2022 DC OCTO Lidar DEM: Washington, DC

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Link to custom download, from the Data Access Viewer (DAV), the lidar point data from which these raster Digital Elevation Model (DEM) data were created.

The Data Access Viewer (DAV) allows a user to search for and download elevation, imagery, and land cover data for the coastal U.S. and its territories. The data, hosted by the NOAA Office for Coastal Management, can be customized and requested for free download through a checkout interface. An email provides a link to the customized data, while the original data set is available through a link within the viewer.

This graphic displays the footprint for this lidar data set.

The lidar data acquisition for DC OCTO was flown to support the creation of a 4 ppsm classified lidar point cloud data set, 1m resolution hydro-flattened bare earth DEM, DSM, nDSM, 1 meter resolution intensity images, and 0.6m contours over the full project area covering the District of Columbia. Due to security requirements in the area, Fugro received waivers to fly in the Flight Restricted Zone (FRZ) and P-56 areas. The lidar acquisition was flown in a single lift on January 24, 2022, and a short reflight on January 26, 2022, at an altitude of 7,950 feet above mean sea level and composed of 22 flight lines, 20 primary lines and two cross ties. All lidar data was collected with a Piper Navajo PA31-50, tail# N62912 and a Leica ALS80 lidar sensor, #130. Due to the known difficulties flying over DC, the ALS80 sensor was selected to take advantage of its flight altitude and speed, minimizing the number of lifts for the various flight restrictions. All lidar was collected in conjunction with airborne GPS.

Rice and Associates, under contract to Fugro USA Land, Inc., successfully established ground control for the DC OCTO project area. A total of 31 survey points were visited, 1 ground control point, 25 NVA checkpoints, and 5 VVA checkpoints. Of those 31 surveys, 18 were only visually observed to maintain control integrity and assure that construction had not altered the point. 13 surveys were done for this project to compile with previously surveyed ground control data. The new surveys used GPS to establish the control network. The ground control was delivered in Maryland State Plane (FIPS1900) meters, with the horizontal datum provided in both NAD1983 and NAD83(2011). The vertical datum was the North American Vertical Datum of 1988 (NAVD88) using GEOID12B. The table below shows which points were used as the control, which were used as NVA checkpoints, and which were VVA checkpoints: NVA - NVA-01 NEW SURVEY,NVA-02 NEW SURVEY,NVA-03,NVA-04-2,NVA-05 NEW SURVEY,NVA-06,NVA-07-2,NVA-08,NVA-09,NVA-10 NEW SURVEY,NVA-11,NVA-12-2 NEW SURVEY,NVA-13,NVA-14 NEW SURVEY,NVA-15,NVA-16,NVA-17,NVA-18,NVA-19 NEW SURVEY,NVA-20; VVA - VVA01 NEW SURVEY,VVA02 NEW SURVEY,VVA03 NEW SURVEY,VVA04 NEW SURVEY,VVA05 NEW SURVEY; GCP - GCP-01-2,GCP-02,GCP-03,GCP-04 NEW SURVEY,GCP-05-2,GCP-06-2. During initial processing, QC and accuracy assessments were run the data in NAD83(2011) datum which is the native coordinate system from the sensor. Following boresight the data was re-projected to NAD83 for delivery per the contract specifications and cut to the delivery extent the control was re-run in the final deliverable projection.

Pre-Processing and Boresight All lidar data went through a preliminary field review to ensure that complete coverage was obtained and that there were no gaps between flight lines prior to leaving the project site. Once back in the office, the data went through a complete iteration of processing to ensure that it is complete, uncorrupted and that the entire project area was covered without gaps. There were three steps to processing: 1) GPS/IMU processing - airborne GPS and IMU data was processed using the airport GPS base station data; 2) raw lidar data processing - the raw data was processed to LAS format flight lines with full resolution output before performing QC. A starting configuration file is used in this process, which contains the latest calibration parameters for the sensor and outputs the flight line trajectories. 3) Verification of coverage and data quality - the trajectory files were checked to ensure completeness of acquisition for the flight lines, calibration lines and cross flight lines. Intensity images were generated for the entire lift and thoroughly reviewed for data gaps in project area. A sample TIN surface was generated to ensure no anomalies or turbulence were present in the data; if any adverse quality issues were discovered, the flight line was rejected and re-flown. The achieved post spacing confirmed against the project specification of 4 ppsm and checked for clustering in point distribution. The review showed that the lidar data exceeded the 2 ppsm post spacing. The lidar data was boresighted using the following steps: 1) The raw data was processed to LAS format flight lines using the final GPS/IMU solution. This LAS dataset was used as source data for boresighting. 2) Fugro proprietary and commercial software was used to calculate initial boresight adjustment angles based on sample areas within the lift. These areas cover calibration flight lines collected in the lift, cross tie and production flight lines. These areas are well distributed in the lift coverage and cover multiple terrain types that are necessary for boresight angle calculation. The results were analyzed and any additional adjustments were completed in the selected areas. 3) Once the boresight angle calculation was completed, the adjusted settings were applied to the flight lines of the lift and checked for consistency. The technicians utilized commercial and proprietary software packages to analyze the matching between flight line overlaps for the entire lift and adjusted as necessary. 4) Vertical misalignment of all flight lines was checked and corrected, as was the matching between data and ground truth. 5) A final vertical accuracy check of the boresighted flight lines against the surveyed ground control points was conducted. The boresighted lidar data achieved a vertical accuracy of 0.052m RMSE (0.102m at 95% confidence) against the 20 NVA checkpoint control locations.

Data Redaction Following the boresight completion, the lidar dataset redaction was conducted under the guidance of the United States Secret Service. All lidar data returns and collected data were removed from the dataset based on the redaction footprint shapefile agreed upon in 2022.

Classified Point Cloud The boresighted lidar data underwent an automated classification filter to classify low noise, high noise, and ground points. To obtain optimum results, the parameters used by the automated classification filter are customized for each terrain type and project. Once the automated filtering was completed, the lidar files went through a visual inspection to ensure that an appropriate level of filtering was used. In cases where the filtering was too aggressive and important terrain may have been filtered out, the data is either run through a different filter within localized area or is corrected during the manual filtering process. A second automatic filter is run for the initial classification on buildings. Following the automatic filters, manual editing was completed in Terrascan software to correct any misclassification of the lidar dataset. All tiles then went through a peer review to ensure proper editing and consistency. When the peer review was completed two additional automatic filters were applied. The first filter was used to classify the vegetation - moving the unclassified points to either the low, medium, or high vegetation classes. The second filter was used to re-classify points inside water bodies to water class, and a 1*NPS buffer around water bodies to ignored ground class. Once the manual inspection, QC, and auto filter is complete for the lidar tiles, the LAS point cloud data was converted into the final deliverable format and the accuracy statistics were re-run to confirm the deliverable accuracy. The LAS was then cut to the final delivery layout and in LAS 1.4 format for delivery. The point cloud was delivered with data in the following classifications: Class 1 - Processed but Unclassified; Class 2 - Bare Earth Ground; Class 3 - Low Vegetation; Class 4 - Medium Vegetation; Class 5 - High Vegetation, Class 6 - Buildings; Class 7 - Low Point (Noise); Class 9 - Water; Class 17 - Bridge Decks; Class 18 - High Noise; Class 20 - Ignored Ground.

The hydro-flattened bare earth Digital Terrain Model (DTM) was generated using the lidar bare earth points and 3D hydro breaklines to a resolution of 1 m. The bare earth points that fell within 1 x NPS along the hydro breaklines were excluded from the DEM generation process and put into class 20 - ignored ground. This is analogous to the removal of mass points for the same reason in a traditional photogrammetrically compiled DTM. This process was done in batch using proprietary software. The technicians then used Fugro proprietary software for the production of the lidar-derived hydro-flattened bare earth DEM surface in initial grid format at 1 m GSD. Water bodies (inland ponds and lakes), inland streams and rivers, and island holes were hydro-flattened within the DEM. Once the initial, hydro-flattened bare earth DEM was generated, the tiles were checked to ensure that the grid spacing met specifications. The surface was also checked to ensure proper hydro-flattening. The entire dataset was reviewed for complete project coverage and then converted to Geotiffs.

The NOAA Office for Coastal Management (OCM) downloaded one file in GeoTiff format from the Open Data DC site (https://opendata.dc.gov/datasets/2022-lidar-hydro-enforced-digital-terrain-model/explore). The bare earth raster file was at a 1 meter grid spacing. The data were in Maryland State Plane NAD83, meters coordinates and NAVD88 (Geoid12b) elevations in meters. OCM copied the raster file to https for Digital Coast storage and provisioning purposes.