
[-defdefine SettingDefaultDatumSection ''-]
<section type='appendix' id='WCGIS_IntroductionToDatums_id'
webfilename='introductiontodatums'>
    <title>
        Introduction to Datums
    </title>
    <section>
        <title>
            What is a Datum?
        </title>
        <para>
            A datum is a detailed survey of a country, continent, or some
            other portion of the Earth's surface. As part of the survey, the
            longitudes and latitudes of a large number of points are measured
            using the best available methods. Once the survey is complete,
            maps of the region can be constructed.
        </para>
        <para>
            Over time, the accuracy of surveying methods has steadily improved.
            Therefore, most regions of the Earth of been surveyed many times,
            with increasingly accuracy. Each of these surveys is referred
            to as a datum. With each new datum, the longitude and latitude
            of a given point on the Earth will change. This change can be
            as large as several hundred meters.
        </para>
    </section>
    <section>
        <title>
            Why are Datums Important?
        </title>
        <para>
            Adopting a new datum is expensive. Maps must be reprinted. Data
            files must be converted. Computer programs must be modified. Therefore,
            at any given time, more than one datum will likely be in use for
            a given region.
        </para>
        <para>
            For example, in the United States, both the North American Datum
            of 1927 (NAD-27) and the World Geodetic System of 1984 (WGS-84)
            are in widespread use. In addition, maps and data in numerous
            lesser-used datums are also available.
        </para>
        <para>
            For this reason, longitude and latitude alone are insufficient
            to describe the location of a point with an accuracy better than
            a few hundred meters. If greater accuracy is required, another
            piece of information is needed: the datum.
        </para>
        <note type='note'>
            <para>
                Even if you are certain that all of your data and maps are in
                a single, consistent datum, you still may need to know what that
                datum is. For example, the datum is required in order to convert
                from Universal Transverse Mercator (UTM) coordinates to longitude
                and latitude. This is because the conversion depends on the ellipsoid
                (a mathematical model of the Earth's shape), and the ellipsoid
                is part of the datum.
            </para>
        </note>
    </section>
    <section>
        <title>
            Doesn't WGS-84 Solve the Datum Problem?
        </title>
        <para>
            Most datums are local, intended to be used for only a portion
            of the Earth's surface. However, in recent decades, improved surveying
            techniques have enabled global datums to be established. These
            datums can be used to accurately map the entire Earth.
        </para>
        <para>
            The most recent, and most accurate, datum is the World Geodetic
            System of 1984 (WGS-84). While future improvements in surveying
            techniques may result in modifications to this datum, these are
            expected to be minor: the coordinates of locations should shift
            by only a few centimeters. Therefore, for most practical applications,
            WGS-84 represents the ultimate datum.
        </para>
        <para>
            Once all maps and data are converted to WGS-84, the problems associated
            with multiple datums will disappear. Therefore, mapping organizations
            throughout the world are converting to WGS-84.
        </para>
        <para>
            Unfortunately, the goal of a single global datum has not yet been
            attained. Converting to a new datum is expensive and time-consuming.
            Therefore, the conversion to WGS-84 is proceeding slowly, and
            may take decades. Certainly for the foreseeable future, the existence
            of multiple datums will be a reality. Anyone who works with geographic
            data must be prepared to deal with this fact.
        </para>
    </section>
    <section>
        <title>
            How &app_name; Converts Between Datums
        </title>
        <para>
            &app_name; can convert between most common datums. The 3-parameter
            Molodensky transformation is used for this purpose. Transformation
            parameters were taken from <italic>Department of Defense World
            Geodetic System 1984&emdash;Its Definition and Relationships with
            Local Geodetic Systems</italic>, NIMA Technical Report TR8350.2,
            Third Edition, Amendment 1, 3 January 2000.
        </para>
    </section>
    <section>
        <title>
            Recommendations
        </title>
        <para>
            If errors of several hundred meters are acceptable, you can ignore
            the entire issue of datums. However, if you require greater accuracy,
            then you must know the datum of any geographic data you use.
        </para>
        <para>
            Whenever you obtain geographic data (maps, computer files, tables
            of locations), make sure that you determine the datum of that
            data. For maps, the datum can often be found printed in the legend
            area of the map. For computer files, the datum is often listed
            in the accompanying documentation. If you cannot determine the
            datum, ask your source. Any reputable source of geographic data
            should be able to supply this information.
        </para>
        <para>
            Similarly, whenever you supply geographic data to others, make
            sure you tell them the datum. Include the datum in any documentation
            you provide.
        </para>
        <para>
            If you are working with computer programs that process geographic
            data, insure that you know how those programs handles datums.
            Some programs (such as &app_name;) allow you to specify the datum
            of your data. Others require that all data be in a fixed datum.
            Consult the program's documentation.
        </para>
        <para>
            If you are working with a global positioning system (GPS), make
            sure you know the datum that your GPS displays locations in. Many
            allow you to select the display datum.
        </para>
        <para>
            If you absolutely cannot determine the datum of your data, then
            WGS-84 should be assumed. However, be aware of the potential errors
            if this assumption is incorrect.
        </para>
    </section>
    &SettingDefaultDatumSection;
</section>
