
<section id='BaseOps_ElevationAndImpedanceChunks_id'
webfilename='topography'>
    <title>
        Editing Elevation and Ground Impedance Data
    </title>
    <para>
        When calculating noise levels, the NMap, AAM, AGM, and RNM noise
        models can 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.
    </para>
    <para>
        BaseOps can create the elevation and ground impedance files for
        a BaseOps case by using data from a variety of publicly available
        sources.
    </para>
    <note type='note'>
        <para>
            Terrain support is not optional when running the AGM noise model;
            it requires that the elevation and impedance grids be present.
        </para>
        <para>
            In addition, AGM requires that the elevation, impedance, and noise
            calculation grids all cover the same geographic area. The easiest
            way to achieve this is to first set the parameters of the <link
            target='#BaseOps_GridChunk_id'>noise calculation grid</link>.
            Then, when creating both the elevation and impedance grids, set
            "Grid Location and Size" to "Same as Noise Grid".
        </para>
    </note>
    <section
    id='BaseOps_ElevationAndImpedanceChunks_CreatingElevationFile_id'>
        <title>
            Creating an Elevation File
        </title>
        <para>
            To work with elevation data, choose Elevation from the <link
            target='#BaseOps_ListPane_id'>object type selector dropdown list</link>.
            Then edit elevation data in the <link
            target='#WCF_ApplicationMainWindow_id'>text pane</link>.
        </para>
        <section
        id='BaseOps_ElevationAndImpedanceChunks_ElevationDataSource_id'>
            <title>
                Elevation Data Sources
            </title>
            <exhibit>
                <img src='BaseOps_ElevationChunk_DataSources_ScreenCap.png'/>
            </exhibit>
            <para>
                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.
            </para>
            <para>
                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.
            </para>
            <para>
                To add a new data source, press the Add New Data Source button
                <img src='intres:BaseOps_AddPlusSignSmallButtonBitmap'/>. The
                Select Elevation Data Source Type dialog box appears.
            </para>
            <exhibit>
                <img
                src='BaseOps_ElevationChunk_SelectDataSourceTypeDialogBox_ScreenCap.png'/>
            </exhibit>
            <para>
                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.
            </para>
            <para>
                To edit a data source, press the Edit button <img
                src='WCF_ThreeDotsButton_Figure.png'/> 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.
            </para>
            <para>
                To delete a data source, select it by clicking on it, then either
                press the Delete Data Source button <img
                src='intres:WCF_DeleteCrossToolbarButtonBitmap'/>, or press <keytext>Ctrl
                + Del</keytext>.
            </para>
            <para>
                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 <img src='intres:WCF_MoveUpToolbarButtonBitmap'/>
                and <img src='intres:WCF_MoveDownToolbarButtonBitmap'/>, or press
                <keytext>Ctrl + U</keytext> and <keytext>Ctrl + D</keytext>. 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.
            </para>
            <section
            id='BaseOps_ElevationAndImpedanceChunks_ImpedanceDataSource_File_id'>
                <title>
                    File Elevation Data Sources
                </title>
                <para>
                    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.
                </para>
                <exhibit>
                    <img
                    src='BaseOps_ElevationChunk_OptionsDialogBox_NED_ScreenCap.png'/>
                </exhibit>
                <para>
                    The following file types are supported:
                </para>
                <unorderedlist>
                    <listitem>
                        <para>
                            <bold>Shuttle Radar Topography Mission (SRTM) File</bold> - 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.
                        </para>
                        <para>
                            The data is provided as a series of files with the extension of
                            <filetext>.hgt</filetext>. 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
                            <filetext>n40w095.hgt</filetext> 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.
                        </para>
                        <para>
                            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 <link>http://dds.cr.usgs.gov/srtm/version2_1/</link>.
                            Documentation is available on this web site describing the SRTM
                            data in detail.
                        </para>
                    </listitem>
                    <listitem>
                        <para>
                            <bold>National Elevation Dataset (NED) GridFloat File</bold> -
                            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.
                        </para>
                        <para>
                            The data is supplied through the USGS's seamless map data server.
                            As of January 2019, the link to the seamless server is <link>https://viewer.nationalmap.gov/advanced-viewer/</link>.
                        </para>
                        <para>
                            The NED data is provided in several formats. BaseOps reads the
                            GridFloat format.
                        </para>
                        <para>
                            Using the seamless server website, define your area of interest
                            and choose to download NED GridFloat data. The seamless server
                            will create a <filetext>.zip</filetext> file for you to download.
                            Unzip the enclosed files from the <filetext>.zip</filetext> file
                            to a directory on your computer. Then, in BaseOps, create a NED
                            elevation data source and select the file with the extension <filetext>.flt</filetext>
                            from among the files you unzipped.
                        </para>
                        <note type='tip'>
                            <para>
                                BaseOps includes a tutorial on using the seamless map data server.
                                Choose <menutext>Tutorial</menutext> from BaseOps' <menutext>Help</menutext>
                                menu, then select the lesson titled "Using the USGS Seamless Map
                                Data Server".
                            </para>
                        </note>
                    </listitem>
                    <listitem>
                        <para>
                            <bold>Noisemap .ELV File</bold> - BaseOps can import legacy Noisemap
                            <filetext>.elv</filetext> elevation files.
                        </para>
                        <para>
                            On the data source's options dialog box, in addition to the name
                            of the <filetext>.elv</filetext> file, you must also specify the
                            longitude and latitude of the reference point that was used when
                            the <filetext>.elv</filetext> file was created.
                        </para>
                        <exhibit>
                            <img
                            src='BaseOps_ElevationChunk_OptionsDialogBox_ELV_ScreenCap.png'/>
                        </exhibit>
                    </listitem>
                </unorderedlist>
            </section>
            <section
            id='BaseOps_ElevationAndImpedanceChunks_ImpedanceDataSource_Manual_id'>
                <title>
                    Manual Elevation Data Source
                </title>
                <para>
                    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.
                </para>
                <para>
                    When a manual elevation data source is selected, all of its data
                    points are displayed in the <link
                    target='#WCF_ApplicationMainWindow_id'>map pane</link>. Use the
                    mouse to add/edit elevation data points on the map.
                </para>
                <exhibit>
                    <img
                    src='BaseOps_ElevationChunk_ManualDataSource_PointsOnMap_ScreenCap.png'/>
                </exhibit>
                <para>
                    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.
                </para>
                <para>
                    To add a data point, select the Add mouse tool <img
                    src='intres:BaseOps_AddTool_ToolbarButton'/>, then click on the
                    map. The Elevation dialog box will appear.
                </para>
                <exhibit>
                    <img
                    src='BaseOps_ElevationChunk_ManualDataSource_ElevationDialogBox_ScreenCap.png'/>
                </exhibit>
                <para>
                    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 <iref target='BaseOps_CaseUnits_id'/>.
                </para>
                <para>
                    To move a point, drag it using the Select and Edit mouse tool
                    <img src='intres:WCF_ArrowMouseCursorToolbarButtonBitmap'/>.
                </para>
                <para>
                    To change a existing point's elevation, double-click on it using
                    the Select and Edit mouse tool.
                </para>
                <para>
                    To delete a point, select it using the Select and Edit mouse tool,
                    then press <keytext>Ctrl + Delete</keytext>.
                </para>
            </section>
        </section>
        <section
        id='BaseOps_ElevationAndImpedanceChunks_DefaultElevation_id'>
            <title>
                Default Elevation
            </title>
            <exhibit>
                <img
                src='BaseOps_ElevationChunk_DefaultElevation_ScreenCap.png'/>
            </exhibit>
            <para>
                For areas where no elevation data source applies, the ground elevation
                is assumed to equal the reference point elevation.
            </para>
            <note type='note'>
                <para>
                    The reference point elevation can also be edited in the Reference
                    Point section of the <link target='#BaseOps_CaseChunk_id'>Case
                    page</link>.
                </para>
            </note>
        </section>
        <section
        id='BaseOps_ElevationAndImpedanceChunks_ElevationGridOptions_id'>
            <title>
                Elevation Grid Options
            </title>
            <exhibit>
                <img src='BaseOps_ElevationChunk_GridOptions_ScreenCap.png'/>
            </exhibit>
            <para>
                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.
            </para>
            <para>
                The <italic>Grid Point Spacing</italic> 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.
            </para>
            <para>
                The <italic>Grid Location and Size</italic> controls the geographic
                area covered by the elevation grid. Typically, you will want the
                elevation grid to completely cover the <link
                target='#BaseOps_GridChunk_id'>noise calculation grid</link>.
            </para>
            <para>
                You have two choices for how to specify the grid location and
                size.
            </para>
            <unorderedlist>
                <listitem>
                    <para>
                        <italic>Same as Noise Grid</italic> - The elevation grid will
                        cover the same geographic area as the <link
                        target='#BaseOps_GridChunk_id'>noise calculation grid</link>.
                        Press the <italic>Go to Noise Grid</italic> button to view and
                        edit the noise grid properties.
                    </para>
                </listitem>
            </unorderedlist>
            <unorderedlist>
                <listitem>
                    <para>
                        <italic>Specify Manually</italic> - In the boxes provided, enter
                        the location and size of the elevation grid.
                    </para>
                    <para>
                        In the <italic>Location of Grid Center</italic> boxes, type the
                        coordinates of the center of the grid, specified with respect
                        to the <link target='#BaseOps_CaseReferencePoint_id'>case reference
                        point</link>. 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).
                    </para>
                    <note type='note'>
                        <para>
                            The grid location is always specified with respect to the case
                            reference point, regardless of the current <link
                            target='#BaseOps_CaseCoordinateSystem_id'>case coordinate system</link>.
                        </para>
                    </note>
                    <para>
                        In the <italic>Grid Size</italic> boxes, type the dimensions of
                        the elevation grid in the east-west and north-south directions.
                    </para>
                </listitem>
            </unorderedlist>
            <note type='note'>
                <para>
                    You can change the units used to specify distance: see <iref
                    target='BaseOps_CaseUnits_id'/>.
                </para>
            </note>
        </section>
        <section
        id='BaseOps_ElevationAndImpedanceChunks_MapDisplay_id'>
            <title>
                Map Display
            </title>
            <exhibit>
                <img src='BaseOps_ElevationChunk_MapDisplay_ScreenCap.png'/>
            </exhibit>
            <para>
                You can choose what is displayed in the map pane when editing
                elevation data. You have two choices.
            </para>
            <unorderedlist>
                <listitem>
                    <para>
                        <italic>Display data sources on map</italic> - BaseOps displays
                        the area covered by each <link
                        target='#BaseOps_ElevationAndImpedanceChunks_ElevationDataSource_id'>elevation
                        data source</link>, tinted in light blue. The area covered by
                        the <link
                        target='#BaseOps_ElevationAndImpedanceChunks_ElevationGridOptions_id'>elevation
                        grid</link> is also displayed, tinted in yellow.
                    </para>
                    <exhibit>
                        <img
                        src='BaseOps_ElevationChunk_MapDisplay_DataSources_ScreenCap.png'/>
                    </exhibit>
                    <para>
                        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 <link
                        target='#BaseOps_ElevationAndImpedanceChunks_DefaultElevation_id'>default
                        reference point elevation</link> 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.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <italic>Display elevation grid on map</italic> - BaseOps display
                        the calculated elevation grid on the map using contours.
                    </para>
                    <exhibit>
                        <img
                        src='BaseOps_ElevationChunk_MapDisplay_ElevationGrid_ScreenCap.png'/>
                    </exhibit>
                    <para>
                        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.
                    </para>
                </listitem>
            </unorderedlist>
            <note type='note'>
                <para>
                    When an <link
                    target='#BaseOps_ElevationAndImpedanceChunks_ElevationDataSource_id'>elevation
                    data source</link> is selected, the <link
                    target='#WCF_ApplicationMainWindow_id'>map pane</link> always
                    displays the geographic limits of that particular data source.
                </para>
            </note>
        </section>
        <section
        id='BaseOps_ElevationAndImpedanceChunks_CreateElevationGrid_id'>
            <title>
                Create Elevation Grid
            </title>
            <exhibit>
                <img
                src='BaseOps_ElevationChunk_CreateElevationGrid_ScreenCap.png'/>
            </exhibit>
            <para>
                Once you have defined the <link
                target='#BaseOps_ElevationAndImpedanceChunks_ElevationDataSource_id'>elevation
                data sources</link> and set the <link
                target='#BaseOps_ElevationAndImpedanceChunks_ElevationGridOptions_id'>elevation
                grid options</link>, 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.
            </para>
            <para>
                The elevation grid file will be named <filetext>CASENAME.elv</filetext>,
                where <filetext>CASENAME</filetext> is the name of the <filetext>.baseops</filetext>
                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 <iref
                target='BaseOps_RunChunk_ElevationAndGroundImpedance_id'/> for
                more information.
            </para>
            <para>
                After creating the elevation grid, BaseOps will automatically
                set the <link
                target='#BaseOps_ElevationAndImpedanceChunks_MapDisplay_id'>Map
                Display option</link> to <italic>Display elevation grid on map</italic>,
                so that the elevation grid contours are displayed on the map.
            </para>
            <note type='important'>
                <para>
                    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.
                </para>
            </note>
        </section>
    </section>
    <section>
        <title>
            Creating a Ground Impedance File
        </title>
        <para>
            Creating a ground impedance file is similar to creating an elevation
            file. Therefore, only the differences will be described here.
            You should read <iref
            target='BaseOps_ElevationAndImpedanceChunks_CreatingElevationFile_id'/>
            before reading this section.
        </para>
        <section>
            <title>
                Entering Ground Impedance Values
            </title>
            <para>
                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.
            </para>
            <para>
                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).
            </para>
            <para>
                In BaseOps, you specify ground impedance values by using a drop-down
                control to select from a list of ground covers.
            </para>
            <exhibit>
                <img
                src='BaseOps_ElevationChunk_GroundImpedanceControl_ScreenCap.png'/>
            </exhibit>
            <para>
                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 <img
                src='intres:BaseOps_AddPlusSignSmallButtonBitmap'/>. The Add New
                Ground Cover dialog box appears.
            </para>
            <exhibit>
                <img
                src='BaseOps_ElevationChunk_AddGroundCoverDialogBox_ScreenCap.png'/>
            </exhibit>
            <para>
                In the boxes provided, enter a descriptive name for the ground
                cover, and the corresponding ground impedance value in units of
                kPa-s/m<superscript>2</superscript>.
            </para>
            <note type='important'>
                <para>
                    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/m<superscript>2</superscript>,
                    which is equivalent to the units of rayl/m.
                </para>
            </note>
            <para>
                To remove a ground cover from the library, select it in the dropdown
                control, then either press the Delete button <img
                src='intres:WCF_DeleteCrossToolbarButtonBitmap'/>, or press <keytext>Ctrl
                + Del</keytext>.
            </para>
        </section>
        <section>
            <title>
                File Ground Impedance Data Sources
            </title>
            <para>
                BaseOps can import ground impedance data from external files.
                The following file types are supported:
            </para>
            <unorderedlist>
                <listitem>
                    <para>
                        <bold>National Land Cover Data (NLCD) 2001 .bil File</bold> -
                        National Land Cover Data is a United States Geological Survey
                        (USGS) product consisting of the best available land cover data.
                    </para>
                    <para>
                        The data is supplied through the USGS's seamless map data server.
                        As of January 2019, the link to the seamless server is <link>https://viewer.nationalmap.gov/advanced-viewer/</link>.
                    </para>
                    <para>
                        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 <filetext>.zip</filetext>
                        file for you to download. Unzip the enclosed files from the <filetext>.zip</filetext>
                        file to a directory on your computer. Then, in BaseOps, create
                        a NLCD ground impedance data source and select the file with the
                        extension <filetext>.bil</filetext> from among the files you unzipped.
                    </para>
                    <para>
                        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.
                    </para>
                    <exhibit>
                        <img
                        src='BaseOps_GroundImpedanceChunk_NLCD2001DataSourceDialogBox_ScreenCap.png'/>
                    </exhibit>
                    <para>
                        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.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <bold>Digital Line Graph (DLG) Hydrography (HYF) File</bold> -
                        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.
                    </para>
                    <para>
                        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 <link>http://www.webgis.com/</link>. 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 <filetext>.zip</filetext>
                        file. Unzip the enclosed <filetext>.opt</filetext> file from each
                        <filetext>.zip</filetext> file to a directory on your computer.
                        Then, create a DLG ground impedance data source for each <filetext>.opt</filetext>
                        file.
                    </para>
                    <para>
                        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.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <bold>Noisemap .IMP File</bold> - BaseOps can import legacy Noisemap
                        <filetext>.imp</filetext> ground impedance files. These are similar
                        to Noisemap <filetext>.elv</filetext> files. See <iref
                        target='BaseOps_ElevationAndImpedanceChunks_ImpedanceDataSource_File_id'/>
                        for more information.
                    </para>
                </listitem>
            </unorderedlist>
        </section>
        <section>
            <title>
                "Calculate From Elevation Grid" Ground Impedance Data Source
            </title>
            <para>
                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.
            </para>
            <exhibit>
                <img
                src='BaseOps_GroundImpedanceChunk_ElevationGridDataSourceDialogBox_ScreenCap.png'/>
            </exhibit>
            <para>
                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/m<superscript>2</superscript>
                is used.
            </para>
            <para>
                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.
            </para>
            <para>
                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 <iref
                target='BaseOps_ElevationAndImpedanceChunks_CreatingElevationFile_id'/>
                for more information.
            </para>
            <note type='tip'>
                <para>
                    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.
                </para>
                <exhibit>
                    <img
                    src='BaseOps_GroundImpedanceChunk_ElevationGridDataSource_Sample_Figure.png'/>
                </exhibit>
            </note>
        </section>
        <section>
            <title>
                Manual Ground Impedance Data Sources
            </title>
            <para>
                A manual ground impedance data source allows you to manually define
                areas with a particular ground cover by clicking on the <link
                target='#WCF_ApplicationMainWindow_id'>map pane</link>. There
                are similarities to a <link
                target='#BaseOps_ElevationAndImpedanceChunks_ImpedanceDataSource_Manual_id'>manual
                elevation data source</link>, but there is one important difference.
            </para>
            <para>
                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.
            </para>
            <para>
                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.
            </para>
            <para>
                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.
            </para>
            <para>
                After adding a new manual ground impedance data source, the data
                source's options dialog box appears.
            </para>
            <exhibit>
                <img
                src='BaseOps_GroundImpedanceChunk_ManualDataSourceDialogBox_ScreenCap.png'/>
            </exhibit>
            <para>
                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.
            </para>
            <para>
                To add a point, select the Add mouse tool <img
                src='intres:BaseOps_AddTool_ToolbarButton'/>, then click on the
                map. The new point will be added to the current polygon. To begin
                a new polygon, hold down the <keytext>Shift</keytext> key while
                you click.
            </para>
            <para>
                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 <img
                src='intres:WCF_ArrowMouseCursorToolbarButtonBitmap'/>.
            </para>
            <exhibit>
                <img
                src='BaseOps_GroundImpedanceChunk_ManualDataSource_PolygonExample_ScreenCap.png'/>
            </exhibit>
            <para>
                To move a point, drag it using the Select and Edit mouse tool.
            </para>
            <para>
                To delete a point, select it using the Select and Edit mouse tool,
                then press Ctrl&nbsp;+&nbsp;Delete.
            </para>
        </section>
    </section>
    <section>
        <title>
            Determining Elevation and Ground Impedance Values at a Location
        </title>
        <para>
            If the mouse is over the <link
            target='#WCF_ApplicationMainWindow_id'>map pane</link>, the elevation
            and ground impedance values at the mouse's location are displayed
            on the status bar. See <iref
            target='WCF_ApplicationMainStatusBar_id'/> for more information.
        </para>
        <exhibit>
            <img src='BaseOps_StatusBar_MouseLocation_ScreenCap.png'/>
        </exhibit>
        <para>
            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.
        </para>
    </section>
</section>
