
<section id='WCGISxWCF_DatumControl_id'>
    <title>
        Datum Control
    </title>
    <para>
        A Datum Control is used to select a geographic datum.
    </para>
    <exhibit>
        <img src='WCF_DatumControl_ScreenCap.png'/>
    </exhibit>
    <para>
        The current datum is displayed in the box. To change it, either
        press the <keytext>Space Bar</keytext>, or press the Select Datum
        button <img src='WCF_ThreeDotsButton_Figure.png'/>. The Select
        Datum dialog box is displayed.
    </para>
    <exhibit>
        <img
        src='WCF_DatumControl_SelectDatumDialogBox_ScreenCap.png'/>
    </exhibit>
    <para>
        The list at the top of the dialog box displays all available datums,
        grouped by continent or ocean (North America, Pacific Ocean, etc.).
        Within these groups, datums are listed alphabetically.
    </para>
    <para>
        Below the list, the following information about the
        selected datum is presented:
    </para>
    <unorderedlist>
        <listitem>
            <para>
                The datum's name
            </para>
        </listitem>
        <listitem>
            <para>
                The applicable geographic region for this datum
            </para>
        </listitem>
        <listitem>
            <para>
                The continent or ocean that contains the datum's applicable region
            </para>
        </listitem>
        <listitem>
            <para>
                The datum's reference ellipsoid
            </para>
        </listitem>
        <listitem>
            <para>
                The method used to convert between this datum and the WGS-84 datum (for
                the 3-parameter Molodensky transformation, the three transformation
                parameters DX, DY, and DZ are displayed)
            </para>
        </listitem>
        <listitem>
            <para>
                An estimate of the worst-case error when converting from this
                datum to WGS-84
            </para>
        </listitem>
    </unorderedlist>
    <para>
        See <iref target='WCGIS_IntroductionToDatums_id'/> for general
        information on datums. See <iref
        target='WCGIS_DEFDATUM_ApplicationOptions_DefaultDatum_id'/>
        for information about the default datum.
    </para>
    <note type='tip'>
        <para>
            Some datums are listed several times, each associated with a different
            area or region. For example, the "North American 1927" datum is
            listed 20 times: "North American 1927 - Alaska (excluding Aleutian
            Islands)", "North American 1927 - Alberta, British Columbia",
            etc. Generally, you should pick the one with the smallest associated region that completely
            encloses your area of interest. For example, if you are working
            with North American 1927 map data for the state of New York, you
            should select "North American 1927 - Contiguous United States
            east of Mississippi River" instead of "North American 1927 - Contiguous
            United States."
        </para>
    </note>
    <note type='tip'>
        <para>
            Datums are often abbreviated using their initials and two-digit
            year. For example, the North American 1927 datum
            is commonly abbreviated as NAD-27.
        </para>
    </note>
</section>

