1. Downloading and Preparing GIS Precipitation Data Layers
Using ESRI ArcMap
Presented by William Merkel
Hydraulic Engineer, NRCS
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2. Create a Directory to Save data
Working with GIS data, it is always best to organize the data into a directory.
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3. Open Internet and go to www.precip.net
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4. Open Data & Products
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5. Select GIS Data Files
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6. Select Data Type, Duration, Recurrence
For this example, select Data Type: “Return period Estimates”, Duration: 30 min, and Recurrence: 10 yr.
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7. Select Save
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8. Select a directory and file name
If more than one grid dataset will be saved in this directory, it is recommended to enter a unique file name at this point. output.asc is a default name.
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9. After Download Complete, Close
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10. Open Documentation and GIS Shapefiles
Download the state/county map of New York and New England from Documentation / Outreach Documents / GIS Shapefiles.
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11. Select Save
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12. Save it in directory
It is recommended to save this file in the same directory as the precipitation layer saved previously. The state/county map need only be downloaded once. It is used as an overlay to the GIS grid for location purposes and used later to assign a GIS projection to the precipitation layer.
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13. After Download Complete, Close
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14. Right Click, Extract to here
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15. Layer named NE_states_counties_WGS72 is unzipped
The map of states and counties in New York and New England is now available to open in a GIS. The map is in the WGS 1972 projection.
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16. Open ESRI ArcMap
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17. Open a new empty map, click OK
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18. Click the Add Data icon
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19. Add map of northeast states
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20. The map should look similar to this.
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21. Change map to No Color
Right click on the purple color in legend to open the color selection window. Changing the map to No Color will show only the state and county boundaries. It will allow the precipitation data to be visible after it is added later.
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22. Map will show state and county boundaries
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23. Open ArcToolbox Window
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24. Select Conversion Tools, To Raster, and ASCII to Raster
Double click the ASCII to Raster command.
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25. Select Input file, Output name, and FLOAT
Use the browse icon to the right of each file name box to select directory and file names. Use the pull-down icon to the right of the Output data type data box to select FLOAT.
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26. After conversion is complete, Close
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27. The precipitation grid is added to the map
The precipitation grid at this point is temporary. The values in the grid need to be divided by 1000 to convert the values to inches.
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28. Select Spatial Analyst Tools, Math, and Divide
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29. Select Input raster, divide by 1000, and Output Raster name
Use the pull-down selection to the right of the Input raster name to select the layer named ASCIITo_outp1. Edit the Output raster name to represent that the grid is 30 minute duration and 10 year recurrence interval.
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30. When the Divide operation is complete, Close
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31. The grid 30min_10yr is now prepared
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32. Change color scheme of the grid, optional
Left click on the color ramp of the layer to open the Select Color Ramp window.
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33. Colors have been changed.
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34. Right click, open layer properties
Open layer properties to see what projection is assigned to the grid.
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35. Spatial Reference (projection) is undefined
To do any more work with this grid, a projection needs to be assigned. In fact, the projection of this grid must be defined to be the same as the map of state/county boundaries that is shown in this ArcMap project. Close properties by clicking OK.
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36. Select Data Management Tools, Projections and Transformations, and Define Projection
Double click to open the Define Projection command window.
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37. Select Input Dataset using pull-down selection
Select Input Dataset using pull-down selection. Click Find Icon to the right of Coordinate System.
Click the icon to the right of the Coordinate System data box to select a projection.
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38. Select Import
Select the Import button so the projection of the state/county map can be used.
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39. Select NE_states_counties_WGS72.shp
Click Add button.
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40. Click Apply
The projection of the state/county map will be assigned to the 30 minute 10 year grid layer.
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41. Click OK
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42. Click OK to start command operation
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43. Close when command is completed.
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44. Open layer properties to check assigned projection
Right click on 30min_10yr grid name and select Properties.
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45. Correct projection is assigned.
The projection is set up in latitude and longitude coordinates. Close properties by clicking OK.
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46. Remove the temporary grid from the map window.
Before the temporary layer (ASCIITo_outo1) can be deleted, it must first be removed from the map. Right click on the layer name and select Remove.
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47. Open ArcCatalog and browse to find asciito_outp1 . Delete it.
Select the asciito_outp1 layer, right click and select Delete.
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48. asciito_outp1 is deleted from computer.
Close Arc Catalog.
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49. Layer preparation is completed.
At this point, the precipitation layer is complete. Now that a projection is assigned, the projection may be changed to another projection by using ARCGIS commands.
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50. Save the Project: Optional
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51. Save the Project: Optional
Select a directory and enter a file name. Close ArcMap after project is saved.
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52. The End
Questions ?
This same process is followed to prepare GIS ASCII grid maps downloaded from the NOAA 14 web site.
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