[gdal-dev] NLCD images and north

[Jo Meder]

Hi David,

On 20/12/2013, at 7:38 AM, David Strip wrote:

> 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.

Right, that makes it very clear, thanks.

> 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.

Our app is landscape renderer/visualiser (not based around real world data) which has a planet as the base scene object. It's a sphere and we use a lat/long geographic coordinate system for georeferencing. As a "near enough" test I reprojected the NLCD data to WGS84/EPSG:4326 and that seems to have done the trick. There is still a ways to go but it's a promising start. I did a quick test render showing the Mt St Helens from the USGS National Map with the 1/9 Arc Second DEMs, several orthophotos and the NLCD data:

http://www.jomeder.com/photos/mshmanydata.jpg

I think the ghosting in areas with the orthophotos is a bit of misregistration between the orthophoto and the DEM. There is one set of shadows from the orthophotos and one from the rendering.

Anyway, thanks for putting me on the right track.

Regards,

Jo Meder