Finding elevation differences between adjacent pixels in ArcGIS Desktop?

Question

Is there a way to determine what the difference in elevation is between two adjacent pixels?

I am using an SRTM DEM to map faults.

I am using the vertical error (>16m) as a criterion for the difference in elevation to accurately map these faults.

There must be a more efficient way to do this than by using the i-tool.

Answer 1

Compute the focal range grid using a 2 x 2 neighborhood. (Use the option where NoData cells are ignored.)

• Any two adjacent pixels will be included within at least one neighborhood. Therefore, if any pair of cells differ by more than 16 m, they will cause at least one surrounding neighborhood to have a focal range exceeding 16 m.

• If the focal range of a neighborhood exceeds 16 m, that means (by definition) there exist two cells differing by at least 16 m. Since the neighborhood is 2 x 2, those cells will be adjacent (perhaps diagonally).

This shows that you can identify all potential fault locations by selecting areas where the focal range exceeds 16 m.

If you don't want to include diagonally adjacent pixels, you can compute two focal range grids, one with a 1 x 2 neighborhood and the other with a 2 x 1 neighborhood, and then take their (local) maximum.

Answer 2

If you really need the discrete difference between adjacent row and/or column pixel values, you can use NumPy's diff function along one row or column axis:

import numpy as np
# Somehow load raster into NumPy, I use GDAL
>>> dtm = np.array([[560.78, 556.04, 559.11],
                    [559.84, 560.77, 560.24],
                    [560.03, 559.82, 560.05]])
>>> print(dtm)
[[ 560.78  556.04  559.11]
 [ 559.84  560.77  560.24]
 [ 560.03  559.82  560.05]]

>>> dzdrow = np.diff(dtm, axis=0)
>>> dzdrow
[[-0.94  4.73  1.13]
 [ 0.19 -0.95 -0.19]]

>>> dzdcol = np.diff(dtm, axis=1)
>>> dzdcol
[[-4.74  3.07]
 [ 0.93 -0.53]
 [-0.21  0.23]]

I generally would have more GDAL Python code here to write the two arrays to two raster files so that I can view them in ArcGIS or wherever.

As expected, the difference along columns output has one fewer column, and the difference along rows output has one fewer row. However, as there are two outputs from this analysis, it can be more difficult to interpret (compared to calculating slope with one output). With this method, the row and column directions are two perpendicular components, so you can't combine them into one raster output.

Answer 3

You could calculate the slope of the raster. Then based on the cell size, you could work out the slope angle which corresponded to a 16m difference in elevation.