[GRASS-user] r.thin ?

[charlie]

 Micha & Markus, thanks for the tips, I will try r.mapcalc to extract the borders by color; you confirmed my concern that the hashing and other artifacts may alter the final result.  If I can't get it to work, I may just start from scratch with v.digit.

--- On Tue, 11/9/10, Micha Silver <micha at arava.co.il> wrote:

From: Micha Silver <micha at arava.co.il>
Subject: Re: [GRASS-user] r.thin ?
To: "charlie" <carowan at yahoo.com>
Cc: grass-user at lists.osgeo.org
Date: Tuesday, November 9, 2010, 1:53 PM





  
  
On 11/09/2010 06:21 PM, charlie wrote:

  
    
      
        
        
        
          
            
              
              
              
                
                  
                    
                    
                    
                      
                        
                          Micha
and Achim, thank you for the replies.  In response to your questions, I
am trying to convert a raster map (a scanned & georeferenced map
with only XY data) with a shoreline and some wetland islands into a
vector map so I can compare gains/losses over time.

                          

A preview of the map is here:
http://historicalcharts.noaa.gov/tiled_jpgs_done/zoomifyURLDrivenWebPage.htm?zoomifyImagePath=579-12-1929

                          

                          
                        
                      
                    
                    
                    
                  
                
              
              
              
            
          
        
        
        
      
    
  



So what you want is the boundaries of the islands? I think that in
order to extract this from the image, you'll have to do some
pre-processing of the image -  you need the raster to show only the
boundaries, and everything else should be 0. From the r.thin man page:

"r.thin assumes that linear features are encoded with positive values
on a background of 0's in the input raster data file. "

So you'll have to query the image for the color value of the boundary
lines, then use an r.mapcalc expression to get only those raster pixels
and set everything else to 0. This might get very tedious since it
seems all the text and horizontal hashing lines are the same color as
the boundary lines....

If you succeed to separate the boundaries, then run the resulting
raster thru r.thin, and finally convert to vector.




  
    
      
        
        
        
          
            
              
              
              
                
                  
                    
                    
                    
                      
                        
                          The
command I used is as follows:

r.thin input=NOAA1929 output=NOAA1929THIN iterations=6000 

                          

I have tried r.thin with 1000 iterations, which did successfully thin
the lines down to the features borders quite well, however only 50% of
the map was completed.

                          

I bumped the iterations up to 6000, it completed in 4200, but it seemed
that it thinned the entire map down to a 1px line.

                          

Thanks

                          

--- On Tue, 11/9/10, Micha Silver <micha at arava.co.il>
wrote:

                          

From: Micha Silver <micha at arava.co.il>

Subject: Re: [GRASS-user] r.thin ?

To: "charlie" <carowan at yahoo.com>

Cc: grass-user at lists.osgeo.org

Date: Tuesday, November 9, 2010, 5:24 AM

                            

                            charlie wrote:

                            

> Hi.  I am currently using Grass 6.4 on linux and beginning the the
process of vectorizing an old NOAA raster map with r.thin. The map is
approximately 12,000 pixels by 8,000 pixels wide.

> 

> According to this post, ( http://osgeo-org.1803224.n2.nabble.com/r-thin-td2718260.html#a2718260
)the iteration variable has to be equal or larger than half the number
of cells of the wider line so I increased the iterations to 6,000, and
it would up finishing in approximately 4,200 passes.

> 

> Previously, I was getting excellent preliminary results, with
fewer (1,000) iterations,  distinct features were visible, however at
least 50% of the map was not completed. 

> Now with an increase to 6,000 iterations r.thin completes, but the
results are a single 1px wide horizontal line.  Im sure I am
overlooking something relatively simple; can anyone give me any advice
on how to best run r.thin in this particular case?

> 

                            

What is it you are trying to do? The r.thin module is designed to make
line features in a raster exactly one pixel wide so that when
converting to a line vector, the line will be clean.

If you run r.thin on a raster that represents areas, and give a high
enough iteration value, the areas will eventually be reduced to lines.

                            

> 

> Thanks!

> 

> Charlie

> 

> 

> <a href="http://www.wetlandresearch.com">Wetlands</a>

> 

> 

> This mail was received via Mail-SeCure System.

>
------------------------------------------------------------------------

> 

> _______________________________________________

> grass-user mailing list

> grass-user at lists.osgeo.org

> http://lists.osgeo.org/mailman/listinfo/grass-user

> 

> This mail was received via Mail-SeCure System.

> 

> 

>   

                            

                            
                          
                          
                        
                      
                    
                    
                    
                  
                
              
              
              
            
          
        
        
        
      
    
  
  

This mail was received via Mail-SeCure System.






-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il
 

 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/grass-user/attachments/20101110/f06f0fab/attachment-0001.html