Frequently Asked Questions#

What is the difference between a cell and a node?#

A cell is a polygon surrounding a node. Nodes on the grid perimeter do not have cells. Cells have area, nodes have coordinates.

Why is my node data a 1d array? I’m using a raster…#

All Landlab data structures have to be compatible with both regular and irregular grids. A 2d structure for node data might make sense for a raster, but it wouldn’t work for an irregular grid - and moreover, there is also no sensible way to represent link data as a 2d raster either, even for a raster. Thus, instead, Landlab stores all data in an order set by node ID number. For a raster, IDs begin at zero in the bottom left corner of the grid, then run along each row in turn. For links, the IDs also start in the bottom left and run across then up, but in this case all vertical links are listed, then all horizontal links. Here’s a sketch summary of this scheme for a 4x5 raster:

NODES:                                    LINKS:
15------16------17------18------19        *--27-->*--28-->*--29-->*--30-->*
|       |       |       |       |         ^       ^       ^       ^       ^
|       |       |       |       |        22      23      24      25      26
|       |       |       |       |         |       |       |       |       |
10------11------12------13------14        *--18-->*--19-->*--20-->*--21-->*
|       |       |       |       |         ^       ^       ^       ^       ^
|       |       |       |       |        13      14      15      16      17
|       |       |       |       |         |       |       |       |       |
5-------6-------7-------8-------9         *---9-->*--10-->*--11-->*--12-->*
|       |       |       |       |         ^       ^       ^       ^       ^
|       |       |       |       |         4       5       6       7       8
|       |       |       |       |         |       |       |       |       |
0-------1-------2-------3-------4         *---0-->*---1-->*---2-->*---3-->*

How do I set the boundary codes for the edges of a grid?#

By default, the boundary nodes around the perimeter of a grid are all open boundaries. For a raster grid, if you want to make one or more sides closed boundaries, use the grid method RasterModelGrid.set_closed_boundaries_at_grid_edges.

The following code snippet sets the southern boundary nodes to be closed:

import landlab

grid = landlab.RasterModelGrid(3, 4)
grid.set_closed_boundaries_at_grid_edges(True, False, False, False)
grid.status_at_node
array([4, 4, 4, 4, 1, 0, 0, 1, 1, 1, 1, 1], dtype=int8)

It’s also possible to set the boundary conditions “by hand”, if you know the ID of the element you’re trying to change: :

mynodes_to_close = z < 0.0  # if z is some elevation field
grid.status_at_node[mynodes_to_close] = grid.BC_NODE_IS_CLOSED
my_fixed_node = mg.find_nearest_node((1.2, 2.3))
my_fixed_node
9
grid.status_at_node[my_fixed_node] = (
    grid.BC_NODE_IS_FIXED_GRADIENT
)  # to fix the node closest to (1.2, 2.3)

Can I import Landlab output into ParaView or VisIt?#

See How do I get netCDF output? below.

How do I get netCDF output?#

At present, Landlab can write output to a netCDF file if you are using a raster grid (support for unstructured grids is coming later). To create netCDF output, use the function landlab.io.netcdf.write_netcdf. This function will write to file

the grid geometry, and
any data arrays that are linked to the grid

this will automatically include any arrays that you created with functions such as landlab.grid.base.ModelGrid.add_zeros, as long as you provided a name for the array as one of the arguments.

How do I assign values from nodes to links?#

Suppose you have a set of values, such as water depths, that are defined at nodes. How do you figure out what the corresponding values would be at the links, so you can multiply these by some other quantity (such as water-surface slope) that is defined on links? Here are some options:

assign the average
assign the upstream value
assign the downstream value

Look at this Tutorial for all the Landlab mappers

How do I test whether my grid is regular or irregular?#

There are a number of cases when designing Landlab components where you’ll want to do something one way if the grid is a raster, or another if it’s a Voronoi-derived type. The way to do this is:

from landlab import RasterModelGrid, VoronoiDelaunayGrid

# ...
if isinstance(mg, RasterModelGrid):
    print("Doing it one way")
elif isinstance(mg, VoronoiDelaunayGrid):
    print("Doing it the other way")
else:
    raise TypeError("Landlab did not recognize your grid type!")

How do I modify boundary conditions for part of the grid where I know the coordinates?#

See this tutorial.

I am having trouble installing Landlab on Ubuntu without Anaconda. What is the fix?#

Andy Wickert (5/16) suggests the following:

“The version of setuptools that comes standard on Ubuntu is out-of-date with respect to Landlab’s Cython code. Here is the fix:”

sudo apt-get install python-setuptools # if you don't have it already
sudo easy_install pip
sudo apt-get remove python-setuptools
pip install setuptools # add "--upgrade" if needed

And then you can cd to landlab and this works:

pip install -e .

Support: How can I ask more questions and get help?#

File an issue at https://github.com/landlab/landlab using the New issue button in the upper right. Tell us about your issue, and we’ll be in touch.

How do I keep in touch with Landlab developments?#

There are a few ways to follow Landlab developments. You can

follow Landlab on Twitter @landlabtoolkit,
“watch” Landlab’s GitHub repository,
file a pull request or an issue at https://github.com/landlab/landlab,

Why are there no other FAQs besides these few?#

Because we need your questions. Please feel free to add your own questions by making a GitHub Issue.