1. INTRODUCTION On April 26th, 2006, LSI personnel and field survey equipment mobilized from Calgary, Alberta to Gimli, Manitoba. Can-West's Cessna 185 airplane arrived on the 27th with LSI's Helix LiDAR system already installed and ready for data collection. The LiDAR data collection survey was done over two large block areas in the Interlakes, Manitoba region for the purpose of creating an accurate terrain model. The entire project was staged out of the Gimli airport because it is the closest airport to the project area with necessary services including fuel and aircraft storage. LiDAR data was collected for the Fisher River project from April 28th until May 5th, 2006. After each mission the airborne GPS and LiDAR data was processed to assure quality and full data coverage with no gaps. The final project deliverables were completed back at LSI's office in Calgary after the entire project area was collected. The following sections of this report describe the field data collection and data processing portions of the Fisher River 2006 LiDAR project. 2. GROUND CONTROL To obtain an accurate airborne GPS solution during each LiDAR data collection mission, LSI utilizes differential GPS (DGPS) surveying techniques. DGPS involves having a static GPS receiver collecting data at a known ground benchmark in the vicinity of the project area simultaneously with the collection of the kinematic GPS on the aircraft. After each mission, during the post processing, the two sets of raw GPS data are combined together resulting in an accurate positioning solution of the aircraft. Prior to mobilization to Manitoba, Lindsay Donnelly of GeoManitoba, Manitoba Conservation and Water Stewardship provided LSI with a map and list of existing ground benchmarks in the Fisher River region. LSI selected many of the existing points and further densified the network with the addition of several newly established points. All of the benchmark locations were chosen based on location with respect to project area, point separation, accessibility and GPS satellite visibility. The GPS network created by LSI and utilized during the project along with the project area is shown below in Figure 1. Additionally, the final horizontal NAD83 UTM Zone 14 locations and CGVD elevations are shown below in Table 1. Figure 1: GPS Control Network Table 1: GPS ground control points used for LiDAR surveys Point ID Description Easting (m) Northing(m) Height (m) 232504 Existing Nail 638190.378 5610952.673 198.138 774032 Gov't Point 638645.916 5596788.419 202.209 82R382 Gov't Point 569246.182 5624317.475 232.229 89M161 Gov't Point 607419.166 5674837.639 209.430 FR06_01 New 6" nail 582005.918 5639881.938 252.971 GIMAIR1 New 6" nail 638420.528 5611028.542 198.010 GIMAIR2 New 6" nail 638453.478 5610970.014 198.072 Figures 2 and 3 show LSI's NovAtel DL-4 GPS receivers set up over the 232504 and FR06_01 GPS stations, respectively. The new point FR06_01 that LSI established was used for the LiDAR survey and the GIMAIR1 and GIMAIR2 points were used for the building calibration survey which is explained below in section 4.3. The new control points were monumented with 6" nails for a temporary period of time, only long enough to support the LiDAR project. Figure 2: NovAtel DL-4 GPS receiver at the Gimli airport (232504) Figure 3: NovAtel DL-4 GPS receiver at FR06_01 3. GPS MISSION PLANNING Before mobilizing to Gimli, a preliminary investigation of the GPS forecast was performed. Using GPS satellite almanacs released weekly by the United States Coast Guard (USCG) Navigation Center, LSI was able to determine the number of satellites that would be visible during LiDAR data collection and the quality of the GPS solution. To ensure accurate GPS positioning, missions were only flown during times with 5 or more satellites visible and a Position Dilution of Precision (PDOP) below 4. The PDOP value is an indicator of the geometry, and hence quality, of the GPS solution at a given location on the Earth. The quality of the GPS-derived position diminishes with increasing PDOP value. The predicted number of visible GPS satellites and PDOP for the Fisher River project area on May 1st, 2006 from 7:00 am to 7:00 pm local time (GMT -5 hours) is shown below in Figure 4. An elevation cutoff angle of 15° was used to eliminate any low satellites from the forecast that could degrade the GPS solution. Figure 4: PDOP and number of visible satellites for Fisher River area on May 1, 2006 4. SYSTEM CALIBRATION 4.1 HELIX Installation The Helix LiDAR system was installed in a Cessna 185 aircraft (registration number C-GAYZ) owned and operated by Can-West Corporate Air Charter. The STC installation approved by the Canadian government was completed in several hours in Calgary prior to departing to Manitoba. Four different parts of the installed HELIX system are shown below in Figure 5 including the laser mount, the GPS antenna mount, the operator navigation screen and the computer chassis. Figure 5: Cessna 185 HELIX installation 4.2 IMU-GPS Antenna Offset Survey Several parameters unique to each aircraft LiDAR installation must be determined in order to produce accurate LiDAR point clouds. On the C-GAYZ aircraft, the phase center of the GPS antenna in relation to the IMU inertial system body reference was precisely measured using conventional survey techniques and least square intersection computations. On April 24, 2006 a total station and prisms were set up at several different points in a controlled hanger environment to measure redundant horizontal and vertical angles and distances from the IMU unit to the GPS antenna. Using the measurements, least squares intersection calculations were completed to determine the accurate three dimensional distances between the IMU and GPS. A portion of the GPS-IMU offset survey is shown in Figure 6 below. Figure 6: Cessna 185 GPS-IMU offset survey 4.3 Calibration Buildings LSI also conducts "building calibration" LiDAR flight missions to allow for the determination of the roll, pitch and heading misalignment angles between the IMU measurement axis and the laser sensor. As the airborne LiDAR survey produces a high density of 3-D points, buildings with relatively flat-planed roofs are selected to provide a basis on which to calibrate the HELIX system. The Gimli Rural District bus maintenance building, located at the Gimli Airport, was chosen as the calibration building for this project, as seen below in the two images in Figure 7. Figure 7: Calibration building at Gimli Airport The six corners of the building's roof were surveyed using a total station, prisms and traditional network survey techniques. First, five ground points surrounding the building were established and tied to control point 232504 using static GPS baseline observations. Observations of the horizontal and vertical angles to the roof corners were then made from these surrounding ground points using the total station and prisms. The UTM coordinates of the roof corners, referenced to 232504, were then estimated by a least squares adjustment of the angle observations. The layout of the ground survey of the calibration building is seen in Figure 8 with a line between two points indicating a vertical/horizontal angle measurement. Points GIMAIR1 through GIMAIR5 represent the ground points while points A through F represent the building roof corners. Figure 9: Ground survey of calibration building Before commencement of the airborne surveys a LiDAR calibration mission was flown over the calibration building. The calibration flight consisted of a series of eight flight lines flown at orthogonal and parallel headings at a flying height of 300 m and 500 m above ground level (AGL) and a speed of 150 kph. Post analysis of the calibration flight data allowed for successful determination of the installation orientation for the HELIX LiDAR system. By fixing the surveyed rooftop coordinates, the LiDAR offset and orientation parameters could be adjusted such that the resulting point cloud data accurately matched the calibration surface. At the beginning and end of each survey data collection mission, parallel passes were made over the calibration building again to allow for verification of system alignment in the data post-processing phase. Local features such as roads, lakes and buildings were also used to check for consistent alignment of the LiDAR data for each flight mission. 5. DATA COLLECTION LiDAR data collection for the project began on April 28, 2006 (Julian Day 118) and finished on May 5, 2006(JD 125) with several down days due to poor weather conditions including rain, snow, and fog. Prior to arriving at the project site, LSI designed 100 flight lines to cover the entire area as delivered in the pre-mission plan in April, 2006. During the project data was collected for all 100 flight lines plus additional flight lines to cover data void areas and calibration passes. Below, in Figure 10 is a spatial image representing the trajectories of the lines flown to collect LiDAR data for the Fisher River project. Figure 10: Flight Line Trajectories All LiDAR data was collected at a flying height of 600 m above ground level (AGL) and approximate forward speeds of 170 kph. The Riegl Q560 scanning laser used in the HELIX system pulsed at a rate of 70 kHz with an effective frequency of 46.2 kHz scanning out to 30 degrees on both sides of nadir. The scanning mirror was controlled to rotate at a frequency that produced 56 scan lines per second, resulting in a dense grid pattern on the ground with an average spacing of 0.85 m between points. Data was collected with multiple returns enabled producing up to 8 returns per pulse, but in most cases there was 4 returns or less. In areas with single pass coverage the laser point return density averaged over 1 point per square meter and in areas with overlap coverage the laser point return density doubled to over 2 points per square meter. Another important characteristic of the Q560 laser that was used for the project is the laser beam divergence of 0.5 mrad. The strong laser can achieve this divergence causing the laser footprint to be only 30 cm wide when it hits the ground from a flying height of 600m AGL. The raw kinematic GPS data, or Airborne GPS (ABGPS) data, was collected for each flight mission at a 1 HZ rate from the GPS antenna located on the wing of the aircraft near the fuselage. Concurrently, NovAtel DL-4 GPS receivers occupying ground control points collected data at a 1 Hz rate, allowing the position of the aircraft to be accurately solved through a post-processed differential GPS solution. A basic flight mission of a maximum of five hours would comprise of collecting GPS, INS and laser data over the building calibration site, ferrying to the project area, collecting the appropriate flight lines and ferrying back to the base for refueling. 6.0 DATA POST PROCESSING 6.1 Airborne GPS All DGPS processing was performed using Waypoint's GrafNav 7.60 software, a thorough GPS processing program capable of producing consistently accurate kinematic DGPS solutions. The most accurate kinematic DGPS results are obtained with a fixed, rather than float, solution. To help fix the integer ambiguities, LSI logged GPS static data from the aircraft and ground station for at least twenty minutes at the start and end of each mission. During all LiDAR missions the aircraft was never separated by more than 35 km from a GPS base station, making it possible to keep an accurate fixed solution throughout each flight. An important element of the GrafNav software is the capability of solving both a forward and reverse solution, which is why there is a static logging session at both the start and end of each mission. The program weighs each solution appropriately and combines them to obtain the most accurate kinematic results. An excellent indicator of the quality of the processed kinematic data is the forward-reverse separation of the solution in the horizontal and vertical directions. Results that produced under +/-10 cm combined separation for this project were deemed as accurate kinematic DGPS solutions. If for any reason during post-processing the solution changed to float or the combined separation went above 10 cm, different variables were examined and manipulated to obtain more accurate results. Several of the examined GPS variables include: standard deviations for the C/A and phase code, elevation masks, kinematic ambiguity resolution parameters, L2 noise models and problematic GPS satellites. In the end, high quality fixed solutions were achieved for all post-processed DGPS trajectories. 6.2 LiDAR Point Clouds A first level of processing was performed in the field for quality control purposes, to verify data collection and to ensure the absence of data gaps. Final data processing was completed at LSI's office in Calgary after the completion of data collection. The determined aircraft GPS position and statistics were combined with the 100 Hz inertial measurement data obtained from the NovAtel SPAN System fixed to the HELIX plate to obtain accurate position and attitude values (roll, pitch and heading) of the sensor base plate during flight. Using proprietary LSI software the position and orientation of the IMU were combined with the plate offset values to determine final geo-referenced positions and orientations for the laser sensor. The laser point clouds were initially generated in the UTM mapping system with ellipsoidal heights for cleaning and classification purposes. Horizontal and vertical alignment checks were done on all of the flight lines utilizing identifiable features within the survey area. These features included river banks, streams, ponds, roads, flat open fields, sharp peaks and buildings. Positional consistency of these features in consecutive laser passes could be compared because the flight lines were designed with side overlap and the adjoining corridors also had areas of overlap. All adjacent laser passes were inspected and found to have proper alignment consistency with respect to each other. After completion of the QC process, the 3D point clouds were subjected to a classification routine where the lowest points are deemed to be ground points and a "bare earth" DEM is established based on terrain characteristics. With the point clouds divided into tiles of manageable sizes, experienced technicians visually inspected each tile and corrected any ground points missed or erroneously classified by the automatic classification routine. Upon completion of the ground point classification, the remaining points were further classified into vegetation, buildings, blunders and non vegetation or non ground points using both automatic software algorithms and manual classification. The final step was to apply the Canadian Geodetic Vertical Datum 1928 (CGVD28) geoid model to the data set allowing for the conversion of ellipsoid heights into orthometric or mean sea-level (MSL) heights. All of the data was broken into 1 km by 1 km tiles for manageable data manipulation and delivery sizes. The names of the tiles represent the UTM position in meters of the southwest corner of the tile; the first three digits are the first thee numbers of the Easting and the last four digits are the first four numbers of the Northing, ex. Tile 5735643 is located at E: 573000, N: 5643000. 7.0 CONCLUSION The LiDAR data for the Fisher River project was delivered in two types of formats; LAS binary and ASCII text. The LAS format contains ten data fields as specified by Manitoba Water Stewardship in the Statement of Work section of the project tender. To reduce file size the LAS format data is stored in binary format and can be used with a supplied extraction program to convert into ASCII format. The two types of ASCII file formats delivered are a "Bald Earth" Digital Terrain Model (DTM) and a "Highest Surface" Digital Surface Model (DSM). The difference between the types of files is the DTM files contain only ground points and the DSM files contain all of the classified points excluding blunder points. In addition to the LiDAR data, trajectory files and MetaData was also delivered as specified by Manitoba Water Stewardship. All of the delivered geo-spatial data is in NAD83 horizontal datum with the UTM Zone 14 mapping projection and orthometric heights with the CGVD28 geoid model. LSI appreciates the opportunity to be involved with Manitoba water Stewardship for the Fisher River project and is available for questions and comments regarding the LiDAR survey.
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5m grid in ascii (x, y, z) format received from Terrapoint Inc. are unzipped and the file extension renamed to .txt. An AML called ld_fmttxtshore.aml was run against the text files to format the files properly. Another AML called ld_lidar_grid.aml is used to create the 5m grid files. The grid files are merged together using MERGE.
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Perimeter of feature in internal units.
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1007 Century Street
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1007 Century Street