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Wasmer Consulting - BaseOps - User's Guide - Editing Elevation and Ground Impedance Data


BaseOps User's Guide, Chapter 12: Editing Elevation and Ground Impedance Data

When calculating noise levels, the NMap, AAM, and RNM noise models can optionally consider the effects of terrain elevation and ground impedance on noise propagation. When running in terrain mode, the noise models read files that contain the ground elevation and ground impedance values at a grid of points covering the area of interest. The format of these files is described in the documentation accompanying NMap, AAM, and RNM.

BaseOps can create the elevation and ground impedance files for a BaseOps case by using data from a variety of publicly available sources.

Creating an Elevation File

To work with elevation data, choose Elevation from the object type selector dropdown list. Then edit elevation data in the text pane.

Elevation Data Sources

To enter elevation data, you define one or more elevation data sources. Each data source defines the ground elevation for a portion of the earth's surface.

A data source can be an external file (for example, a Shuttle Radar Topography Mission elevation file from NASA). Alternatively, an elevation data source can be a set of manually entered elevation data points.

To add a new data source, press the Add New Data Source button . The Select Elevation Data Source Type dialog box appears.

Select the type of data source you would like to add, then press OK. The data source is added to the list. The types of data sources are described below.

To edit a data source, press the Edit button next to that source. A dialog box appears that allows you to edit the data source's options. The options available will depend on the type of data source.

To delete a data source, select it by clicking on it, then either press the Delete Data Source button , or press Ctrl + Del.

To move a data source up or down in the list, select the data source by clicking on it, then either press the Move Up and Move Down buttons and , or press Ctrl + U and Ctrl + D. The order in which data sources are listed is important. When calculating the elevation at a location, BaseOps searches the elevation data source list for the first data source that applies to that location. If two data sources overlap (i.e., if they both supply an elevation at a given location), then the elevation provided by the topmost data source in the list is used.

File Elevation Data Sources

Most of the data sources are file data source, meaning that they import data from an external file. The options dialog box for each file data source contains a box where the imported file's name is entered.

The following file types are supported:

Manual Elevation Data Source

A manual elevation data source allows you to define the ground's elevation by manually entering elevation points on a map. This allows you to specify elevation data when no external data is available for import. It also allows you to selectively override portions of imported elevation data.

When a manual elevation data source is selected, all of its data points are displayed in the map pane. Use the mouse to add/edit elevation data points on the map.

The region covered by the manual data source consists of the convex hull surrounding all of the points you have defined. This area is tinted with light blue on the map. Using interpolation, BaseOps can calculate the elevation at any location inside the blue-tinted area.

To add a data point, select the Add mouse tool , then click on the map. The Elevation dialog box will appear.

Enter the ground elevation at the new data point's location, then press OK. You can change the units used to specify the elevation: see Setting the Case's Physical Units.

To move a point, drag it using the Select and Edit mouse tool .

To change a existing point's elevation, double-click on it using the Select and Edit mouse tool.

To delete a point, select it using the Select and Edit mouse tool, then press Ctrl + Delete.

Default Elevation

For areas where no elevation data source applies, the ground elevation is assumed to equal the reference point elevation.

Note:

The reference point elevation can also be edited in the Reference Point section of the Case page.

Elevation Grid Options

Elevation information is supplied to the NMap, AAM, and RNM noise models using an elevation grid file, which contains the ground elevations at a grid of regularly spaced locations. You can choose the geographic area covered by this grid, and its resolution.

The Grid Point Spacing is the distance between adjacent grid points (i.e., the grid resolution). Smaller distances increase the ability of the grid to resolve detailed topographic features, but increase noise model run times.

The Grid Location and Size controls the geographic area covered by the elevation grid. Typically, you will want the elevation grid to completely cover the noise calculation grid.

You have two choices for how to specify the grid location and size.

Note:

You can change the units used to specify distance: see Setting the Case's Physical Units.

Map Display

You can choose what is displayed in the map pane when editing elevation data. You have two choices.

Note:

When an elevation data source is selected, the map pane always displays the geographic limits of that particular data source.

Create Elevation Grid

Once you have defined the elevation data sources and set the elevation grid options, press the Create Elevation Grid button. BaseOps will read each of the file data sources, perform any necessary calculations, and write the elevation grid file.

The elevation grid file will be named CASENAME.elv, where CASENAME is the name of the .baseops case file. When the elevation grid is created, BaseOps will automatically enter this file name into the Elevation File section of the Run options. See Elevation and Ground Impedance for more information.

After creating the elevation grid, BaseOps will automatically set the Map Display option to Display elevation grid on map, so that the elevation grid contours are displayed on the map.

Important:

If you create the elevation grid, then edit any of the elevation grid data, press the Create Elevation Grid button again to update the elevation grid.

Creating a Ground Impedance File

Creating a ground impedance file is similar to creating an elevation file. Therefore, only the differences will be described here. You should read Creating an Elevation File before reading this section.

Entering Ground Impedance Values

Ground impedance is a measure of how acoustically soft or hard the ground cover is at a location. Water, for example, is acoustically hard compared to, say, a grass-covered field, so sound will propagate with less attenuation across water.

Typically, you will not know the precise numeric value of the ground impedance at a particular location. Rather, you will know the type of ground cover (for example, bare rock, or deciduous forest).

In BaseOps, you specify ground impedance values by using a drop-down control to select from a list of ground covers.

BaseOps ships with a library of common ground covers. You can edit this library. To add a new ground cover to the library, press the Add button . The Add New Ground Cover dialog box appears.

In the boxes provided, enter a descriptive name for the ground cover, and the corresponding ground impedance value in units of kPa-s/m2.

Important:

In older versions of some noise modeling literature, the units of ground impedance have been erroneously referred to as rayl. This is incorrect. This correct units are kPa-s/m2, which is equivalent to the units of rayl/m.

To remove a ground cover from the library, select it in the dropdown control, then either press the Delete button , or press Ctrl + Del.

File Ground Impedance Data Sources

BaseOps can import ground impedance data from external files. The following file types are supported:

"Calculate From Elevation Grid" Ground Impedance Data Source

A "calculate from elevation grid" ground impedance data source analyzes the elevation grid to determine ground impedance values. In the options dialog box for this data source, a threshold elevation is specified.

For locations where the elevation is less than or equal to the threshold elevation, the ground cover is assumed to be water, and a ground impedance value of 100,000 kPa-s/m2 is used.

For locations where the elevation is greater than the threshold elevation, no ground impedance values are supplied by this data source. For these locations, BaseOps will search for other applicable data sources in the data source list. If none are found, the default ground impedance is used.

If a "calculate from elevation grid" ground impedance data source is used, then the elevation grid must be available when the ground impedance grid is created. See Creating an Elevation File for more information.

Tip:

The "calculate from elevation grid" ground impedance data source is useful for airfields located near large bodies of water (the ocean, or large lakes) where no source of more detailed ground cover data is available.

Manual Ground Impedance Data Sources

A manual ground impedance data source allows you to manually define areas with a particular ground cover by clicking on the map pane. There are similarities to a manual elevation data source, but there is one important difference.

In a manual elevation data source, each point you add specifies the elevation at that point. BaseOps then interpolates to find intermediate elevations between those points.

In a manual ground impedance data source, you define a single ground impedance value for the entire data source. The points you add on the map define one or more polygons enclosing the area where that ground impedance value applies. BaseOps does not interpolate to find intermediate ground impedance values. Typically, this is what you want. Unlike elevation, which usually varies smoothly, ground impedance values usually vary in a stepwise fashion. For example, consider a parking lot surrounding by a grass lawn. At the edge of the parking lot, the ground cover changes instantaneously from grass to concrete.

There are occasional situations where ground impedance does change gradually. For example, a lake in the middle of a grassy field may have a reedy boundary with a ground impedance intermediate between water and grass. If such areas are large enough to be significant, they can be dealt with by defining a new ground cover representing the reedy area.

After adding a new manual ground impedance data source, the data source's options dialog box appears.

Select the ground cover for this data source and, optionally, provide a descriptive name describing the source. Then use the mouse to add ground impedance data points on the map. The points define one or more polygons enclosing the area that has the ground cover you selected.

To add a point, select the Add mouse tool , then click on the map. The new point will be added to the current polygon. To begin a new polygon, hold down the Shift key while you click.

Arrowheads indicate each polygon's direction. It is important to know a polygon's direction when you add a point to it --- the new point is added directly after the selected point. The selected point is highlighted in yellow. To select a point, click on it using the Select and Edit mouse tool .

To move a point, drag it using the Select and Edit mouse tool.

To delete a point, select it using the Select and Edit mouse tool, then press Ctrl + Delete.

Determining Elevation and Ground Impedance Values at a Location

If the mouse is over the map pane, the elevation and ground impedance values at the mouse's location are displayed on the status bar. See Status Bar for more information.

If the elevation and/or ground impedance grids have not been created (or if they are out of date), then the default elevation and/or ground impedance values are displayed.


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