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<div class="moz-cite-prefix">Hi Simon,<br>
<br>
Surely if your image is georeferenced, then it will have been done
so to a CRS (not necessarily a projected one though). However, if
you don't know the CRS (and it appears that QGIS cannot extract it
from the GeoTiff if it is prompting you), but you do know the
georeferenced units used, then you can achieve what you ask by
disabling on the fly reprojection in QGIS. This has the effect of
disregarding CRS information, and treating the coordinates of the
data as cartesian (even if they are geographic). Distance and area
are measured on the plane. If you set the measurement units under
Project > Project Properties > General to 'Map units', then
they will be the units of the data, regardless of what QGIS
actually thinks they are.<br>
<br>
Cheers,<br>
<br>
Andy<br>
<br>
On 06/05/2016 13:33, simonc8 wrote:<br>
</div>
<blockquote cite="mid:1462538027587-5264924.post@n6.nabble.com"
type="cite">
<pre wrap="">I want to use QGIS to make measurements such as distances and angles on a
flat scaled image which I have as a georeferenced tiff. I don't want to use
any projection - just a straightforward orthogonal linear scale in metres.
QGIS seems to require that I choose a projection. What would be the best
choice for my needs? Can I make a custom projection with no ellipsoid or
datum?
If I import my geotiff and don't specify a projection QGIS assumes my units
are in degrees rather than metres, which will make measurement of distances
and angles inaccurate.
Grateful for assistance.
--
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<br>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
Andy Harfoot
GeoData Institute
University of Southampton
Southampton
SO17 1BJ
Tel: +44 (0)23 8059 2719
Fax: +44 (0)23 8059 2849
<a class="moz-txt-link-abbreviated" href="http://www.geodata.soton.ac.uk">www.geodata.soton.ac.uk</a>
</pre>
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