[gdal-dev] NLCD images and north

[Jo Meder]

Hi David,

Thanks for your help. I hadn't replied before now because I'm on the digest and I was waiting for it to come through. It never did! Seems like I'm not getting all the digests for some reason.

On 10/12/2013, at 6:04 PM, David Strip <gdal at stripfamily.net> wrote:

> I downloaded some NLCD test data (roughly Washington DC, for what it's
> worth) using the NationalMap viewer and I see the apparent rotation, but
> actually it's not rotated. I suspect you are seeing the same phenomenon.
> Your download includes, among other files, a .tif file with the image,
> and a .tfw file that defines the "world" in terms of the projection
> defined in the .prj file. In my  experiment the .tfw file shows no
> rotation (entries 2 and 3 are equal to 0). However, the projection is an
> Albers Equal Area projection, so the image boundaries don't align to the
> tif boundaries. If you open your file in something like qgis and set the
> projection to the appropriate UTM zone or similar projection, you'll
> probably get what you're expecting - a north-south rectangle containing
> your data. 

I gave it a try and got that result too.

> At least that's what happened when I tried it. (And of course
> that's what it should be according the .tfw file)

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?

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.

Regards,

Jo Meder