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<div class="moz-cite-prefix">Hi,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">On 27/10/2020 06:18, Adrian via Talk-GB
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:59236119.5828334.1603779538301@mail.yahoo.com">
<pre class="moz-quote-pre" wrap="">I agree with Rob that the misalignment of 5m is obvious if you look at Hugh Town (Scilly). Both if you compare with the OSM data and if you compare with the tracklogs that have been uploaded to OSM. So this transformation won't do. I think we need to go for the look-up table.</pre>
</blockquote>
<p>Whilst the details of your <span class="module__title__link">geodesy
is impressive but way beyond my expertise, over ten years of OSM
survey traces suggests another factor to be wary of when
comparing sub-10m position sources. <br>
</span></p>
<p><span class="module__title__link">Using a Garmin Oregon 550 as a
baseline, Oregon 650 and 750 consistently give location between
4-8m North North West in Northumberland - the tool may influence
the measurement beyond your accuracy.<br>
</span></p>
<p><span class="module__title__link"><span
class="module__title__link">For resilience, I map with at
least two GPSr on my bike handlebars and regularly upload both
tracks to OSM and use both to better position mapping and any
layers such as imagery. </span>Over the years I've used five
or six Garmin GPSr. None are even close to a 'proper'
differential total station, however with datum/ spheroid set to
WGS 84, the same tools and JOSM workflow show the offset. </span><span
class="module__title__link"><span class="module__title__link">Changing
GPS/ GLONASS or WAAS/ EGNOS seems to have less impact than the
choice of Garmin unit (same settings across devices). Firmware
updates have changed motion compensation when changing
direction fast, but the offset remains.<br>
</span></span></p>
<p><span class="module__title__link"><span
class="module__title__link">The trouble will be is without
device data in tracklogs there's no way to separate random
from systematic offsets (even if you had them...) - you can
only average all data.</span></span><span
class="module__title__link"><br>
</span></p>
<p><span class="module__title__link"><br>
</span></p>
<p><span class="module__title__link">Thanks for your interesting
work - I remember tales from Registers of Scotland of an OS
baseline survey error that 'moved' the East coast by many meters
proving 'You Are Here' is hard to quantify!<br>
</span></p>
<p><br>
</p>
<pre class="moz-signature" cols="72">James
--
James Derrick
<a class="moz-txt-link-abbreviated" href="mailto:lists@jamesderrick.org">lists@jamesderrick.org</a>, Cramlington, England
I wouldn't be a volunteer if you paid me...
<a class="moz-txt-link-freetext" href="https://www.openstreetmap.org/user/James%20Derrick">https://www.openstreetmap.org/user/James%20Derrick</a>
</pre>
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