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Surface Analysis

Surface Analysis

Gemini Terrain has the following functions for surface analysis:

  • Slope maps
  • Aspect maps
  • Elevation zone maps
  • Visibility maps
  • Sun/shadow maps

Slope maps show the slope in the terrain. Slope maps will, for example, easily show where you should not build houses and where we can build on large lots or small lots.

Example of slope map

Aspect maps show slope direction. This can, for example, be interesting in connection with climate/sun conditions, housing planning, agriculture, and other purposes.

Example of aspect map

Elevation zone maps show in a clear way the different elevation intervals in the area.

Example of elevation zone map

Visibility maps show areas that can be seen from one or more observation points or lines. Visibility maps are useful when you want to find out how visible objects are from a given point, for example finding the most hidden areas for parking lots or the best areas for communication towers.

Example of visibility map

The example above shows areas that have sea views. Other examples of use can be to find the most hidden areas for parking lots or the best areas for communication towers.

Sun/shadow maps show areas that have shadow and areas that have sun. The map shows the situation for a user-determined time.

Example of sun/shadow map

Sun/shadow maps are created for fixed times, typically:

  • Vernal equinox (March 20–21)
  • Autumnal equinox (September 22–23)
  • Summer solstice (June 20–21)
  • Winter solstice (December 21–22)

The equinoxes are the only days in a year where day and night are almost equally long, hence the name. From sunset to sunrise or vice versa, almost exactly 12 hours will pass.

Summer solstice is the day with the longest daylight hours. When the sun is in the south at midday, it has its greatest height above the horizon in the year. Similarly, winter solstice has the shortest daylight hours. In everyday language, we often say that "the sun turns" at these times.

Info

See also the section on 3D visualization. We can also display sun/shadow areas in render mode.

Basis for Analysis

To be able to perform the analyses, we must have a terrain model in the drawing, that is, an application layer that is defined as a layer. We can run the analysis for the entire terrain model area or limit the area for the analysis with a polygon.

The program performs the analysis on a triangle model, either directly by using the triangle network in the application layer or indirectly by triangulating the grid model calculated from the application layer (this triangulation happens automatically when the analysis is performed).

If we use the grid model, we can set the desired density of the grid in the dialog for Terrain settings. Note that one of the application layers that defines the layer must be active before we can perform the analysis.

If we use the triangle network, the application layer with the triangle network must be active before we can perform the analysis.

Advantages of Grid

  • Gives better visual impression, since the grid will "smooth out" sharp slope differences
  • Takes terrain interventions into account (roads/excavation pits etc.)
  • Avoids having to triangulate the contour map

Disadvantages of Grid

  • Slightly jagged borders between polygons
  • More resource-intensive (grid size can become a problem)

Advantages of Triangle Network

  • Smooth, correct lines that coincide with contour lines
  • Works quickly on large datasets

Disadvantages of Triangle Network

  • Sharp peak triangles create noise in the polygons

Above we have briefly described the various analysis functions, as well as what we must have as a data basis. To perform these analyses, Gemini Terrain has a dedicated dialog where we select analysis and set up criteria for it.

Procedure

  1. Make the application layer with the terrain model (grid or triangle network) active
  2. Mark the polygon that delimits the area for the analysis and select Tools - Create thematic map (landscape analysis)... from the context menu (right-click)
  3. Select the current analysis and set settings
  4. Click Execute to run the analysis
  5. Create application layer, preferably use a predefined template for the analysis
  6. Paste result polygon into the layer