Hi Simon,
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.
Cheers,
Andy
On 06/05/2016 13:33, simonc8 wrote:
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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Andy Harfoot
GeoData Institute
University of Southampton
Southampton
SO17 1BJ
Tel: +44 (0)23 8059 2719
Fax: +44 (0)23 8059 2849
www.geodata.soton.ac.uk
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