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Specifications for Grid Calculations

Grid Specification

Both, the receiver spacing in x- and y-direction and the receiver height are defined in the grid properties. By default, a receiver height of 4 m relative to the ground is predefined.

If you want to restrict a grid calculation just to a certain area, this can be performed by the object "Calculation Area" on the toolbox. With several calculation areas in a project, all active calculation areas are included in the grid calculation.

Object „Calculation Area“ in the toolbox

Grid Appearance

For the appearance of the calculated grid the dialog Appearance (Grid menu) offers the following dialog sections:

  • section „Show“: selection of display mode
  • section „Color Palettes“: selection of color palettes, globally for all evaluation parameters or individually for each evaluation parameter.
  • Level Range and Line Width: This section corresponds with the options available in former releases of CadnaA. The display options resulting from former CadnaA-files will be displayed for color palette „(file)“

Principally, there are three types of grid to appear:

Lines of Equal Sound Level Areas of Equal Sound Level Raster with Oversampling

For large limits and a - with respect to calculation time - coarse grid spacing the mode "Raster" is recommended (default setting).

By use of oversampling the boundaries between the individual colored areas can be smoothed when using the setting „Raster“. This is caused by an interpolation between the grid points actually calculated, though and therefore can lead to a deviation between the calculated level at a fixed receiver point and the sound level on the grid at the same location.

Applying the default configuration settings, the calculation at the grid points does not differ from the calculation at fixed receiver points.

Color Palettes

On the dialog section „Color Palettes“ the color palettes for up to four evaluation parameters and for the ground grid are selected. A color palette is a library object defining the class width, the number of classes and the color to used. With the default setting, the color palette „(default)“ selected for evaluation parameter 1 is applied to all further evaluation parameters and the ground grid.

Note

Detailed information on the definition and use of color palettes is provided in the CadnaA reference manual, Color Palettes. In this introductory manual, just the color palettes as delivered per default in the global library are used.

Example

In this example, the horizontal grid using a calculation area is calculated for a point source.

  • Enter a point source with PWL = 100 dB(A) via the keyboard at (x, y)=(100, 100).
  • Enter a receiver point at (x, y)=(200, 100) in the same way.
  • Scroll down using the vertical scroll bar and the horizontal scroll bar to the left, in order to display the area near the origin in the main window.
  • Select the object "Calculation Area" from the toolbox with the mouse.
  • Draw a calculation area either with the help of the mouse or about the keyboard. Use the following corner coordinates:(x1, y1)=(50, 150); (x2,y2)=(250,150) (x3, y3)=(250, 50); (x4, y4)=(50, 50)
  • Start the grid calculation via the Grid menu, command Calc Grid.

Note

In CadnaA, grid calculations are started separately from receiver point calculations (via the pocket calculator-symbol).

The color palette „(default)“ makes use of the grid appearance setting „Raster, Oversampling=1“:

Selecting a Color Palette

To select a color palette from the global library proceed as follows:

  • Open the dialog Grid Appearance from the Grid menu.

  • Click on the file selector symbol and select by a mouse click from the library table Color Palette (global) the palette with the name „pal_35_85_step_5“ and click the OK button.

  • Clicking the button „Apply“ on dialog Grid Appearance displays the effect caused by the new color palette immediately.

The use of progressive colors is defined for this color palette:

Grid Appearance using the global Color Palette pal_35_85_step_5

Interpolation

The intermediate grid values are interpolated from the grid points. So, grid values can deviate from the receiver point values at those locations. The difference depends on:

- the grid resolution, and
- the distance of the interpolated point from the source.

Example: comparing the level at a receiver 46.1 dB(A) with the interpolated level at the same location on the grid (see status bar, on the lower right): 46.5 dB(A)

Note

The text box (label) showing the level at the receiver point was generated using the command Generate Label from the receiver’s context menu of (via right mouse button, attribute LP1).

  • Double-click alternatively on the receiver to display the daytime level (on dialog Receiver).
  • Select on menu Grid|Appearance for „Grid Points“ the option „Values".

The figure above shows the parameter LP1 at the receiver point [46.1 dB(A)] as well as the value calculated on the grid by means of interpolation on the status bar at the location of the mouse’ arrow head [46.5 dB(A)]. The difference is 0.4 dB in this example.

Level Range

This option is deactivated per default. If it is activated, the specifications for lower/upper limit and class width apply only with the options „Lines of Equal Sound Level“ or „Areas of Equal Sound Level“ selected.

Note

The option „Level Range“ overrides the settings of the color palette selected and should, therefore, not be used with new definitions of the grid appearance. Use color palettes instead.

Progressive Colors

This option may have three modes: deactivated (no tick mark), activated (black tick mark), neutral (grey tick mark). In the neutral mode (default setting) the setting for „Progressive Color“ of the respective color palette applies. Thus, if a progressive color has been defined in the color palette it will be used, and vice versa. With this option deactivated, color palettes with progressive colors show use distinct color with transition. With this option activated, color palettes without progressive colors show them, instead.

Import of old Palette-Files

This button enables to load files with color palettes (file extension *.PAL) from former CadnaA releases. The imported color palette is shown on the list box for the evaluation parameter 1 and simultaneously copied to the local library of color palettes.

Displaying grid points & grid values

Furthermore, grid points and grid values can be displayed:

Dialog Grid Appearance: options for grid points

  • displaying the grid points (grid spacing 10x10m, grid not displayed):

  • displaying the grid values (rounded to integers):

Saving the grid

The following grid formats can be saved (via menu Grid|Save as):

CadnaA, ASCII, LIMA, NMGF

The following grid formats can be loaded (via menu Grid|Open):

CadnaA, ASCII, AUSTAL, ESRI, Immi, LIMA, NMGF (from NMPlot, TNM, INM) and SoundPlan.

By multiple selection several grid files can be opened at once (e.g. with segmented grids).