[GRASS-user] r.thin ?

[charlie]

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

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>
> 
> 
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