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Convert Vector Topographic Maps into Raster DEMs

Convert Vector Topographic Maps into Raster DEMs

Topographic data is frequently available in both vector and raster formats. While each format has its advantages, ENVI offers several tools for extracting additional information from raster digital elevation model (DEM) data. For example, you can use raster DEMs to do the following:

Use Convert Contours to DEM to triangulate a raster grid for the DEM from the vector records’ nodes. It is recommended that the output pixel size be set no smaller than the approximate sampling distance of the vector nodes. If the output pixel size differs significantly from the node sampling distance, the vector contour lines are automatically made more dense in an attempt to reduce interpolation artifacts. All vector records contained in the file are used unless their corresponding elevation attribute value falls outside the valid elevation range you specified.

The vector data must be an ENVI vector format (.evf) file. The .evf file must contain an attribute that defines the elevation of each vector contour line. If the vector topography is in an external format file (such as a shapefile), open the data in ENVI and the import routine prompts you to convert the file into an .evf.

  1. From the Toolbox, select Terrain > Convert Contours to DEM. The Select Input EVF File dialog appears.
  2. Select an .evf file containing topographic contour lines and an elevation attribute, and click Open. If ENVI does not find an associated attribute file, the Choose the Associated Attribute File dialog appears. Otherwise, skip the next step.
  3. Select a .dbf file and click Open. The Convert Elevation Contours to Raster DEM dialog appears.
  4. From the Elevation Attribute Column drop-down list, select the attribute that contains the elevation of the vector contours.
  5. If desired, enter a Valid Elevation Range in the same units as the elevation attributes. ENVI omits vectors whose elevation falls outside of the valid range when constructing the DEM.
  6. Enter an Output Pixel Size and Output Data Type. You can change the pixel size before you click OK to begin processing. It is recommended that you do not set the output pixel size to a value smaller than the approximate sampling distance between the nodes that define the vector contours.
  7. Use the Gridding Interpolation Method and the Extrapolate Edge of Image toggle buttons to set the gridding parameters.
  8. To spatially subset the output DEM, click one of the following buttons:
    • Map: To restrict the DEM output to an area defined in map coordinates. The Spatial Subset by Map Coordinates dialog appears.
    • File: To restrict the output DEM to the same area as an existing georeferenced file. The Choose a File on Which to Base the Spatial Subset dialog appears.
  9. In the Select Output Projection area, choose a map projection for the output DEM. This does not have to be the same projection as the input vector data.
  10. Click OK. The DEM Output Parameters dialog appears.
  11. Examine the information displayed under the Gridded DEM Output Image title to make sure it is accurate. If you need to change any of the output parameters, including the pixel size, click Change Output Parameters.
  12. Select output to File or Memory.
  13. Click OK. ENVI adds the resulting output to the Layer Manager.

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