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<p>I meant my question seriously, not hypothetically.... I assume all these boundary posts are tagged with something like "source:location=MA State Data Set 2015-01-19"? If not, how is a mapper to compare his "correctness" to the existing "correctness"?</p>
<p>Without this provenance information in the database, the accuracy is unknown to other mappers, and may in fact be worse (for example an armchair mapper digitising from small scale map).</p>
<p>--colin</p>
<p>On 2015-09-11 20:02, Greg Troxel wrote:</p>
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<div class="pre" style="margin: 0; padding: 0; font-family: monospace"><br /> Colin Smale <<a href="mailto:colin.smale@xs4all.nl">colin.smale@xs4all.nl</a>> writes:<br /><br />
<blockquote type="cite" style="padding: 0 0.4em; border-left: #1010ff 2px solid; margin: 0">How does the chap with the GPS on his smartphone know that the old<br /> coordinate of 50.000 is "more correct" than his own measurement?</blockquote>
<br /> This is generally a hard question, but if you're using a phone, and the<br /> value you see is within 20m (or maybe 10m) of what's in the db, then you<br /> shouldn't change it unless you've thought about why your new value is<br /> better. And arguably data with higher-than-normal accuracy deserves a<br /> note.<br /><br /> Around me, town boundaries are defined by the actual positions of<br /> granite markers. Massaschusetts publishes a dataset with coordinates<br /> determined from a surveying long ago, reckoned forward to a modern<br /> coordinate system. Those coordinates are in OSM. I've visited a number<br /> of these, and of course my handheld GPS receiver gets different numbers.<br /> But they are all close, and I have no reason to believe my measurements<br /> are better. In fact, I think my measurements are worse. So I'm<br /> certainly not changing the values in the database.<br /><br /> So really, people just have to understand the expected errors of their<br /> measurements.</div>
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