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Wasmer Consulting - NMPlot - Adding Noisemap and INM Grids

Adding Noisemap and INM Noise Grids with NMPlot

Currently, there are two computer programs that are commonly used to calculate aircraft noise levels around airports: the United States Air Force's Noisemap, and the United States Federal Aviation Administration's Integrated Noise Model (INM). Both of these programs can calculate noise from both civilian and military aircraft. However, when calculating the noise at joint civilian-military airports, it is often easier to use Noisemap for military aircraft and INM for civilian aircraft. The result is two independent grids: the INM grid, containing the civilian noise levels, and the Noisemap grid, containing the military noise levels.

Wasmer Consulting is often asked if NMPlot can be used to sum these two grids: i.e., to create a new grid that contains the total (civilian + military) noise levels. The answer is yes. However, since INM does not include georeferencing information in its grid files, the process is somewhat involved, as you must manually add the missing georeferencing information.

Step by Step Instructions

  1. You will need...

    • The grid file created by Noisemap. The file will be named <casename>.grd, where <casename> is the name of your noise analysis case. It is created when you run Noisemap. If there is a .bps file with the same name, you will need it also.

    • The grid file created by INM. This file will be named nmplot.grd. It is created when you use INM to display noise contours.

    • A reference point for the INM grid. A reference point is a location whose coordinates are known in both a) degrees of longitude and latitude, and b) INM's X-Y coordinate system. Pick a location near the center of the INM grid. The end of one of the runways is often a good choice.

    • The NMPlot application, available at http://wasmerconsulting.com/nmplot.htm.

    • A text editor application. Microsoft Window's Notepad accessory should work fine.

  2. You must convert the INM grid into a text format that you can easily edit. Start NMPlot, and open the INM grid. Choose Save As from the File menu. The Save As dialog box is displayed. Choose NMAGF ASCII Grid Format for the save as type. Press Save, overwriting the old INM grid file. Close NMPlot.

  3. Using a text editor, open the INM grid. Search for a line that begins with the text "{CART". For example...

    {CART 0.0 0.0 0 0 FEET 0}

    If this line does not exist, add it.

  4. Edit this line so that it specifies the coordinates of your INM reference point. The first pair of numbers should be the east longitude and north latitude of the reference point, in decimal degrees. The second pair of numbers should be the X and Y coordinates of the reference point.

    For example, suppose that your INM reference point has a longitude of 118 degrees west, a latitude of 34 degrees north, a X coordinate of 3000, and a Y coordinate of 1000. Then, your edited CART line should look like this.

    {CART -118.0 34.0 3000.0 1000.0 FEET 0}


    The longitude is in degrees east, so west longitudes are negative.

  5. Save the modified INM grid, then close the text editor. The INM grid now contains georeferencing information.

  6. Start NMPlot, then open the INM grid.

  7. Choose Combine Grids from the Grid menu. The Combine Grids dialog box appears.

  8. For File containing second grid, enter the name of the Noisemap grid file.

  9. For Method used to combine the data points, choose Add Noise Decibels.

  10. For The defined area polygon of the new grid should be, choose the intersection of the two existing grids' defined area polygons.

  11. For both of the Data points settings, choose not be included in the grid.

  12. For If a data point in the current grid is located within, enter the desired tolerance, in meters. A suggested value is one half the grid point spacing in your Noisemap grid. For example, if your Noisemap grid spacing is 1000 feet, enter 500 feet (152 meters).


    The tolerance is set in meters. Noisemap grid spacings are often expressed using feet, so you will need to manually convert the units.

  13. Press the OK button. The grids are summed.

  14. Choose Save as from the File, the save the grid to a new file. This is the combined grid, containing the sum of the Noisemap and INM noise levels. You can now use NMPlot to plot contours of the combined noise.


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