Why bring NDFD grids into a GIS? | Using NDFD grids for pre-hurricane landfall decision making | Download the data | Convert the data to an ESRI shapefile | Import an ESRI shapefile into ArcGIS | Load Legends into ArcMap to view GRIB2-derived shapefiles | Convert a message to ESRI Spatial analyst or GrADS file format | Open an FLT file in ArcGIS
One of the most attractive components of the National Digital Forecast Database is the geospatial nature of the data. The database is based on a 5km national grid that covers the Continental U.S. and separate grids that cover Hawaii, Alaska, the U.S. Virgin Islands, and Puerto Rico. Each 5km grid cell has forecast information so that a point specific forecast can be generated for any cell based on geographic coordinate location. Besides being able to just view the grids using algorithms to create graphics of the gridded information, more advanced users may want to perform analysis with gridded data to answer some real-world problems. Bringing the NDFD gridded files into a GIS enables the user to not only display the forecast elements with other georectified datasets such as satellite imagery, aerial photographs, and planimetric layers such as political boundaries, road networks, buildings, etc , but also to query the data and use it for geospatial analysis. The result is essentially a real-time and forecast decision support tool for making critical decisions based on various weather elements. This is the case for marine interests as well, as the NDFD grid extend offshore several miles and deliver wave and wind information. Using the NDFD data in a GIS gives the flexibility to deal with either Raster versions of the NDFD grids (floating point grids), or vector versions (ESRI shapefiles) as well as other formats that are just recently becoming importable into GIS like NetCDF files and GRADS. The best way to illustrate the power of using NDFD grids in a GIS can best be presented via a use case scenario given below.
During the 2004 hurricane season NDFD forecast elements were used to display experimental forecast elements such as wind speed, wave heights, and quantitative precipitation forecasts along with hurricane forecast tracks in a map format that was used by DHS/FEMA for risk analysis at the FEMA Region IV Regional Operation Center and Disaster Field Office. For hurricanes Charley, Frances, Ivan, and Jeanne maps were created showing NDFD forecast elements overlaid on relevant political boundaries, roads, shoreline, and major population centers. The elements gave DHD/FEMA risk analysts an idea of the spatial coverage and intensity of forecast winds and wave heights. These maps were used to create the Presidential Briefing Package in which DHS/FEMA briefs the executive branch on potential damages that will be incurred from the approaching storm so as to pre-secure federal resources for response and aid through a presidential disaster declaration. This is an example of how having the NDFD grids converted to GIS significantly benefited the decision making process for weather driven natural disasters such as hurricanes. See Figure 1 and 2 below.
1. Instructions for downloading the data
Note: For more information on the inventory chart, see the Degrib: Man Pages and Full Tutorial. A similar document is also available in a portable document format. NDFD Tkdegrib and GRIB2 Data Download Tutorial (PDF).
e. Click on the Choose File Type menu at the bottom left corner of the window and choose SHP (Figure 5).
f. Manually type the output file name into the OUTPUT Filename dialog box or click the Recommend button to have tkdegrib choose one (Figure 6).
g. Now choose the Type of .shp file (Point, Small Polygon, or Large Polygon).
Note: For more information on types of shapefiles download the NDFD Tkdegrib and GRIB2 Data Download Tutorial (PDF).
h. Check the box for Verbose .shp file if the values for latitude, longitude, and X and Y are needed. A non-verbose shapefile will generate point identification and data, but not the latitude, longitude, and X and Y (Figure 7).
A. | B. |
i. The default values for the box in the bottom right corner for Round data to will default to a number when Recommend for Output File is clicked. Most weather elements will have a default of 0. The default for Units (when possible) can be accepted or changed, depending on personal preferences. The values for both Force Major Earth Axis and Force Minor Earth Axis are more advanced options and should be left as 0 for general purposes (Figure 8).
Note: The Round data to value will show its default when the Recommend button is complete. Most of the weather elements will default as 0; the exceptions are snow and waveh, which will default as 1, and qpf, which will default as 2.
j. Then click the Generate .shp file button at the bottom center of the window.
d.The shapefile will now appear in the Display view on the left side of the window, and the Map view will appear in the right side of the window. Both the Display tab or Table of Contents and the Map view tab should already be selected (Figure 10).
d. In the Import Symbology dialog box, select Import symbology definition from an ArcView 3 legend file (*.avl) and click the browse button (Figure 12).
e. Navigate to the /ndfd/degrib16/arcview/ folder.
f. Select the /point_legend or /poly_legend, depending on which type of shapefile was generated, and highlight the individual weather element that the shapefile addresses.
g. Click the Open button (Figure 13).
h. In the Import Symbology dialog box, under What do you want to import? select Complete symbology definition and click OK (Figure 14).
i. Accept the Value Field default value in the Import Symbology Matching Dialog box and click OK (Figure 15).
j. In the Layer Properties dialog, there should be different colors for the Symbol and different numbers for the Value. Click OK (Figure 16)
k. Notice the symbology is now applied to the shapefile in both the Display view and the Map view (Figure 17).
e. Click on Choose File Type in the lower left section of the window and choose FLT from the drop-down menu (Figure 19).
f. In the Grid drop-down menu, choose one of the three options: Projected: Original GRIB; Coverage: Nearest Point; or Coverage: Bi-Linear.
Note: For more information see full tutorial (LINK).
g. To create the GrADS .ctl file, check the box for Create GrADS .ctl file (Figure 20).
h. The other three options (Start at Upper Left, use NDFD Weather code, and M.S.B. First) are more advanced options and do not need to be selected for general purposes.
i. In the OUTPUT Filename dialog box, type the output file name or click Recommend to have tkdegrib choose one (Figure 21).
j. The default values for the box in the bottom right corner for Round data to and Units (when possible) can be accepted or changed, depending on personal preferences. The values for both Force Major Earth Axis and Force Minor Earth Axis are more advanced options and should be left as 0 for general purposes (Figure 22).
Note: The Round data to value will show its default when the Recommend button is complete. Most of the weather elements will default as 0; the exceptions are snow and waveh, which will default as 1, and qpf, which will default as 2.
k. Click the Generate .flt file button at the bottom of the window.
a. Open a new window in ArcMap and click on the ArcToolbox icon . This will open the ArcToolbox window (Figure 23).
b. Expand the Conversion Tools option in the Favorites window.
c. Expand the To Raster option. Then double-click on the Float to Raster selection (Figure 24).
d. In the Float to Raster window, click on the browse button next to Input floating point raster file (Figure 25).
e. Find the .flt file which was generated from the NDFD DataDownload tool and open it so that it appears under Input floating point raster file (Figure 26).
f. The program should create an Output Raster location. Click OK (Figure 27).
g. There will be an indication that the conversion is taking place and when it is finished. Click Close when it says "Completed" in the upper left corner (Figure 28).
h. The new Float to Raster file is now a binary raster file that has been converted to a floating point grid and can be used for raster analysis in ArcGIS (Figure 29).
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