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running-the-chi-analyses.asc

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Running a chi analysis

Like the Analysis Driver from the previous chapter, the chi tool is run using a parameter file. We tend to put the parameter file in the same directory as the data. We find that it is much simpler to work with one terminal window open in the directory with the source code (/path/to/LSDTopoTools/Git_projects/LSDTopoTools_ChiMudd2014/driver_functions_MuddChi2014/) and another terminal window open in the directory with the data (/path/to/LSDTopoTools/Topographic_projects/LSDTT_workshop_data/).

Looking at parameter file

The parameter file for the chi tool is called Workshop_ChiTool.driver. It is set up to run on the small WA.bil raster. If you want to run on the larger WAWater.bil raster you just need to change the filename in the parameter file.

The parameter file looks like this:

# Parameters for performing chi analysis
# Comments are preceeded by the hash symbol
# Documentation can be found here:
# http://lsdtopotools.github.io/LSDTT_book/#_chi_analysis_part_3_getting_chi_gradients_for_the_entire_landscape

# These are parameters for the file i/o
# IMPORTANT: You MUST make the write directory: the code will not work if it doens't exist.
read path: /LSDTopoTools/Topographic_projects/LSDTT_workshop_data/
write path: /LSDTopoTools/Topographic_projects/LSDTT_workshop_data/
read fname: WAWater
channel heads fname: NULL

# Parameter for filling the DEM
min_slope_for_fill: 0.0001

# Parameters for selecting channels and basins

threshold_contributing_pixels: 2000
minimum_basin_size_pixels: 500
test_drainage_boundaries: false

# Parameters for chi analysis
A_0: 1
m_over_n: 0.45
threshold_pixels_for_chi: 2000

n_iterations: 10
target_nodes: 80
minimum_segment_length: 10
sigma: 10.0
skip: 2

# The data that you want printed to file
only_check_parameters: false
print_stream_order_raster: true
print_DrainageArea_raster: true
print_segmented_M_chi_map_to_csv: true
print_simple_chi_map_to_csv: true

There is a whole bunch of stuff in here, which you should mostly ignore. Here are the relevant lines:

  1. The lines that control the location of the files to read and write are here:

    read path: /LSDTopoTools/Topographic_projects/LSDTT_workshop_data/
write path: /LSDTopoTools/Topographic_projects/LSDTT_workshop_data/
read fname: WA

    You need to update these lines with your own directory structures. The read name: gives the prefix of the DEM. If you want the bigger dem, replace WA with WAWater.

    Updating your parameter files the easy way

    You can update the read and write paths to reflect your own directory structure by running EITHER

    python LSDTopoToolsSetup.py -id 0 -cp True

    OR

    python LSDTopoToolsSetup.py -id 1 -cp True

    You should use the same -id flag you used when you first set up LSDTopoTools.

  2. These lines control how dense the channel network will be when you run the code:

    threshold_contributing_pixels: 2000
minimum_basin_size_pixels: 500
test_drainage_boundaries: false

# Parameters for chi analysis
A_0: 1
m_over_n: 0.45
threshold_pixels_for_chi: 2000

    The threshold_pixels_for_chi parameter should not be larger than the threshold_contributing_pixels parameter. If these parameters are small you will get many channels, if they are large you will get fewer channels. minimum_basin_size_pixels should not be larger than threshold_contributing_pixels.

    If you keep A_0 equal to 1, then the chi slope will be the same as the channel steepness index, k_sn.

    # The data that you want printed to file
only_check_parameters: false
print_stream_order_raster: true
print_DrainageArea_raster: true
print_segmented_M_chi_map_to_csv: true
print_simple_chi_map_to_csv: true

    1. The only_check_parameters is for bug testing: if true it will make sure your parameters will not immediately lead to a crash.

    2. The print_stream_order_raster will print a raster showing where the stream network is.

    3. The print_DrainageArea_raster will print a raster showing the drainage area.

    4. The two csv parameters are to print files that can both be read by either a GIS or by excel. The segmented is more complete, it has all the information of the simple_chi_map and more; the simple_chi_map only has the chi coordinate and location of the channels.

Some extra chi mapping parameters

The default of the chi mapping program is to only extract the chi values and slopes from the biggest basin. You can get the chi slopes and values for all basins by adding the following line to the parameter file:

only_take_largest_basin: false

You can print the chi values to a raster (rather than csv) with the line:

print_chi_coordinate_raster: true

Running the analysis with the parameter file

  1. If you’ve done the analysis driver exercise, this should be familiar.

  2. Go into the directory where you compiled the chi tool and run:

    $ ./chi_mapping_tool.exe /path/to/LSDTopoTools/Topographic_projects/LSDTT_workshop_data/ Workshop_ChiTool.driver

  3. The program should spit out a bunch of text, and afterwards you will be left with some new files, which we will look at in QGIS in the next section.