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On 03/31/2012 07:42 AM, Tony Bigbee wrote:
<blockquote
cite="mid:CAHBUhLWFZrDGo2My=QbnBn5hbfvXYXh4z2wzuGW6yESb5dN9ag@mail.gmail.com"
type="cite">
<pre wrap="">Hi,
I reported a possible bug over two weeks ago regarding the scale bar
and map units but there has been no response to the bug. I've found
postings about properly setting the map units, projection units, and
the scale bar, but cannot resolve the problem. Here is the bug report
(including a test data file):
<a class="moz-txt-link-freetext" href="http://hub.qgis.org/issues/5183">http://hub.qgis.org/issues/5183</a></pre>
</blockquote>
<br>
If I understand correctly this test.geojson is in the un-projected
WGS84 CRS.<br>
In this case, the map units will be (correctly) in degrees. Since
the width of the rectangle is only a very small fraction of a
degree, you need to set the "segment size" to something like 0.001.<br>
I then changed the *project* CRS (without making any changes to
test.geojson) to UTM zone 14 (EPSG:32614), and zoomed to the layer.
Now I opened the map composer, and added a scale bar. WIth segment
size 10 (that's now 10 meters since the unit in UTM is meters) I got
a good looking scalebar. <br>
<br>
Not sure I understand what the problem is?<br>
<br>
<blockquote
cite="mid:CAHBUhLWFZrDGo2My=QbnBn5hbfvXYXh4z2wzuGW6yESb5dN9ag@mail.gmail.com"
type="cite">
<pre wrap="">
I've also discovered that generating areas of relatively small
polygons (100x100m) is not reported in square meters (using WGS 84 /
EPSG:4326) via Vector:Geometry Tools:Export/Add Geometry columns. I
wonder if the two issues are related. I'm also curious about how the
area is calculated (does the Export process convert to some equal area
projection?)</pre>
</blockquote>
<br>
To do area calculation correctly *always* reproject to a projected
CRS. (Right click on the layer, "Save As..." and choose a suitable
CRS). The area calculations in an unprojected CRS (like WGS84) will
be in square degrees, and furthermore will be wrong. QGIS does not
calculate area on a sphere, only on a planar CRS, so any length or
area calculations done in an unprojected CRS will be misleading.<br>
HTH<br>
<br>
<br>
<blockquote
cite="mid:CAHBUhLWFZrDGo2My=QbnBn5hbfvXYXh4z2wzuGW6yESb5dN9ag@mail.gmail.com"
type="cite">
<pre wrap="">
Thanks.
Tony Bigbee
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</pre>
</blockquote>
<p><br>
</p>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
GIS Consultant, Arava Development Co.
<a class="moz-txt-link-freetext" href="http://www.surfaces.co.il">http://www.surfaces.co.il</a></pre>
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