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2011 U.S. Department of Agriculture- Natural Resources Conservation Service (USDA-NRCS) Topographic Lidar: Northwest Connecticut

browse graphicThis kmz file shows the extent of coverage for the 2011 USDA Northwestern Connecticut lidar data set.
Earth Eye collected LiDAR data for approximately 1,703 square kilometers that partially cover the Connecticut counties of Litchfield and Fairfield. The nominal pulse spacing for this project was no greater than 1 point every 0.7 meters. Dewberry used proprietary procedures to classify the LAS according to project specifications: 1-Unclassified, 2-Ground, 7-Noise, 9-Water, 12-Overlap. Dewberry produced 3D breaklines and combined these with the final LiDAR data to produce seamless hydro flattened DEMs for the 1,742 tiles (1000 m x 1000 m) that cover the project area.

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    Distribution Formats
    • LAZ
    Distributor Distributor information not available
    Point of Contact Barbara Alexander
    USDA/NRCS > Natural Resources Conservation Service, U.S. Department of Agriculture
    (860) 871-4046
    Associated Resources
    • Lidar Final Report
    • U.S. Corp of Engineers (USACE)
    • USDA/NRCS > Natural Resources Conservation Service, U.S. Department of Agriculture
      • publication: 2013-11-01
      Data Presentation Form: Digital image
      Dataset Progress Status Complete
      Data Update Frequency: As needed
      Purpose: The purpose of this LiDAR data was to produce high accuracy 3D elevation products, including tiled LiDAR in LAS 1.2 format, 3D breaklines, and 1 m cell size hydro flattened Digital Elevation Models (DEMs). This data was produced for the U.S. Corp of Engineers and USDA-NRCS Connecticut for use in projects dealing with conservation planning, design, research, floodplain mapping, dam safety assessments, and hydrologic modeling.
      Time Period: 2011-12-13  to  2011-12-19
      Spatial Reference System:
      Spatial Bounding Box Coordinates:
      N: 42.051671
      S: 41.588247
      E: -72.996434
      W: -73.513386
      Spatial Coverage Map:
      • Topography/Bathymetry
      • Elevation
      • Remote Sensing
      • Light Detection and Ranging
      • Lidar
      • LAS
      • Breaklines
      • Bare earth
      • USA
      • Connecticut
      • Litchfield County
      • Fairfield County
      Use Constraints No constraint information available
      Fees Fee information not available.
      Lineage Statement Lineage statement not available.
      • Earth Eye
      • Dewberry - Geospatial Services Group
      Processing Steps
      • Data for the U.S. Corp of Engineers High Resolution LiDAR Data Acquisition & Processing for Portions of Connecticut project was acquired by Earth Eye, LLC. The project area included approximately 657 contiguous square miles for portions of Connecticut including a buffer of 200 meters. LiDAR sensor data were collected with an Optech ALTM3100EA LIDAR System. No imagery was requested or delivered. The data was delivered in the UTM coordinate system, meters, zone 18, horizontal datum NAD83, vertical datum NGVD88, Geoid 09. Deliverables for the project included a raw (unclassified) calibrated LiDAR point cloud, survey control, and a final control report. The calibration process considered all errors inherent with the equipment including errors in GPS, IMU, and sensor specific parameters. Adjustments were made to achieve a flight line to flight line data match (relative calibration) and subsequently adjusted to control for absolute accuracy. Process steps to achieve this are as follows: Rigorous LiDAR calibration: all sources of error such as the sensor's ranging and torsion parameters, atmospheric variables, GPS conditions, and IMU offsets were analyzed and removed to the highest level possible. This method addresses all errors, both vertical and horizontal in nature. Ranging, atmospheric variables, and GPS conditions affect the vertical position of the surface, whereas IMU offsets and torsion parameters affect the data horizontally. The horizontal accuracy is proven through repeatability: when the position of features remains constant no matter what direction the plane was flying and no matter where the feature is positioned within the swath, relative horizontal accuracy is achieved. Absolute horizontal accuracy is achieved through the use of differential GPS with base lines shorter than 25 miles. The base station is set at a temporary monument that is 'tied-in' to the CORS network. The same position is used for every lift, ensuring that any errors in its position will affect all data equally and can therefore be removed equally. Vertical accuracy is achieved through the adjustment to ground control survey points within the finished product. Although the base station has absolute vertical accuracy, adjustments to sensor parameters introduces vertical error that must be normalized in the final (mean) adjustment. The minimum expected horizontal accuracy was tested during the boresight process to meet or exceed the National Standard for Spatial Data Accuracy (NSSDA) for a Horizontal accuracy of 1 meter RMSE or better and a Vertical Accuracy of RMSE(z) 9.25 cm.
      • Dewberry utilizes a variety of software suites for inventory management, classification, and data processing. All LiDAR related processes begin by importing the data into the GeoCue task management software. The swath data is tiled according to project specifications (1,000 m x 1,000 m). The tiled data is then opened in Terrascan where Dewberry uses proprietary ground classification routines to remove any non-ground points and generate an accurate ground surface. The ground routine consists of three main parameters (building size, iteration angle, and iteration distance); by adjusting these parameters and running several iterations of this routine an initial ground surface is developed. The building size parameter sets a roaming window size. Each tile is loaded with neighboring points from adjacent tiles and the routine classifies the data section by section based on this roaming window size. The second most important parameter is the maximum terrain angle, which sets the highest allowed terrain angle within the model. Once the ground routine has been completed a manual quality control routine is done using hillshades, cross-sections, and profiles within the Terrasolid software suite. After this QC step, a peer review and supervisor manual inspection is completed on a percentage of the classified tiles based on the project size and variability of the terrain. After the ground classification corrections were completed, the dataset was processed through a water classification routine that utilizes breaklines compiled by Dewberry to automatically classify hydrographic features. The water classification routine selects ground points within the breakline polygons and automatically classifies them as class 9, water. In addition to classes 1, 2, and 9, there is a Class 7, noise points. This class was only used if needed when points could manually be identified as low/high points. The fully classified dataset is then processed through Dewberry's comprehensive quality control program. The data was classified as follows: Class 1 = Unclassified. This class includes vegetation, buildings, noise etc. Class 2 = Ground Class 7= Noise Class 9 = Water The LAS header information was verified to contain the following: Class (Integer) GPS Week Time (0.0001 seconds) Easting (0.01 foot) Northing (0.01 foot) Elevation (0.01 foot) Echo Number (Integer 1 to 4) Echo (Integer 1 to 4) Intensity (8 bit integer) Flight Line (Integer) Scan Angle (Integer degree)
      • The NOAA Coastal Services Center (CSC) received topographic files in LAS V1.2 format. The files contained lidar elevation measurements, intensity values, scan angle values, return information, flightline information, and adjusted standard GPS time. The data were received in UTM Zone 18N, NAD83 coordinates and were vertically referenced to NAVD88 using the Geoid09 model. The vertical units of the data were meters. CSC performed the following processing for data storage and Digital Coast provisioning purposes: 1. The LAS header's total point count did not match the actual number of points contained in several files. The header counts were repaired using LASTools' Lasinfo. 2. The Global Encoding Bit of each LAS file was set to '1' to reflect the use of Adjusted Standard GPS Time. (NOTE: Tile 18TXM6117 does not contain GPS Time information) 3. The topographic las files were converted from orthometric (NAVD88) heights to ellipsoidal heights using Geoid09. 4. The topographic las files were converted from a Projected Coordinate System (UTM Zone 18N) to a Geographic Coordinate System (NAD83). 5. The topographic las files' horizontal units were converted from meters to decimal degrees. 6. The data were converted to LAZ format.
      • The NOAA National Geophysical Data Center (NGDC) received lidar data files via ftp transfer from the NOAA Coastal Services Center. The data are currently being served via NOAA CSC Digital Coast at The data can be used to re-populate the system. The data are archived in LAS or LAZ format. The LAS format is an industry standard for LiDAR data developed by the American Society of Photogrammetry and Remote Sensing (ASPRS); LAZ is a loseless compressed version of LAS developed by Martin Isenburg ( The data are exclusively in geographic coordinates (either NAD83 or ITRF94). The data are referenced vertically to the ellipsoid (either GRS80 or ITRF94), allowing for the ability to apply the most up to date geoid model when transforming to orthometric heights.

      Metadata Last Modified: 2013-12-31

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