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:

Shuttle Radar Topography Mission (SRTM) File - The Shuttle Radar Topography Mission collected elevation data using a radar system that was flown by NASA on a space shuttle mission. The coverage is worldwide between the latitudes of 56° south to 60° north.

The data is provided as a series of files with the extension of .hgt. Each file covers one degree of longitude and one degree of latitude. The files follow a naming convention based upon the latitude and longitude of the southwest corner of the area covered by the file. For example, a SRTM file named n40w095.hgt contains elevation data for the portion of the earth's surface between 40° and 41° north latitude and between of 95° and 94° west longitude.

As of October 2009, the latest version of the SRTM data is 2.1, and the official NASA web site for downloading version 2.1 SRTM data is http://dds.cr.usgs.gov/srtm/version2_1/. Documentation is available on this web site describing the SRTM data in detail.
National Elevation Dataset (NED) GridFloat File - The National Elevation Dataset is a United States Geological Survey (USGS) product consisting of the best available elevation data. It is available for the United States and its territories.

The data is supplied through the USGS's seamless map data server. As of October 2009, the link to the seamless server is http://seamless.usgs.gov.

The NED data is provided in several formats. BaseOps reads the GridFloat format.

Using the seamless server website, define your area of interest and choose to download NED GridFloat data. The seamless server will create a .zip file for you to download. Unzip the enclosed files from the .zip file to a directory on your computer. Then, in BaseOps, create a NED elevation data source and select the file with the extension .flt from among the files you unzipped.

Tip:

BaseOps includes a tutorial on using the seamless map data server. Choose Tutorial from BaseOps' Help menu, then select the lesson titled "Using the USGS Seamless Map Data Server".
Noisemap .ELV File - BaseOps can import legacy Noisemap .elv elevation files.

On the data source's options dialog box, in addition to the name of the .elv file, you must also specify the longitude and latitude of the reference point that was used when the .elv file was created.

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.

Same as Noise Grid - The elevation grid will cover the same geographic area as the noise calculation grid. Press the Go to Noise Grid button to view and edit the noise grid properties.

Specify Manually - In the boxes provided, enter the location and size of the elevation grid.

In the Location of Grid Center boxes, type the coordinates of the center of the grid, specified with respect to the case reference point. Typically, the reference point is located near the center of the area of interest, so the grid center location will usually be close to (0, 0).

Note:

The grid location is always specified with respect to the case reference point, regardless of the current case coordinate system.

In the Grid Size boxes, type the dimensions of the elevation grid in the east-west and north-south directions.

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.

Display data sources on map - BaseOps displays the area covered by each elevation data source, tinted in light blue. The area covered by the elevation grid is also displayed, tinted in yellow.

In this example, there are three elevation data sources, covering the northwest, southwest, and southeast potions of the elevation grid. The northeast portion of the elevation grid is not covered by any data source. In that area, the default reference point elevation will be used when constructing the elevation grid. Typically, you will want the entire elevation grid covered by a data source, so you would probably want to add another data source that covers the northeast area.
Display elevation grid on map - BaseOps display the calculated elevation grid on the map using contours.

The elevation grid will be displayed only if there is an up-to-date elevation grid available. If you have not created the elevation grid yet, or if you have edited the elevation grid data since last creating the grid, then the map will display a note reminding you to create the elevation grid.

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:

National Land Cover Data (NLCD) 2001 .bil File - National Land Cover Data is a United States Geological Survey (USGS) product consisting of the best available land cover data.

The data is supplied through the USGS's seamless map data server. As of October 2009, the link to the seamless server is http://seamless.usgs.gov.

Using the seamless server website, define your area of interest and choose to download "NLCD 2001 Land Cover" data in the BIL format. The seamless server will create a .zip file for you to download. Unzip the enclosed files from the .zip file to a directory on your computer. Then, in BaseOps, create a NLCD ground impedance data source and select the file with the extension .bil from among the files you unzipped.

BaseOps ships with a table mapping each of the NLCD land cover codes to a reasonable ground impedance value. You can view and, if desired, edit this table.

In the Ground Impedance column, enter the desired ground impedance value associated with each of the NLCD ground cover codes. The description of each ground cover code, as supplied by the USGS, is displayed for reference.
Digital Line Graph (DLG) Hydrography (HYF) File - Digital Line Graph (DLG) is a map data format developed by the United States Geological Survey (USGS) for storing vector map data. The DLG standard defines several categories of map data, including the hydrography (HYF) category, which pertains to water features. BaseOps can read DLG hydrography files and assign appropriate ground impedance values to those areas where a water feature is present.

BaseOps can read 1:24000 and 1:100000 scale, level 3, optional format UTM DLG files. As of October 2009, the most convenient way to obtain DLG data for the United States was through the WebGIS website at http://www.webgis.com/. Choose to download Digital Line data in the Standard (UTM) format. On the map, drill down to the state and then county desired, then download each of the hydrography DLG files, which will be compressed in a .zip file. Unzip the enclosed .opt file from each .zip file to a directory on your computer. Then, create a DLG ground impedance data source for each .opt file.

You will also need to specify the geographic datum for each DLG file. For DLG files created by the USGS, the datum will be North American 1927.
Noisemap .IMP File - BaseOps can import legacy Noisemap .imp ground impedance files. These are similar to Noisemap .elv files. See File Elevation Data Sources for more information.

"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.