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<p>On 2020-05-08 14:09, Greg Troxel wrote:</p>
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<div class="pre" style="margin: 0; padding: 0; font-family: monospace">Martin Koppenhoefer <<a href="mailto:dieterdreist@gmail.com">dieterdreist@gmail.com</a>> writes:<br /><br />
<blockquote type="cite" style="padding: 0 0.4em; border-left: #1010ff 2px solid; margin: 0">Am Fr., 8. Mai 2020 um 03:22 Uhr schrieb Greg Troxel <<a href="mailto:gdt@lexort.com">gdt@lexort.com</a>>:<br /><br />
<blockquote type="cite" style="padding: 0 0.4em; border-left: #1010ff 2px solid; margin: 0"> 3) Look up the data sheet and mark it as ele:datum=NGVD29 or<br /> ele:datum=NAVD88 as it turns out.</blockquote>
<br /> IIRR, in another mail, you wrote that the difference between these 2 is<br /> less than a meter, can you confirm this, or did I understand you or<br /> remember wrong?</blockquote>
<br /> Yes,it typically is.<br /><br /> So let me ask you again, since you keep declining to answer this:<br /><br /> Please give an example of an elevation of a real thing that is<br /> meaningfully different in one of these "regional datums" (established<br /> by a country's survey agency) compared to WGS84 height above geoid.<br /> Identify the regional datum, and identify two values linked with a<br /> rigorous transformation (such as national survey agencies publish).</div>
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<div class="pre" style="margin: 0; padding: 0; font-family: monospace">As I mentioned before, the national datums of the Netherlands and Belgium differ by over 2m, which for everything connected to water is very significant. Waterways often form the border, with bridges that cross the border. You cannot use a map/chart (at last for tidal waters) if you don't know what datum it uses.</div>
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<div class="pre" style="margin: 0; padding: 0; font-family: monospace">In OSM we often leave out "obvious" annotations, giving rise to a kind of "default" (such as maxspeed in km/h). But there is always a way of making it explicit, for those who feel the need. In this case we may agree to define "ele" as relative to the "local datum" or WGS84 or whatever, but we must always provide a system for making that explicit, and (preferably) a means to derive the intended basis for values that are not explicitly qualified.</div>
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