Multiple Quadrangles\\\\\ Single Quadrangles\\\\\ Bay Area DEMs

Multiple Quadrangles

1. Use the National Elevation Dataset for areas covered by multiple quads (free < 100MB)
   
2. After unzipping with WinZIP, these grids arrive ready to be used in ESRI software.
3. Metadata may be found here.
   

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Single Quadrangles in SDTS Format

1. Download your DEM from MapMart or GIS Data Depot for a single quadrangle
2. Navigate to the quad of interest.
   Note: These files are compressed using Tar/Gzip utilities and will have a name like  ***tar.gz.
3. Unzip the ***tar.gz files using WinZip.
4. Import the data into a GIS client:
   Select your program: ArcView 3.x or ArcGIS Desktop

ArcGIS Desktop Directions

1. Open ArcToolbox
2. Under Conversion Tools > Import to Raster, choose 'SDTS Raster to Grid'
3. In the Input Prefix box, navigate to the folder with the ****.ddf files, and choose the 4 digit input file (ex:1158). Note that the path for the .ddf files may not contain any spaces.
4. Choose the Z-value field name for the Record No. box, i.e. "ELEVATION".
5. Select an output folder and name the Grid.
6. Keep Convert Values and Use Data Dictionary checked.
7. Hit OK.
8. You may then add this DEM Grid to ArcInfo Desktop or ArcView 3.x
   
   

ArcView 3.x Directions:

    1. Edit the ***IDN.DDF file

- First make a COPY of  the ****IDEN.DDF file (for safe keeping).
- Use a text editor to open the ****IDEN.DDF file.
- Replace the words "SRPE: RASTER PROFILE and EXTENSIONS" with "SDTS RASTER PROFILE ***"
- You must also enter 16 spaces following SDTS RASTER PROFILE________________*** (yes, include the asterisks)
- Save the file to the same location.

     2. Using the SDTS Raster to Grid Utility

- Choose Start > Programs > ESRI >ArcView GIS 3.x > SDTS Raster to Grid
- In the Input Prefix box, enter the complete path name to the directory with the ****.ddf files . Add a backslash (\) and type the four character transfer prefix (ex: c:\GIS\Shastadata\Rasters\1158).
- In the Output Grid box, enter the complete path + a new file name for the DEM output (ex: c:\GIS\Shastadata\Rasters\ECanyon).
- Leave the Raster Object Record Number box blank.
- Under the Data Dictionary Modules section, click Keep DD.
- In the Void and Fill section, be sure that the Convert Null Values button is selected.
- Hit OK.  It will then create an ESRI GRID file and place it in the output directory you created.

     3. Adding your DEM GRID to ArcView:

- Start ArcView. Under File/Extensions, turn on the Spatial Analyst Extension.
- Open a new View window.
- Select View > Add Theme
- Migrate to your output folder, choose Grid Data Source from the drop down box and select your grid.
- The new DEM grid will show up as a theme in the table of contents of the view.
- Place a check mark next to the theme name to draw it in the view window.
- You can then delete the *.ddf files.

It took me a few attempts to get this new procedure to work, but it DOES WORK, stick with it!

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Bay Area DEMs

1. Bay Area 30M DEMs (already in ".dem" format) are available for download from the USGS BARD DEM page.
2. Navigate to the desired file, click to download the *.gz file, and unzip using WinZip.
3. If you are using ArcView 3.x, it is necessary to first turn on the Spatial Analyst extension. Then go to File > Import Data Source, choose "USGS DEM", and navigate to your *.dem file.
4. If you are using ArcGIS Desktop, you must first open ArcToolbox. Select Conversion Tools > To Raster > DEM to Grid, and specify the input .dem and output grid names. Click OK. Then, open ArcMap and go to File > Add Data and navigate to the grid file.
   

Return to menu

ArcGIS Desktop Instructions

ArcView 3.x Instructions

1. Double click to unzip the .gz file you downloaded.  You will be prompted with this window:

2. Enter the file extension ".dem"
3. Open ArcToolbox
4. Under Conversion Tools > Import to Raster, choose 'DEM to Grid'
5. In the 'USGS DEM' box, navigate to the folder with the ****.dem 250K grid file
6. Select an output folder and name the Grid.
7. Hit OK.
8. You may then add this DEM Grid to ArcInfo Desktop or ArcView 3.x

Return to menu

A. Download the relevant GTOPO30 files by right clicking and saving the link of the tile's name.

B. Unzip the *.tar.gz file (see WinZip decompression instructions)

C. Follow the import instructions below for your software, either ArcView 3.x, ArcGIS Desktop or ArcInfo Workstation:

ArcView 3.x Instructions

1. Download the gtopoimp.ZIP extension to ArcView.
2. Unzip the gtopoimp.zip  file.  From within Winzip, open the gtopoimp.txt file.
3. Read the gtopoimp.txt file for instructions about where to extract the other files, and instructions on how to use this simple extension.

ArcGIS Desktop Instructions

1. Rename the *.dem to *.bil
2. Open ArcCatalog
3. Navigate to the *.bil created in Step 1.
4. Right-click on the *.bil, click Export, and click the Raster to Different Format option. Specify the location to which you will save the new grid, and ensure the Save As Type is set to ESRI GRID.
5. Open ArcMap and add the grid created in Step 4.
6. On the ArcGIS Spatial Analyst toolbar, click Spatial Analyst and select Options.
7. Click the General tab and set the working directory.
8. Click the Extent tab and set the analysis extents to Same as layer 'your grid'.
9. Click the Cell Size tab and set the cell size to Same as layer 'your grid'. Click OK.
10. Navigate to ArcGIS Spatial Analyst and select the Raster Calculator. Enter the following expression, substituting your grid for [in_grid]. Include the square brackets.
    

    CON([in_grid] >= 32768,[in_grid] - 65536,[in_grid])

    Click Evaluate.
11. The ocean areas (-9999) can be converted to NODATA using the SETNULL function. Insert the Calculation from Step 10 for [Calculation]. Include the square brackets.

    setnull([Calculation ] == -9999, [Calculation ])

    Click Evaluate.
12. Save the final output to disk by right-clicking the Calculation result from Step 11 and selecting Make Permanent.
    

ArcInfo Workstation Instructions

1. Ensure the DEM files have been renamed with bil extension (see related FAQ.)
2. Run Imagegrid. Imagegrid does not support the conversion of signed image data; therefore the negative 16-bit signed
   values will not be interpreted correctly without the formula in step #3.
3. Run the following formula in Grid:
   out_grid = con(in_grid >= 32768, in_grid - 65536, in_grid)
   The statistics of the Grid should now match those listed in the .STX file. NOTE: The GTOPO30 data masks the oceans
   with -9999 values. If desired, use the Setnull function and set these masked values to: Nodata.

Return to menu

1. Start Winzip; Go to the "Options" menu and select "Configuration". In the miscellaneous tab, make sure that the "Tar File Smart CR/LF conversion" option is turned OFF.
2. Go back to the main Winzip menu and select File/Open Archive. Select your ***tar.gz file and click "Open".
3. Answer "Yes" when Winzip asks you: "Should Winzip decompress it to a temp folder and open it?"
4. Extract all of the individual *.ddf  files listed by clicking the extract button and choosing a unique folder on the hard drive.