# 1.4-terrain-mesh

Bane Sullivan
%matplotlib inline
from pyvista import set_plot_theme
set_plot_theme('document')

# #Terrain Following Mesh

Use a topographic surface to create a 3D terrain-following mesh.

Terrain following meshes are common in the environmental sciences, for instance in hydrological modelling (see Maxwell 2013 <https://www.sciencedirect.com/science/article/abs/pii/S0309170812002564>_ and ParFlow <https://parflow.org>_).

In this example, we demonstrate a simple way to make a 3D grid/mesh that follows a given topographic surface. In this example, it is important to note that the given digital elevation model (DEM) is structured (gridded and not triangulated): this is common for DEMs.

# sphinx_gallery_thumbnail_number = 3
import pyvista as pv
import numpy as np
from pyvista import examples

dem = examples.download_crater_topo()
dem

Now let's subsample and extract an area of interest to make this example simple (also the DEM we just load is pretty big). Since the DEM we loaded is a :class:pyvista.UniformGrid mesh, we can use the :func:pyvista.UniformGridFilters.extract_subset filter:

subset = dem.extract_subset((500, 900, 400, 800, 0, 0), (5, 5, 1))
subset.plot(cpos="xy")

Now that we have a region of interest for our terrain following mesh, lets make a 3D surface of that DEM:

terrain = subset.warp_by_scalar()
terrain
terrain.plot()

And now we have a 3D structured surface of the terrain! We can now extend that structured surface into a 3D mesh to form a terrain following grid. To do this, we first our cell spacings in the z-direction (these start from the terrain surface). Then we repeat the XYZ structured coordinates of the terrain mesh and decrease each Z level by our Z cell spacing. Once we have those structured coordinates, we can create a :class:pyvista.StructuredGrid.

z_cells = np.array([25] * 5 + [35] * 3 + [50] * 2 + [75, 100])

xx = np.repeat(terrain.x, len(z_cells), axis=-1)
yy = np.repeat(terrain.y, len(z_cells), axis=-1)
zz = np.repeat(terrain.z, len(z_cells), axis=-1) - np.cumsum(z_cells).reshape(
(1, 1, -1)
)

mesh = pv.StructuredGrid(xx, yy, zz)
mesh["Elevation"] = zz.ravel(order="F")
mesh
cpos = [
(1826736.796308761, 5655837.275274233, 4676.8405505181745),
(1821066.1790519988, 5649248.765538796, 943.0995128226014),
(-0.2797856225380979, -0.27966946337594883, 0.9184252809434081),
]

mesh.plot(show_edges=True, lighting=False, cpos=cpos)