[gdal-dev] NLCD images and north

[Kurtis Nelson]

On 12/19/2013 2:02 PM, David Strip wrote:
> On 12/19/2013 11:38 AM, David Strip wrote:
>> On 12/19/2013 2:18 AM, Jo Meder wrote:
>>> 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?
> Ignore this part of my previous reply. It's not correct and reflects
> confusion on my part.
>> 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.
> End region to ignore
>> 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.
>>> 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.
>> 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.
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.
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