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<div class="moz-cite-prefix">On 12/19/2013 2:02 PM, David Strip
wrote:<br>
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<blockquote cite="mid:52B350F1.8090700@stripfamily.net" type="cite">
<pre wrap="">On 12/19/2013 11:38 AM, David Strip wrote:
</pre>
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<pre wrap="">On 12/19/2013 2:18 AM, Jo Meder wrote:
</pre>
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<pre wrap="">Can you clarify that last part for me please? Are you saying that the data should be aligned north up based on the .tfw file? The geotransform from GDAL also suggested there was no rotation. Or is it just that the data is correct for the projection it is in?
</pre>
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<pre wrap="">Ignore this part of my previous reply. It's not correct and reflects
confusion on my part.
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<pre wrap="">The .tfw file is saying the is indeed aligned north up, in the sense
that pixel (0,0) is due north of pixel (1,0). That is, the columns of
the array are aligned north-south.
</pre>
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<pre wrap="">End region to ignore
</pre>
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<pre wrap="">Once again, open the raster in qgis. You get an apparently rotated patch
that is slightly tapered towards the top. This is how the projection
renders for this part of the world (Washington, DC). Now click
View->Decorations->North Arrow, click Enable North Arrow and click Set
Direction Automatically. Click OK. The resulting north arrow does not
point vertically, but rather is parallel to the edges of the patch,
again validating the north-up nature of the rasters.
</pre>
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<pre wrap="">So far all the data I've used has been north up and seems to have been projected in UTM or something like it. I need to make things transparent to the user so I wonder if what I should be doing is reprojecting all the data to a custom projection for our world. The reprojection would handle the cases like this NLCD data.
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<pre wrap="">Hopefully someone will chime in here with a suggestion how to address
your problem to make this easier for your users. Reprojecting your data
to a projection that doesn't have the visual effect of the Albers seems
to be the right way to go so as not to confuse your users, but what
projection to use is a complicated question that has a lot to do with
the use case and what areas of the world you're working in. Each
projection introduces it own forms of distortion (or alternatively, each
preserves specific characteristics). The choice of projection will
depend on which characteristics are most important to preserve, the
scale/extent of the maps your users work in, and the sorts of analyses
(formal or informal) that they will be performing.</pre>
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To re-iterate what David mentioned - The NLCD data are projected
using an Albers Conical Equal Area projection centered on the
conterminous US (-96 degrees). If you are displaying the data
relative to a UTM frame it will appear increasingly rotated as you
get further from the center of the projection space. The data can
be re-projected to UTM or other such suitable coordinate system
based on the size and location of your study area and the desired
purpose of your data. However, realize that re-projecting raster
data will result in modified pixel values which, when working with
thematic data, can affect your analyses depending on the level of
error introduced and again what you plan to use the data for.
<blockquote cite="mid:52B350F1.8090700@stripfamily.net" type="cite">
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<pre wrap="">
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