
<section id='NMPlot_OverlayingGrids_id'>
    <title>
        Overlaying Grids
    </title>
    <para>
        Using Grid Overlay <link
        target='#WCGIS_MapBackground_Layers_id'>background layers</link>,
        you can overlay the plot elements from two or more grids. For
        instance, you can create a plot that displays contours from two
        different grids. This is useful for comparing grids representing
        alternative scenarios.
    </para>
    <para>
        As an example, assume that you have two grids. The first represents
        the existing noise levels around an airport, while the second
        represents the predicted levels a few years in the future, after
        air traffic has increased. You create a plot that displays both
        the current and predicted noise level contours. This plot clearly
        shows how far the noise contours are expected to grow. See the
        sample plot included with NMPlot, <filetext>Sample Plot - Contour
        Overlay.nmp</filetext>, for an example.
    </para>
    <exhibit>
        <img src='NMPlot_Figure_SamplePlot_ContourOverlay.png'/>
    </exhibit>
    <para>
        To create a plot overlaying two grids, follow these steps.
    </para>
    <orderedlist>
        <listitem>
            <para>
                Create a new plot, and set its <link
                target='#NMPlot_PrimaryGrid_id'>primary grid</link> equal to the
                first of the grids you want to overlay.
            </para>
        </listitem>
        <listitem>
            <para>
                Add a new Grid Overlay background layer to your plot. See <iref
                target='WCGIS_MapBackground_Layers_id'/> for instructions.
            </para>
        </listitem>
        <listitem>
            <para>
                Click on the new layer's Options button. The Grid Overlay Options
                dialog box appears.
            </para>
            <exhibit>
                <img src='NMPlot_ScreenCap_GridOverlayOptionsDialogBox.png'/>
            </exhibit>
        </listitem>
        <listitem>
            <para>
                The Grid Overlay Options dialog box looks very similar to the
                <link target='#NMPlot_PlotOptionsDialogBox_id'>Plot Options dialog
                box</link>. In fact, with the exception that some pages are missing,
                it is nearly identical. Set the options for the overlay just as
                you would set the options for a regular plot. Set the overlay
                grid (which is the Grid Overlay's equivalent to a plot's <link
                target='#NMPlot_PrimaryGrid_id'>primary grid</link>) equal to
                the second of the grids you want to overlay.
            </para>
            <note type='note'>
                <para>
                    The missing pages on the Grid Overlay Options dialog box contain
                    options that are not applicable to an overlay. For example, the
                    <link target='#NMPlot_PlotOrientation_id'>Plot Orientation</link>
                    page is missing, as it is nonsensical to have the overlay oriented
                    differently than the main plot.
                </para>
            </note>
        </listitem>
    </orderedlist>
    <note type='tip'>
        <para>
            You can overlay as many grids as you wish. Take care, however,
            as overlaying more than two sets of contours can result in a cluttered
            plot that is difficult to interpret. A plot overlaying contours
            from several grids works best if you make one set of contours
            very bold (use thick, solid lines in a dark color), and make the
            rest of the contours more subtle (use thin, possibly dashed lines
            in lighter colors).
        </para>
        <exhibit>
            <img src='NMPlot_Figure_OverlayingMultipleContourSets.png'/>
        </exhibit>
    </note>
</section>
