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Generating a Grid from Noise Contours

Starting from lines of equal sound level imported from a file, a level grid can be generated in CadnaA. The resulting grid may - for example - be summed up energetically with an existing grid (see Grid Arithmetic).

For example, the noise contours are imported from the DXF file format as terrain contours with their heights corresponding to the level. Afterwards, a ground grid is generated from these terrain contours.

Examples

Lines with height = level

Proceed as follows based on an empty CadnaA file:

Example

Path: Examples\Immissions\grid.dxf

  • Select via the File|Import menu the import format „AutoCAD DXF“.
  • Specifiy via the „Options“ button the lines to get imported as terrain contours (Layer name = *).

  • After import, delete the terrain contour being an extension of the runway to both sides.
  • Select - if required - the command „Zoom to Limits“ from the toolbox.
  • Convert the outer, rectangular polygon into a calculation area via its context menu.
  • Select the „Ground“ grid from the list box on the symbol bar.
  • If necessary, specify the class width and upper and lower limits and grid spacing (menu Grid|Appearance and Specification).
  • Calculate the ground grid via the Grid|Calc Grid menu.

Note

No source is required when just calculating the ground grid.

Calculated ground grid based on imported terrain contours

Now, the ground level grid complies with a „level grid“ that can be saved and combined with other grids.

  • Save the ground grid via Grid|Save as menu - for example with the file name aircraft.cnr.
  • Now, deactivate the terrain contours using the Modify Objects dialog.
  • Enter a point source with PWL=100 dB(A) and a road with reasonable emission, both at the perimeter of the ground grid (i.e., at low levels).
  • Select the evaluation parameter „Day“ and recalculate the grid.
  • From the Grid menu, open the Grid Arithmetic dialog and load the grid aircraft.cnr in the line „R1“.
  • Enter the following expression for „Day“ to generate the new summation grid:r0++r1gHere r0 denotes the current grid (for the point source and the road) and r1g the stored ground grid for the aircraft noise.

After clicking on OK the calculation of the new sum grid will be executed.

As can be seen, the grid calculation also generates (invalid) levels outside the calculation area. This is due to the calculation areas are internally defined and handled as rectangular areas. Thus, non-axis-parallel calculation areas show invalid grid points outside which have to be deleted.

  • To this end, select the command Delete Grid Points|outside Polygon from the context menu of the calculation area.

Thereafter, the final summation grid looks like this:

Lines with height as attribute

In case the level value is not available by a height information, but within an object attribute, either as text or as a numerical value (for example as part of the ID), it is possible to extract the numerical value.

Example

Path: Examples\Immissions\grid_HA_within_ID.cna

  • Open the referenced file.
  • Double-click on a contour line.

The level is part of the ID. The z column in the Geometry dialog is zero.

Height line with level 55 dB in the ID

On the table Contour Line it can be checked that this applies to all existing terrain contours.

In this example, proceed as follows:

  • Open the table Contour Line (if not already).
  • Click with the right mouse button in the „ID“ column and select the context menu command Change Column.
  • On the dialog, in the „Replace Strings“ section, enter „Find what“:The string to be searched for is enclosed in parentheses and can thus be addressed in the field „Replace with“. The expression „\2“ returns the first expression within parentheses from the „Find What“ box, the rest is lost.

Note

For more information on the Change Column dialog see Changing Column Content.

Subsequently, the values in column ID are transferred to the height attribute HA.

  • Right-click on an empty space in the main window and select the context menu command Modify Objects.
  • Select the action „Change Attributes“ for the object type „Contour Line“ and confirm with OK.
  • On the dialog Modify Attribute, select the attribute HA (Height of fist Point) and enter the string ID into input box „Arithmetic, New Value“.

Note

Alternatively, the attribute can be selected via the double-arrow key.

  • Close the dialog with OK and confirm the following dialog with „All“.
  • Now, display the 3D view by clicking the respective icon on the symbol bar.

The last action updates all height information, also transferring the values of the attribute HA to the z column in the geometry table.

  • Check this by double-clicking on a contour line and then clicking the „Geometry“ button.
  • Now, proceed as described in section „Lines with height = level“ to calculate the ground grid. In this example, the ground grid was calculated with a grid spacing of 10 x 10 m.

Terrain contours representing noise contours (iso-dB-lines)

Level grid (areas with equal sound level): The calculation area was generated from a duplicate of the outer noise contour using the Convert to command.