2022 AK DGGS Lidar DEM: Twentymile River Watershed, AK

Lidar-derived elevation data for the Twentymile River watershed, Southcentral Alaska, collected August-October 2022, Raw Data File 2023-3, uses aerial lidar to produce a classified point cloud, digital surface model (DSM), digital terrain model (DTM), and intensity model of the Twentymile River watershed, Southcentral Alaska, during snow-free ground conditions from August to October 2022. The survey provides snow-free surface elevations for trail planning and assessing avalanche hazards, among other objectives. Ground control data were collected on August 31, 2022, and aerial lidar data were collected on multiple days from August 29 to October 14, 2022, and subsequently processed in a suite of geospatial processing software. These products are released as a Raw Data File with an open end-user license. All files can be downloaded from the Alaska Division of Geological & Geophysical Surveys website (http://doi.org/10.14509/30959).

The orthoimage and elevation data were collected for assessing coastal hazards and changes.

>classified_points: Classified point cloud data are provided in LAS format. Data are classified in accordance with ASPRS 2019 guidelines and contain return and intensity information. The average pulse spacing for non-noise returns is 26.9 cm and the average pulse density is 13.8 pts/m2. For ground points, the average point spacing is 59.6 cm and the average density is 2.82 pts/m2. Within the survey area and the scientific priority area, the average pulse spacing and density is 20.7 cm and 23.24 pts/m2, respectively, and the average ground point spacing and density is 64.4 cm and 2.41 pts/m2.

>dsm: The DSM represents surface elevations, including heights of vegetation, buildings, powerlines, etc. The DSM is a single-band, 32-bit GeoTIFF file of 50-centimeter resolution, delivered in 1 km by 1 km tiles. No Data value is set to -9999.

>dtm: The DTM represent bare earth elevations, excluding vegetation, bridges, buildings, etc. the DTM is single-band, 32-bit float GeoTIFF file, of 50-centimeter ('DTM'). No Data value is set to -9999.

>dtm_detail: The detailed DTM represents bare earth elevations, excluding vegetation. The DTM is single-band, 32-bit float GeoTIFF file, of 20-cm resolution. This higher-resolution DTM file covers an area identified as a scientific priority. No Data value is set to -9999.

>lidar_intensity: The lidar intensity image describes the relative amplitude of reflected signals contributing to the point cloud. Lidar intensity is primarily a function of scanned object reflectance in relation to the signal frequency, is dependent on ambient conditions, and is not necessarily consistent between separate scans. The intensity image is a single-band, 16-bit unsigned GeoTIFF file of 0.5-meter resolution. No Data value is set to -9999.

>lidar_las_index: The LAS index file is a shapefile with polygons that serve as an index to the spatial location and extent of each point cloud (LAS) bin.<

The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

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.

This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys

Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

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.

We did not measure horizontal accuracy for this collection.

We measured a mean offset of -0.0048 m between 24 control points and the point cloud. Because of trivial offset, we did not perform any vertical transformation of the lidar point data. The non-vegetated vertical accuracy (NVA) of the point cloud ground class was found using a Triangulated Irregular Network approach. We calculated the project NVA to have a root mean square error (RMSE) of 6.7 cm. We evaluated the relative accuracy for this dataset as the interswath overlap consistency and measured it at 8.4 cm RMSE.

This data relaease is complete.

We visually inspected the imagery for data errors such as shifts, and seamline mismatches. Data quality is consistent throughout the survey, save for a gap in coverage in a deep valley in the northwest part of the survey area, and over some glaciated areas where there were few returns due to ground cover conditions. Ground point density varies throughout the survey and is illustrated in the acconpanying report. We provide higher resolution DTM files for the area of scientific interest.

Data is backed up to cloud storage.

Data were collected by the Alaska Division of Geological and Geophysical Surveys (AK DGGS). The data were downloaded by the NOAA Office for Coastal Management (OCM) and processed to make it available for custom download from the NOAA Digital Coast Data Access Viewer (DAV) and for bulk download from S3 AWS.