2023 CT GIS Office Hydro-Flattened Bare Earth DEM: Connecticut Statewide

Product: These are Digital Elevation Model (DEM) data for CT Statewide GIS Services as part of the required deliverables for the CT Statewide GIS Services Lidar project. Class 2 (ground) lidar points in conjunction with the hydro breaklines were used to create a 2 foot hydro-flattened Raster DEM.

Geographic Extent: CT Statewide covering approximately 5,241 square miles. Dataset Description: CT Statewide GIS Services Lidar project called for the Planning, Acquisition, processing and derivative products of lidar data to be collected at a nominal pulse spacing (NPS) of 0.35 meter; coastal areas were acquired to achieve >=20 ppsm, and all other areas were acquired to achieve >=14 ppsm. Project specifications are based on the U.S. Geological Survey National Geospatial Program Base Lidar Specification, Version 2024 rev. A. The data was developed based on a horizontal projection/datum of NAD83 (2011), State Plane Connecticut, U.S. Survey Feet and vertical datum of NAVD88 (GEOID18), U.S. Survey Feet. Lidar data was delivered as processed Classified LAS 1.4 files, formatted to 23,381 individual 2,500 ft x 2,500 ft tiles, and as tiled bare earth DEMs; tiled to the same 2,500 ft x 2,500 ft schema. Maximium Surface Height Raster and Swath Separation Image tiles 025645_se, 080655_ne, 085655_ne, 085655_nw, 090655_nw, 110660_se, and 135665_se do not exist in the deliverables due to all of the points in the corresponding LAS tiles being flagged as withheld. LAS file 110660_se is all water and contains no lidar returns, this file does not exist in the deliverables.

Ground Conditions: Lidar was collected in Spring 2023, while no snow was on the ground and rivers were at or below normal levels. In order to post process the lidar data to meet task order specifications and meet ASPRS vertical accuracy guidelines, Dewberry established a total of 38 ground control points that were used to calibrate the lidar to known ground locations established throughout the CT Statewide GIS Services project area. An additional 247 independent accuracy checkpoints, 145 in Bare Earth and Urban landcovers (145 NVA points), 102 in Tall Grass and Brushland/Low Trees categories (102 VVA points), were used to assess the vertical accuracy of the data. These checkpoints were not used to calibrate or post process the data.

To acquire detailed surface elevation data for use in conservation planning, design, research, floodplain mapping, dam safety assessments and elevation modeling, etc. Classified LAS files are used to show the manually reviewed bare earth surface. This allows the user to create Intensity Images, Breaklines and Raster DEM. The purpose of these lidar data was to produce high accuracy 3D hydro-flattened Digital Elevation Model (DEM) with a 2 foot cell size. These raw lidar point cloud data were used to create classified lidar LAS files, intensity images, 3D breaklines, hydro-flattened DEMs as necessary.

Raster File Type = TIFF

Bit Depth/Pixel Type = 32-bit float

Raster Cell Size = 2 Foot

Interpolation or Resampling Technique = Triangulated Irregular Network

Required Vertical Accuracy = 19.6 cm NVA

Any conclusions drawn from the analysis of this information are not the responsibility of NOAA, the Office for Coastal Management or its partners

Data is available online for bulk or custom downloads

None

Users should be aware that temporal changes may have occurred since this data set was collected and some parts of this data may no longer represent actual surface conditions. Users should not use this data for critical applications without a full awareness of its limitations.

Only checkpoints photo-identifiable in the intensity imagery can be used to test the horizontal accuracy of the lidar. Photo-identifiable checkpoints in intensity imagery typically include checkpoints located at the ends of paint stripes on concrete or asphalt surfaces or checkpoints located at 90 degree corners of different reflectivity, e.g. a sidewalk corner adjoining a grass surface. The xy coordinates of checkpoints, as defined in the intensity imagery, are compared to surveyed xy coordinates for each photo-identifiable checkpoint. These differences are used to compute the tested horizontal accuracy of the lidar. As not all projects contain photo-identifiable checkpoints, the horizontal accuracy of the lidar cannot always be tested.

Lidar vendors calibrate their lidar systems during installation of the system and then again for every project acquired. Typical calibrations include cross flights that capture features from multiple directions that allow adjustments to be performed so that the captured features are consistent between all swaths and cross flights from all directions. This data set was produced to meet ASPRS Positional Accuracy Standards for Digital Geospatial Data (2014) for a 41 cm RMSEx/RMSEy Horizontal Accuracy Class which equates to Positional Horizontal Accuracy = +/- 1 meter at a 95% confidence level

This data set was produced to meet ASPRS Positional Accuracy Standard for Digital Geospatial Data (2014) for a 10-cm RMSEz Vertical Accuracy Class.

Datasets contain complete coverage of tiles. No points have been removed or excluded. A visual qualitative assessment was performed to ensure data completeness. There are no void areas or missing data. The raw point cloud is of good quality and data passes Non-Vegetated Vertical Accuracy specifications.

Data covers the entire area specified for this project.