Include discussion of static & dynamic CRS

bp/index.html

@@ -2298,7 +2298,12 @@ <h4 class="subhead">Why</h4>
                 <p>The Earth is a complicated shape (neither spherical nor flat!):</p>
                 <p>For each (Earth-based) <a>coordinate reference system</a>, the topographical surface of the Earth is approximated to a <em>geodetic <a>datum</a></em> that is described using an <a>ellipsoid</a>. The trouble with approximation is that nothing is perfect everywhere, which means that compromise is inevitable. Some datums, like WGS 84, provide a reasonable (but not highly accurate) fit everywhere on the Earth, while other datums (such as the European Terrestrial Reference System 1989 &mdash; as used by <a href="https://epsg.org/crs_4258/ETRS89.html">ETRS89 / EPSG:4258</a>) provide a better fit in a given region at the expense of accuracy elsewhere.</p>
                 <p><a>Spatial data</a> is often <em>projected</em> from the curved surface of the Earth onto a flat plane (e.g. a computer screen or a topographical map) to make it easier to compute distances between positions and calculate areas. There are many choices of projection (e.g. equirectangular, mercator, stereographic, orthographic etc.), each of which is designed for particular tasks. As with <a>datums</a>, projections are often chosen to better support regional, national or local needs.</p>
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                 <p>It is also worth noting that as a living planet, the Earth continues to change its shape; for example, continental drift moves Australia north-eastwards several centimeters each year and New Zealand shifts in multiple directions. To retain accuracy, <a>datums</a> need to be adjusted from time to time &mdash; as is the case of the New Zealand Geodetic Datum (NZGD2000) that is frequently revised to take account of earth deformations.</p>
+		<p>Geodesists refer to those coordinate reference systems which are continually updated so that the same set of coordinates refers to the same place on (or near) the surface of the earth as 'plate-fixed' or 'static coordinate reference systems'; others (such as WGS 84) are 'earth-fixed' or 'dynamic' (a set of coordinates will not resolve to the same spot on the moving surface of the earth).</p>
+		<p>If your intended application requires a combination of positional accuracy and persistence over time, then use a static coordinate reference system or ensure that each set of coordinates is accompanied by the date it was measured.</p>
+		<p>See https://www.youtube.com/watch?v=IKM-bR6SwVs for further explanation.</p>
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                 <p>Sometimes we don't want to measure relative to the surface of the Earth at all:</p>
@@ -2400,6 +2405,7 @@ <h4 class="subhead">Benefits</h4>
           <section class="axioms">
             <h4 class="subhead">Why</h4>
             <p>The <a>geometry</a> of <a>Spatial Things</a> is described using position coordinates; for example, <a>latitude</a> and <a>longitude</a>. Because coordinates describe a position relative to a <a>datum</a> (e.g. zero latitude is the equator and zero longitude is the prime meridian &mdash; often the Greenwich Meridian), it is important to understand both the datum and the units that are used for coordinates along with the order which the coordinate axes are defined: the <a>coordinate reference system</a> (CRS). <a>Spatial data</a> is published in a wide variety of CRS. This variety can create confusion and inconsistencies in using and interpreting spatial data. Unless the CRS is known, errors are likely to be introduced when determining the position and extent of a <a>Spatial Thing</a> on the Earth and this makes comparing or combining spatial data from different sources extremely problematic.</p>
+	    <p>Where the application requires accuracy over time but the data uses a dynamic coordinate system, you will also need to provide the 'epoch' - the data the coordinates were recorded; see <a href="bp-crs-choice">Best Practice 7</a>.
           </section>
           <section class="outcome">
             <h4 class="subhead">Intended Outcome</h4>