Filtering and interpolating DEM values

You can use the filters available under Filtering and Interpolation to eliminate failed or incorrect values in a digital elevation model (DEM). You can apply each filter repeatedly and in different combinations to achieve a cumulative effect. You can also apply the filter to the entire DEM, or limit the operation to a specific area.

To apply filtering and interpolation to a DEM

In the DEM Editing window, under Filtering and Interpolation, click the filter you want to use:
- Remove Noise Filter: Noise refers to pixels containing distorted or failed values. Because pixels adjacent to failed pixels also tend to contain incorrect values, the Remove Noise Filter uses two filters to identify failed pixel values and their surrounding pixels:
  - The first filter calculates the average and variance of the eight elevation values immediately surrounding each pixel, excluding failed and background pixels. If the center pixel is more than two standard deviations away from the average, it is replaced with the failed value.
  - The second filter counts the number of failed values immediately surrounding each pixel. If five or more failed pixels border the center pixel, the center pixel is also set to a failed value.
- Erode holes: Because pixels adjacent to failed pixels also tend to contain incorrect values, the Erode holes filter replaces the eight pixels around each failed pixel with the failed value. When you apply the filter under a mask, the mask enlarges to cover any additional pixels replaced by the failed value.
- Smooth DEM: A Gaussian filter calculates the weighted average of all the pixels in a 3-x-3-pixel frame and assigns the value to the center pixel in the frame. Failed and background pixel values are not replaced by the filter and are not used in the Gaussian calculation.
- Median Filter: Ranks the pixel values within a 5-x-5-pixel frame according to brightness. The median is the middle value of those image pixel values, which is then assigned to the pixel in the center of the frame.
- Interpolate: Replaces failed values with an estimate weighted by distance and calculated from the valid pixels surrounding the failed pixel or pixels. The algorithm used to calculate the estimate is adequate for small areas of less than 200 pixels, but is not recommended for larger areas.
Select one of the following options:
- Entire DEM: applies the filter to the entire DEM
- Use Mask: limits the filter to a specific area
To update the DEM, click Apply.