2018 Town of Nantucket Lidar: Nantucket Island, MA

Product: These lidar data are processed Classified LAS 1.4 files, formatted to 1,577 individual 1250 ft x 1250 ft tiles; used to create intensity images, 3D breaklines and hydro-flattened DEMs as necessary.

Geographic Extent: Nantucket County Massachusetts, covering approximately 91 square miles.

Dataset Description: The Nantucket project called for the Planning, Acquisition, processing and derivative products of lidar data to be collected at a nominal pulse spacing (NPS) of 0.7 meter. Project specifications are based on the U.S. Geological Survey National Geospatial Program Base Lidar Specification, Version 1.2. The data was developed based on a horizontal projection/datum of NAD83 (2011), State Plane, Feet and vertical datum of NAVD88 (GEOID12B), Feet. Lidar data was delivered as processed Classified LAS 1.4 files, formatted to 1,577 individual 1250 ft x 1250 ft tiles, as tiled Intensity Rasters, and as tiled bare-earth DEM; all tiled to the same 1250 ft x 1250 ft schema.

Ground Conditions: Lidar was collected in early to mid 2018, 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, Sanborn Map Company, Inc. established a total of 20 ground control points that were used to calibrate and validate the lidar to known ground locations established throughout the project area.

In addition to these lidar point data, the bare earth Digital Elevation Models (DEM) created from the lidar point data are also available. These data are available for custom download at the link provided in the URL section of this metadata record.

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, and hydro-flattened DEMs as necessary.

CONTRACTOR: Sanborn Map Company, Inc SUBCONTRACTOR: Geomni Lidar data were acquired Geomni. All follow-on processing was completed by the prime contractor.

The following are the USGS lidar fields in JSON:

{

"ldrinfo" : {

"ldrspec" : "USGS-NGP Base Lidar Specification v1.2",

"ldrsens" : "Leica CityMapper",

"ldrmaxnr" : "15",

"ldrnps" : "0.48",

"ldrdens" : "4.4",

"ldranps" : "0.43",

"ldradens" : "5.5",

"ldrfltht" : "2250",

"ldrfltsp" : "145",

"ldrscana" : "40",

"ldrscanr" : "61.6",

"ldrpulsr" : "306",

"ldrpulsd" : "4",

"ldrpulsw" : "0.53",

"ldrwavel" : "1064",

"ldrmpia" : "1",

"ldrbmdiv" : "0.25",

"ldrswatw" : "581",

"ldrswato" : "25",

"ldrgeoid" : "National Geodetic Survey (NGS) Geoid12B"

},

"ldraccur" : {

"ldrchacc" : "0.5",

"rawnva" : "0.227",

"rawnvan" : "20",

"clsnva" : "0.216",

"clsnvan" : "20"

},

"lasinfo" : {

"lasver" : "1.4",

"lasprf" : "6",

"laswheld" : "Withheld (ignore) points were identified in these files using the standard LAS Withheld bit.",

"lasolap" : "Swath "overage" points were identified in these files using the standard LAS overlap bit.",

"lasintr" : "16",

"lasclass" : {

"clascode" : "1",

"clasitem" : "Processed, but Unclassified"

},

"lasclass" : {

"clascode" : "2",

"clasitem" : "Bare Earth Ground"

},

"lasclass" : {

"clascode" : "7",

"clasitem" : "Low Noise"

},

"lasclass" : {

"clascode" : "9",

"clasitem" : "Water"

},

"lasclass" : {

"clascode" : "10",

"clasitem" : "Ignored Ground"

},

"lasclass" : {

"clascode" : "17",

"clasitem" : "Bridge Decks"

},

"lasclass" : {

"clascode" : "18",

"clasitem" : "High Noise"

}

}}

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 and 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.

The required accuracy (ACCz) is: 0.643 ft at a 95% confidence level, derived according to NSSDA, i.e., based on RMSE of 0.328 ft in the bare earth and urban land cover classes. The Vertical Accuracy was tested with 20 control points located in open terrain and urban (non-vegetated) areas. The control points were distributed throughout the project area and were surveyed using GPS techniques. See survey report for additional survey methodologies. Elevations from the unclassified lidar surface were measured for the x,y location of each control point. Elevations interpolated from the lidar surface were then compared to the elevation values of the surveyed control points.

AccuracyZ has been tested to meet 0.643 ft or better Vertical Accuracy at 95% confidence level using RMSE(z) x 1.9600 as defined by the National Standards for Spatial Data Accuracy (NSSDA); assessed and reported using National Digital Elevation Program (NDEP)/ASRPS Guidelines. Tested 0.216 feet at a 95% confidence level (or 3.5 cm RMSE) using RMSE(z) x 1.9600 as defined by the National Standards for Spatial Data Accuracy (NSSDA). The Vertical Accuracy of the classified lidar point cloud swath files was calculated against TINs derived from the final calibrated and controlled swath data using 20 control points located in Open Terrain and Urban land cover classes.

These LAS data files include all data points collected. No points have been removed or excluded. A visual qualitative assessment was performed to ensure data completeness. No void areas or missing data exist. The raw point cloud is of good quality and data passes Vertical Accuracy specifications.

Data covers the entire area specified for this project.

Data is backed up to tape and to cloud storage.

This data was provided by Mass GIS to the NOAA OCM for custom distribution by the NOAA Digital Coast Data Access Viewer (DAV) and bulk distribution via S3 AWS service.