2011 FEMA Lidar DEM: Cordova, Alaska

Bare Earth LiDAR point cloud data provided for this project is compliant with FEMA Guidelines and Specifications Procedure Memorandum 61.

LiDAR derived products for two areas of interest within the Cordova, Alaska area, consisting of 32 square miles, were developed to support coastal or riverine H&H analysis and floodplain boundary delineation.

The NOAA Office for Coastal Management (OCM) downloaded this data set from this AK DGGS site:

https://elevation.alaska.gov/

These files were processed to make the data available for custom and bulk download from the NOAA Digital Coast Data Access Viewer (DAV). The total number of files downloaded and processed was 1. No metadata record was provided with the data. This record is populated with information from the FEMA Starr technical and QA reports downloaded from the Alaska Dept. of Geological and Geophysical Surveys Elevation Portal.

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

The purpose of these terrain datasets are to represent the topography of a watershed and/or floodplain environment for Cordova, Alaska coastal or riverine hydraulic and hydrologic modeling. All terrain data collected for hydrologic analysis, hydraulic analysis, floodplain boundary delineation, and/or testing of floodplain boundary standard compliance meets the requirements outlined in FEMA Appendix A: Guidance for Aerial Mapping and Survey and FEMA Procedure Memorandum 61: Standards for LiDAR and Other High Quality Digital Topography.

Appendix M of FEMA Guidelines and Specifications for FEMA Flood Hazard Mapping Partners contains a detailed description of the data themes and references to other relevant information.

No warranty expressed or implied is made by FEMA regarding the utility of the data on any other system nor shall the act of distribution constitute any such warranty.

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be aware that temporal changes may have occurred since this data set was collected and some parts of this data may no longer represent actual surface conditions.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.

Fundamental Vertical Accuracy (FVA) checkpoints are located only in open terrain, where there is a high probability that the sensor will have detected the ground surface without influence from surrounding vegetation and/or buildings. Checkpoints are located on flat or uniformly sloping terrain and at least five (5) meters away from a change in slope. Checkpoints are located randomly across the acquisition area. At least 20 FVA points were collected for each test.

Consolidated Vertical Accuracy (CVA) checkpoints are collected randomly across different land use types using the ASPRS NSSDA land cover types. The points are located in flat areas with no substantial elevation breaks within a five meter radius. The CVA assessment incorporates a representative sample of the FVA assessment points into the dataset to save on the total number of points collected. CVA points were not collected for any land class comprising less that 10% of the total project area; this may have resulted in less than 4 land classes being collected in a particular area. At least 15 CVA points were collected and 5 FVA points used, for a total of at least 20 points for the CVA testing. All points were collected at three times the accuracy of the surface being checked. Thus to check a 24.5 cm surface the points were collected accurate to 8 cm.

Tests were conducted when processing by the LiDAR vendor was complete and points were called for. The surveyor provided the point coordinates in an excel spreadsheet to the LiDAR vendor. The LiDAR vendor found the corresponding elevation from a surface created from the LiDAR points, filled in the spreadsheet and returned it to the surveyor who compared the elevation of the LiDAR data with that of the accuracy check point, calculated the difference and reported their findings both in terms of RMSEz and at the 95% confidence level (computed as RMSEz x 1.9600). LiDAR datasets passing the quality control checks were delivered to STARR for quality assurance approval.

Tested 36.0 cm consolidated vertical accuracy at 95th percentile in: open terrain, forest terrain, and urban terrain. The Root Mean Square Error for the elevation differences between GPS control points and LiDAR points is 15.8 cm calculated with 43 supplemental vertical accuracy points (SVA).

The vertical accuracy requirements based on flood risk and terrain slope are met with 10.2 cm and 36.0 cm for both FVA and CVA testing. The mandatory requirements for the highest specification for vertical accuracy, 95% confidence levels are < 24.5 cm for FVA and <36.3 cm for CVA.

Independent review of submitted FVA and CVA survey data with vendor provided LAS files. Survey data points containing field collected GPS elevation values were buffered by 10 meters. LiDAR points contained within the buffered areas are selected and used to create a TIN. The TIN facet z value closest to the x and y control point location is compared to the height of the survey point. The height difference is evaluated statistically and compared to the submitted FVA and CVA testing results to insure the vertical accuracy meets project expectations.

Consolidated Vertical Accuracy (CVA) equal to the 95th Percentile calculated against the bare earth surface in all ground cover classes was 0.36 meters.

Fundamental Vertical Accuracy (FVA) equal to the 95% confidence level (RMSE[z] x 1.9600) calculated against the point cloud (unclassified LAS) was 0.102 meters.

Elevation data has been collected for all areas inside project boundaries. No gaps as defined by FEMA Procedural Memo No. 61 are known to exist within the dataset.

The terrain data have been confirmed as complete with content as specified by FEMA Procedural Memo No. 61.

Data is backed up to tape and to cloud storage.

Data were collected by Aerometric, Inc. for FEMA. OCM downloaded the data from the Alaska DGGS Elevation Portal and processed the data to be available for custom and bulk download from the NOAA Digital Coast Data Access Viewer (DAV).