The Export ECW command allows the user to export any loaded raster, vector, and elevation grid data sets to an ECW file. ECW files are highly compressed and great for storing things like satellite imagery. There is no size restriction on exported ECW files, so you can store many terabytes worth of imagery within a single highly compressed ECW file.
The Sample Spacing section allows the user to select the grid spacing to use when generating the file. The default value is the average of the grid spacings of all the currently loaded raster and elevation overlays.
If the Always Generate Square Pixels option is checked, the smaller of the specified x and y resolutions will be used for both the x and y resolution.
Selecting Always Generate Square Pixels ensures that the resultant image file will look good even in software that is not able to deal with pixels that aren't square.
To specify the spacing in units other than those of the currently selected view/ export projection, press the Click Here to Calculate Spacing in Other Units button.
Specify the X and Y spacing of the output pixels in any of the selected units. These pixel dimensions will be translated into the units of the display projection during export. The Use Current Screen Pixel Size button will update the X and Y values to the dimensions of the map display. Use Last Exported Sample Spacing will update the X and Y values to the most recent export resolution.
If you would like to generate an uncompressed format file, simply slide the Target Compression Ratio slider all the way to the left (1:1 target compression ration).
- Default RGB Layout- This option generates a 3 band RGB ECW with 8-bits per band
- Grayscale- This option generates a 1 band image with 256 scales of gray ranging from black to white.
Metadata File Formats to Generate
Select any additional metadata or projection files to be produced with the ECW export.
- Nearest Neighbor - simply uses the value of the sample/pixel that a sample location is in. When resampling an image this can result in a stair-step effect, but will maintain exactly the original color values of the source image.
- Bilinear Interpolation - determines the value of a new pixel based on a weighted average of the 4 pixels in the nearest 2 x 2 neighborhood of the pixel in the original image. The averaging has an anti-aliasing effect and therefore produces relatively smooth edges with less stair-step effect.
- Bicubic Interpolation - a more sophisticated method that produces smoother edges than bilinear interpolation. Here, a new pixel is a bicubic function using 16 pixels in the nearest 4 x 4 neighborhood of the pixel in the original image. This is the method most commonly used by image editing software, printer drivers, and many digital cameras for resampling images.
- Box Average (3x3, 4x4, 5x5, and 7x7) - the box average methods simply find the average values of the nearest 9 (for 3x3), 16 (for 4x4), 25 (for 5x5), or 49 (for 7x7) pixels and use that as the value of the sample location. These methods are very good for resampling data at lower resolutions. The lower the resolution of your export is as compared to the original, the larger "box" size you should use.
- Filter/Noise/Median (2x2, 3x3, 4x4, 5x5, 6x6 and 7x7) - the Filter/Noise/Median methods simply find the median values of the nearest 4 (for 2x2), 9 (for 3x3), 16 (for 4x4), 25 (for 5x5), 36 (for 6x6) or 49 (for 7x7) pixels and use that as the value of the sample location. This resampling function is useful for noisy rasters, so outlier pixels do not contribute to the kernel value. Some common sources of raster noise are previous compression artifacts or irregularities of a scanned map/image.
- Box Maximum (3x3, 4x4, and 5x5) - the box maximum methods simply find the maximum value of the nearest 9 (for 3x3), 16 (for 4x4), 25 (for 5x5), or 49 (for 7x7) pixels and use that as the value of the sample location. These methods are very good for resampling elevation data at lower resolutions so that the new terrain surface has the maximum elevation value rather than the average (good for terrain avoidance). This method behaves the same as the average on raster/imagery layers. The lower the resolution of the export file is as compared to the original, the larger "box" size that should be used.
Save Map Layout
The distance scale, elevation legend, and coordinate grid will be saved to the file (except when exporting elevation grids) if they are configured to show up in the main display.
Save Vector Data if Displayed
Any loaded vector data that is configured to show up in the main display will be saved to the image file (except when exporting elevation grids).
Interpolate to fill small gaps in data
Any small areas with missing data will be filled in by interpolating the surrounding valid data using an IDW method. This is useful for filling small gaps between adjacent tiles or small holes in elevation data.
Make Background (Void) Pixels Transparent:
An alpha channel will be added to the file to indicate which pixels should be treated as transparent.
Include Projection/Datum in Header
The projection information will be saved to the ECW header.
Save ECW Projection Name as EPSG:XXXX
The projection and datum name saved in the ECW header will be formatted as ESPG:XXXX