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Grid Lines

Use Grid Lines to overlay one or more grids on an image. Grids can be pixel-based or map-coordinate and/or latitude/longitude based (for georeferenced images). Each image display can have its own set of grids, which display in the Image, Scroll, and Zoom windows.


Tip
Use ENVI's QuickMap feature to quickly overlay grid lines, titles, declination diagrams, North arrows, and borders on georeferenced images. For details, see Creating QuickMaps.

Figure 4-10: A Pixel-based Image Grid

Figure 4-10: A Pixel-based Image Grid

Figure 4-11: Multiple Grid (Pixel, Map, Geographic) Coordinates on a Georeferenced Image

Figure 4-11: Multiple Grid (Pixel, Map, Geographic) Coordinates on a Georeferenced Image

Pixel-Based Image Grids

Images that have not been referenced to map coordinates can only have pixel-based grids. In this case, the grid spacing is specified in pixels (from (1, 1) in the upper-left corner) and the grid lines are labeled with the pixel coordinates.

Georeferenced Image Grids

Georeferenced images have grids that are based on map coordinates or geographic coordinates (latitude/longitude). The image is assumed to already be in the appropriate map projection prior to displaying the grid. The grid spacing is specified in projection units (normally meters), based on the reference pixel given in the image header file. The grid coordinates are labeled with the appropriate map coordinates.

Overlaying Grid Lines

  1. Select Overlay  Grid Lines. The Grid Line Parameters dialog appears and the image is re-displayed with a virtual border if one is not already applied.

    2.  

  2. Select the desired check boxes next to the Window label at the bottom of the dialog to plot the grid lines in the Image window and/or the Scroll window.

    3.  

  3. To select the grids to display, click the On/Off arrow toggle buttons next to the labels Pixel Grid, Map Grid, and Geographic Grid.

    4.  

    If the image is not georeferenced, then the Map and Geographic coordinate grids are not available. If your image is georeferenced, both the Map and Geographic grids are turned on initially.

     


    Note
    To hide or show the Grid Line Parameters dialog without erasing your classes, see Showing and Hiding Overlay Dialogs and Layers.

Setting Pixel and Map Grid Spacing

Grid spacing is in number of pixels for pixel-based grids and in meters for map coordinate grids.

In the Grid Line Parameters dialog, enter values into the text boxes labeled Grid Spacing.

Setting Geographic Grid Spacing

  1. In the Grid Line Parameters dialog, click on the DMS « DD button to change between degrees-minutes-seconds and decimal-degrees for the geographic grids.

    2.  

  2. In the Spacing text boxes, enter the desired spacing in degrees-minutes-seconds or decimal-degrees.

    3.  

  3. Click Apply to plot the grids on the image.

Grid Line Options

Use the Options menu in the Grid Line Parameters dialog to change the characteristics of all three grids and to change the display borders.

Changing Borders

To change the display borders size and color:

  1. Select Options  Set Display Borders.

    2.  

  2. Enter the desired border size for the left, top, right and bottom in pixels.

    3.  

  3. Select the desired color from the Border Color menu.

Editing Grid Characteristics

To change colors, label size and font, and other grid characteristics:

  1. Within the Grid Line Parameters dialog, select Options  Edit Pixel/Map/Geographic Grid Attributes.

    2.  

  2. When the Edit Map Attributes dialog appears, select from the following steps to change the characteristics of the labels, lines, box, and corners.
    • To turn on and off the plotting of the specific grid elements on the displayed image, toggle the On/Off arrow toggle buttons next to each element name.

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    • To plot grid labels outside the image in the virtual border, turn on Labels.

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      Ticks are also placed along the edges of the image adjacent to the labels.

       

    • To change the color, click Colors and select a color.

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    • To change the thickness of the label, enter a value in the Thick text box.

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    • To change the font of the labels, select from the Font menu. For more information on fonts, see Using Other TrueType Fonts with ENVI.

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    • To change the size, enter a value in the Charsize text box.

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    • To control the distance of the labels from the edge of the image for each grid type, use the Dist parameter. Larger numbers are farther from the edge.

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    • The X-axis Labels and Y-axis Labels arrow toggle buttons toggle between horizontal (H) or vertical (V) display of labels for both the X and Y grid axes independently.

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    • To plot X and Y grid lines, turn on the Lines option.

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    • To draw a Neat Line box around the image, turn on the Box option.

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    • To plot tick marks at each grid intersection, turn on Corners.

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    • To set the line type for the grid and box lines (solid, dotted, dashed, etc.), select from the Style menu.

      •  

    • To set the thickness of the items, enter the value in the Thick parameter box.

     

  3. Click OK when the grid line attributes are set.

    4.  

  4. Click Apply to plot the grid lines on the image.

Managing Grid Files

Use the File pull-down menu to save and restore grid setups.

Saving Grid Parameters

To save the grid parameter settings to a file:

  1. Select File  Save Setup.

    2.  

  2. When the Output Grid Parameters dialog appears, enter an output filename.

    3.  


    Note
    Grid setup files should be saved with the file extension .grd for consistency, but may be saved using other extensions.

Restoring Saved Grid Setups

  1. Select File  Restore Setup.

    2.  

  2. When the file selection window appears with all .grd files listed by default, select the grid file.

    3.  

  3. Click OK.

Saving Grid Lines

See Display Output Options for instructions to burn Grid Lines into images. The virtual border is automatically added to the image and the grid labels are burned in around the outside of the image.

  ENVI Online Help (August 12, 2005)