Analyzing Terrain Topography

Terrain topography is a part of the terrain analysis: it allows to reveal information that is essential to manage several topics such as water supply or soil erosion. You can display ridges, valleys, slope vectors, watershed segmentation and water drop path on a terrain. You can also export watershed data into an .xml file.

All the analyzing commands described below are available in the Terrain Topography Analysis command from the Prepare section of the action bar.

All modes can be combined to obtain a maximum of information.

This task shows you how to:

Compute Ridges and Valleys

You can lay out ridges and valleys on a terrain.

  1. Expand the Ridges and Valleys section and click Hide/Show all valleys and/or Hide/Show all ridges to display these objects on the selected terrain.
  2. Keep the Extended mode option selected to extend all ridges/valleys segments to get longer ones and link them together when possible.

    By default, the option is selected. Two sliders are available to manage the extension of the Minimum length (m) and Minimum slope (%) for valleys and ridges. The minimum value is set by default, in meters. The distance range computed according to the model and the slope range is from 0 to 100%.

  3. To launch computation, click Apply.

    A progress bar is displayed during the computation. You can interrupt the treatment by pressing Escape.

    By default, ridges are displayed in orange and valleys in blue. You can change color by clicking the color segment under the command: .



  4. Click a valley or a ridge in the 3D area to display specific information about the selected object.

    A contextual toolbar appears in which you use commands to hide it, change its color or name.

    When ridges/valleys are computed, some incorrect or disrupted mesh areas are detected and displayed in the shape of red overhanging triangles:

    A new feature called Terrain Topography.1 is created in the tree. When you leave the command, double-click the feature and the Terrain Topography Analysis dialog box opens.

  5. To hide/show all the ridges and/or valleys, click in the Terrain Topography Analysis dialog box.

Compute Slopes

You can make steering arrows appear on a terrain to show slope direction.

  1. Select a terrain mesh or a polyhedral feature.
  2. Expand the Slopes section and click Hide/Show all maximum slope vectors to display slopes on the selected terrain.

    A progress bar is displayed during the computation. You can interrupt the treatment by pressing Escape.



    The default color of the arrows is yellow. You can change color by clicking the color segment under the command. The arrows represent the maximum slope on each triangle of the input terrain mesh. The slope's segment has a point at its beginning (origin of the arrow), and not at its end (end of the arrow).

    When slopes are computed, some incorrect mesh areas are detected and displayed in the shape of red overhanging triangles.

    A new feature called Terrain Topography.1 is created in the tree. When you leave the command, double-click the feature and the Terrain Topography Analysis dialog box opens.

  3. To hide/show all the slopes, click in the Terrain Topography Analysis dialog box.

Compute Watershed Segmentation

You can compute and reveal watershed distribution.

A watershed is an area of land that drains all the streams and rainfall to a common outlet such as a river or a bay. Ridges that separate two watersheds are called the drainage divide. All the precipitation on opposite sides of a drainage divide will flow into different drainage basins.

  1. Select a terrain mesh or a polyhedral feature.
  2. Expand the Watershed Segmentation section and click to display basin division.
    Note: A message appears telling that the terrain will be hidden to prevent any overlay with the computation results.
  3. To launch computation, click Apply.

    A progress bar is displayed during the computation. You can interrupt the treatment by pressing Escape.

    You obtain:

    • Colored areas for each basin.
    • A black point, called outlet, at the base of each basin.



    Notes:

    The water paths do not cross the basins' frontiers (lighter color):



    If you click a basin surface, a contextual toolbar appears with the following information: the name of the basin (editable), depth, Perimeter, area and the longest Water Path. And two buttons allow you to hide/show the basin or its longest path and, to change their color.

    A new feature called Terrain Topography.1 is created in the tree. When you leave the command, double-click the feature and the Terrain Topography Analysis dialog box opens.

  4. To merge the basins according to a depth value, set a new Depth threshold in the dedicated box and compute watershed segmentation.

    The default value of the depth threshold is 1000mm. If a value is set, this new value is saved for the next use of Terrain Topography Analysis. When the value is updated, a preview of merged basins is displayed in highlighted mode on the terrain.



  5. To export the watershed data into an xml file, see the last section.
  6. To hide/show all the basins, click in the Terrain Topography Analysis dialog box.

Compute Water Drop Path

You can compute all the trajectories of a water drop starting from a point on the terrain.

  1. Select a terrain mesh or a polyhedral feature.
  2. Expand the Water Drop Path section and click Enable/Disable Water Path .
  3. Hover over the terrain surface in the 3D area.
    The starting position of the water drop path is given by the pointer. Water path is displayed in real time: each time the pointer moves, the path is updated.

    By default, the water path is displayed in blue. You can change color by clicking the color segment under the command.

  4. Click the terrain surface to keep the water path polyline.

    You can display several water paths.

    A new feature called Terrain Topography.1 is created in the tree. When you leave the command, double-click the feature and the Terrain Topography Analysis dialog box opens.

  5. To remove all the water paths, click .
  6. To hide/show all the water drop paths, click in the Terrain Topography Analysis dialog box.

Compute a Catchment Area

You can select a predefined geometry (point(s), polyline, curve or closed polyline) as an input to display a catchment area. Thus you can see a water drop path on a smaller portion of the terrain.

This command can be combined with the Watershed segmentation computation.

  1. Select a terrain mesh or a polyhedral feature.
  2. Expand the Catchment Area section.
    The Compute Catchment Areas from selected objects command is automatically selected.
  3. To detect water paths in a specific area, select a point, a polyline, a curve or a closed polyline.

    You can select several points to display several water drop areas.

    Note: A message informs you that the terrain transparency will change to allow a better visibility of the localized catchment area.
  4. Optional: To set the depth of the selected areas, change the Depth threshold value.

    This value is also shared with the Watershed segmentation computation.

  5. To launch computation, click Apply.

    A progress bar is displayed during the computation. You can interrupt the treatment by pressing Escape.

    The catchment area is the union of all the water drop paths which go through the input.

    The other areas of the terrain are made opaque to provide a better visibility for the computed area.

    Notes:
    • To cancel transparency on terrain, do one of the following:
      • Hide the catchment areas by clicking .
      • Click OK: transparency is cancelled and catchment areas are still visible.
      • Select the terrain Properties and edit the transparency coefficient in the Graphic tab.
    • Watershed segmentation can be executed in parallel. An advice message appears: it is recommended to hide the segmentation or catchment area results for better performance.
    Selected Feature Catchment Area Result
    Point
    Polyline
    Curve
    Closed polyline (the inputs are the outlines that are highlighted as blue)

    The result is inside the highlighted outlines, indicated by the arrows:
  6. Click the catchment area in the 3D area to display specific information about it: the name, input type (point, polyline, curve, closed polyline), perimeter in m, and area in m2 of the catchment area.

    You can change its color or name, and hide it by clicking .

    A new feature called Terrain Topography.1 is created in the tree. When you leave the command, double-click the feature and the Terrain Topography Analysis dialog box opens.

  7. To open the list of the selected geometry features in a Catchment Area panel, click and, if necessary do the following:
    • Select another feature in the tree or in the 3D area and its name is added to the list.
    • Click to remove a feature from the list.
    • Click next to a selected object for other actions, for example to reframe on it, to hide/show it, to look at its properties.
  8. To export the watershed data into an xml file, see the last section.
  9. To hide/show all the catchment areas, click in the Terrain Topography Analysis dialog box.

Manage Terrain Topography Information

Several actions are available at the bottom of the Terrain Topography Analysis dialog box to hide/show or reset information. After computing the watershed segmentation, you can also export the watershed data into an xml file that can be read in Excel.

By default, all basins are exported. If some basins are selected, you export only on these basins.

  1. To hide/show any topography contextual toolbars, click .
  2. To reset the color of basins, longest paths, water paths, ridges, valleys or slopes, click .
  3. To export watershed data into an xml file, click .
    The following parameters, belonging to the basins, are written in the file: name, profile, perimeter, area, the length of the longest path, the 3D coordinates of the lowest point/of the highest point/of the outlet point, and the basin color (RGB format).