Converting raster landcover data to shapefiles

[Boyd]

This is really puzzling. I opened the sourcefile that I downloaded in both GM 11 and GM 12 and it looks clean (like the viewer screenshot above). I then tried exporting sections of it and they also look clean.

But somehow I suspect compression must be the culprit. I am going to want to re-do a lot of stuff now and get it right. How frustrating! 

[Seldom]

I got my original IMG (graphic) and TIF files from the Seamless Server perhaps a year ago.  Zooming tight to both of them shows well defined square pixels.  Is it possible you got a JPG or JPG2000?

[maps4gps]

Boyd,  
  Looks like that second file may have been reprojected.  Earlier today I was thinking it might have something to do with reprojecting the source in Abers to Geographic; however, it check out OK.  Something is not right with yours.    A compression algorthym not specifically intended to preserve all the original values could produce such results.  

When I downloaded the data a few years ago, it was via the Seamless server and defining 2x6 degree areas.  Did not expect to make you day, but certainly did not want to break it.

Seldom_sn,
  I last looked at the data about 2 1/2 years ago.  I will see if I can find that list - I do remember the values GM gave could be a few units different, but nothing close to another classification type so there was no confustion with which was which.
  I did not have much luck yesterday finding the webpages on NLCD data I remember from years ago.  Not even a reference to the 2007 data that was in the works then.

[Boyd]

You know, I think I know how this happened. I believe I resampled the images at somewhat lower resolution after reading that article I linked to earlier

USGS encourages users to aggregate the data into 3 x 3 or 5 x 5 pixel blocks (in other words, to decrease spatial resolution from 30 meters to 90 or 150 meters)

But that was in reference to the 1992 imagery, so I shouldn't have done it with the 2001 imagery which is obviously a lot cleaner. Resampling would certainly create a problem like the one I'm seeing.

Aaargh  Well, thanks guys for being persistent in pointing out this problem. Now I'll need to do a bunch of stuff again, but should get much better results.

[edit]I also thought about re-projection being a problem, but after trying it doesn't seem to be. It was definitely the resampling.

[maps4gps]

maps4gps, do you have a link to a site that shows the RGB assignment for each NLCD landcover type?  Right now I'm assigning them by eye, assuming that Dark Green (028,102,051) is Evergreen, etc.

I do not think this is the site I used; however these seem to be about right:
http://www.mmm.ucar.edu/people/duda/files/cat.tbl

[Seldom]

Thanks maps4gps, as you say, they are not quite what GM reports, but they're generally only one or two bits off.

[ddabcd277]

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