Terrain complexity is calculated according to IEC 61400-1 edition 3 or 4. The planes are fitting in order to minimise the sum of the squares of the differences between the plane and the underlying terrain. By default, the terrain is sampled every 10 m but this can be changed in the terrain complexity settings.
The fitted planes have to meet the following conditions:
Except that the terrain can fail the deviations limit in an area up to 5.H2 where H is the hub height of the turbine. In this version the planes pass through the turbine base and so the plane fitting becomes trivial.
Users can opt to output the fitted planes as well as the residuals between the plane and the underlying terrain. However, these planes can take up a lot of memory.
For IEC61400-1 Edition 3 Amendment 1, terrain complexity is mostly a matter of pass or fail with a small region in between.
For IEC61400-1 Edition 4, terrain complexity is much more a matter of degree and terrain complexity is classified into None, Low, Medium or High dependent upon two different metrics over a variety of tests.
The settings for using draft edition 4 are similar to those for edition 3 except that the planes can only be output for one set of tests at a time. This is because the testing does not stop when a test is failed and the tests areas tend to overlap.
For draft edition 4, the fitted planes do not need to fit through the turbine base and so a different plane fitting algorithm was needed. This analytical solution proved to be faster and so has been adopted for edition 3 as well now (although still respecting the edition 3 stipulation that planes must pass through the turbine base).
In order to assess whether a turbine is situated in complex terrain, and how complex that terrain is, the terrain slope index (TSI) and the terrain variation index (TVI) must be computed for each and every plane test.
The terrain complexity can be output as a detailed report in its own right, but it can also be used to inform turbulence structure correction parameters (Cct) which can then be used in the calculation of the effective TI.
