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<div>I had similar trouble with the distance measure tool. I don't have details at hand, but my project in Spokane, WA area seemed fine. New project in Coeur d'Alene, ID area was giving distances higher than Google Earth and Bing. I took the car and drove some of the routes, noting mileage posts, comparing to my odometer. I already knew my odometer was reading about 10% high. I found my mapping in Idaho needed about 65% correction! I suspect difference in projections and conversions when using different projection sources.</div><div><br></div><div>My advice is to verify in the field. Don't rely on aerial scaling.</div><div><br></div><div>I found I can adjust the map scale in the map composer. Under feature (individual?) property, the scale bar can be tweeked, using the inverse of the correction factor. I guess it makes the bar appear longer, thus when compared to map, lengths measure shorter. This probably has no effect on area measurements, unfortunately. I had feet numbers overlapping each other, so I manually covered them and inserted aprox. mileage numbers over the scale bar.<br></div><div>-Dave<br></div><div><br></div>
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On Friday, April 13, 2018, 6:14:59 AM PDT, Randal Hale <rjhale@northrivergeographic.com> wrote:
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<p>This has me puzzled. So I may have screwed up a very large job
with incorrect acres and I was doing what I thought I needed to be
doing. I'm probably doing this wrong - but if I'm doing this wrong
there's a large chance more people are doing the same. <br>
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<p>This is all in QGIS 3.0.1<br>
</p>
<p>I have a forester who is working on a tract of land. There was a
very hurried cruise they needed to do. <br>
</p>
<p>Data is in WGS84 and set to EPSG:2240 in QGIS. <br>
</p>
<p>Change the ellipsoid to None under project properties: <br>
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<li>using the identify button are 237 acres. <br>
</li>
<li>I get roughly 194 acres with the measure tool<br>
</li>
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<p>Changing the ellipsoid to GRS1980 (which QGIS Defaults to): <br>
</p>
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<li>I get 196 acres with the identify tool. <br>
</li>
<li> I use the measure tool I get 194 acres</li>
</ul>
<p>I am assuming the 194/196 measurements are the exact same and due
to human error on my part. PostGIS gives me 196 acres. <br>
</p>
<p>I guess (as I type this out) my correct course of action should
be to leave the ellipsoid in GRS1980 and do my work - BUT - I
thought I could drop back to none/planimetric if I set the
projection to EPSG:2240 in QGIS.</p>
<p>Can anyone clue me in - I'm preparing for much yelling from the
client in about 15 minutes as I was off about 40 acres which was a
costly mistake. I'm not confident currently in my area
measurements using QGIS and I need to get confident. <br>
</p>
<p>Randy<br>
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<pre class="yiv7939219252moz-signature">--
Randal Hale
<a rel="nofollow" class="yiv7939219252moz-txt-link-abbreviated" ymailto="mailto:rjhale@northrivergeographic.com" target="_blank" href="mailto:rjhale@northrivergeographic.com">rjhale@northrivergeographic.com</a>
<a rel="nofollow" class="yiv7939219252moz-txt-link-freetext" target="_blank" href="https://www.northrivergeographic.com">https://www.northrivergeographic.com</a>
(423)653-3611
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