ELEVATION. The Base Map

Transcription

1 ELEVATION The Base Map

2 Basic Elevation Types: Points X, Y, Z Irregular distribution Cumbersome Typical sources Lidar Stereo photography Ground survey ASCII, LAS formats

3 Basic Elevation Types: Contour Lines of Constant Height Intuitive Loss of detail Can be pricey Shapefile, DXF, etc. VS

4 Basic Elevation Types: TIN Triangulated Irregular Network Surface/Mesh from points Nonstandard formats Breaklines

5 Basic Elevation Types: Raster Regular grid Many ways to create Common digital elevation model (DEM) Common formats

6 Spatial Accuracy Accuracy is defined as the closeness of an estimated value to a standard or accepted (true) value of a particular quantity (National Digital Elevation Program). Horizontal and vertical accuracy is the positional accuracy in relation to a specified horizontal and vertical datum. Vertical accuracy is the primary factor for specifying the quality of elevation data. Accuracy requirements depend on the intended application; high vertical accuracy is normally required for floodplain management applications.

7 Spatial Accuracy Accuracy Standards and Guidelines National Map Accuracy Standards (NMAS) National Standard for Spatial Data Accuracy (NSSDA): ublications/ International Hydrographic Association Standards for Hydrographic Surveys: National Digital Elevation Program Guidelines for Digital Elevation Data:

8 Spatial Resolution Spatial resolution, or horizontal resolution, is expressed as the size of the pixels (or cell size). The coarser the resolution (i.e., the larger the pixel size), the more ground area represented by a single pixel. The elevation value for each pixel is an averaged value; this value will be affected by the accuracy of the input data. The accuracy of the resulting maps will be no better than that of the input data. USGS 7.5 ` Digital Elevation Model (DEM) LIDAR

9 Elevation Breaklines A breakline is a feature that is used to represent an abrupt change in the smoothness of the surface Breaklines can represent features such as streams, rivers, dams, roads, buildings, etc., which can influence coastal inundation modeling and mapping results. There is a greater need for breaklines for elevation data having a lower resolution, where these types of features cannot be easily identified. TIN created from mass points TIN of the same area created from mass points and breaklines

10 Accuracy Standards FEMA Accuracy Standards Digital topographic data accuracy requirements are based on NSSDA standards, using NMAS terminology regarding map scale and contour interval Digital Elevation Requirements Two-foot contour interval for flat terrain: 95% of the elevations in a data set will have an error equal to or smaller than 1.2 feet. Four-foot contour interval for rolling to hilly terrain: 95% of the elevations in a data set will have an error equal to or smaller than 2.4 feet.

11 Technologies to Collect Elevation Data Ground survey (cm) Stereo aerial photography (10 cm) Lidar (10 cm) IfSAR/InSAR (1 m)

12 Elevation Data Resources U.S. Geological Survey (USGS) National Elevation Dataset Lidar sites: Digital Coast ( CLICK (lidar.cr.usgs.gov) Louisiana Lidar (atlas.lsu.edu) North Carolina Lidar ( Puget Sound Lidar Consortium (pugetsoundlidar.ess.washington.edu) Others?

13 USGS National Elevation Dataset (NED) Seamless.usgs.gov 30m, 10m resolution Some 3m data ~ 2.5m RMSE z nationally Incorporating lidar National coverage

14 USGS NED Source Tool ned.usgs.gov/usgs_gn_ned_dsi/

15 USGS CLICK Data Availability lidar.cr.usgs.gov

16 Digital Coast Data Availability

17 Topographic and Bathymetric Data Inventory

18 National Digital Elevation Program Project Tracker Project types - Planned - In work - Recently completed Federal State (NSGIC) Harvested by Geospatial One Stop (GOS)

19 When It Comes to Flood Mapping or Sea Level Rise Projection, Elevation Data Is the Key National Research Council report Mapping the zone: Improving Flood Map Accuracy Topographic/bathymetric data is the most important factor in determining the accuracy of flood maps Climate Change Science Program SAP 4.1, Coastal Elevations and Sensitivity to Sea-Level Rise Sea level rise mapping requires high-resolution elevation data (Current national datasets {30m DEMs} are not adequate)

20 Importance of Elevation Data Profile in Charleston, South Carolina 30-meter NED data (1 arc second) 10-meter NED data (1/3 arc second) Lidar data NED National Elevation Dataset

21 In Charleston, South Carolina Vulnerable Areas Are Lower Than We Thought Up to 3 meters (10 feet) LOWER!! The average difference between the 10m NED and the lidar is more than 2.5m (8ft).

22 DEM Accuracy and Inundation Mapping 3m lidar DEM (VA=~20cm) 10m National Elevation Dataset DEM (VA=~1m) 0.5m Sea Level Rise 0.5m Sea Level Rise + 1 Root Mean Square Error (RMSE)

23 Build Best Terrain Available NED Digital Coast - JABLTCX Raw Points/Breaklines

24 Obtain Data and Create DEM

25 Enforce Hydro- Connectivity

26 Enforce Hydro- Connectivity

27 Enhance for Better Visualizations

28 5-meter Resolution Resolution Requirements 10-meter Resolution

29 Communicate Mapping Confidence Visualize the mapping confidence of inundation area based on uncertainty of elevation data and MHHW tidal surface.