[GRASS-user] interpolating images with coastal areas

[Edgar Pimiento]

Hi Bulent,


Yes, I think it is better to treat the sea area as NULL data. One way to 
interpolate the DEM is to use a mask corresponding to the land area, another way 
is to replace the values of the sea with null data in the DEM.

Best,

Edgar Pimiento


________________________________
From: Bulent Arikan <bulent.arikan at gmail.com>
To: Edgar Pimiento <e_pimiento at yahoo.com>
Sent: Mon, March 21, 2011 9:20:11 AM
Subject: Re: [GRASS-user] interpolating images with coastal areas

Hi Edgar,
Thank you for the explanation and suggestion. If I understand you correctly, you 
suggest converting elevation values zero and below to NULL and use that 30m 
resolution imagery for interpolation. So, this map should be used as MASK.
I hope I got it right? 


Best,
Bulent


On Sun, Mar 20, 2011 at 3:14 PM, Edgar Pimiento <e_pimiento at yahoo.com> wrote:

Hi,
>
>
>The problem is probably the data for the sea of the ASTER images. In some cases 
>I have seen negative values for sea areas and extremely high values, errors 
>also, in mountainous areas. For interpolating I think  it is better to treat the 
>sea areas as no data values. Use the  land area as mask in the interpolation. 
>You can also check the ASTER elevation data comparing with the corrected version 
>of 90-m SRTM from http://srtm.csi.cgiar.org/
>
>
>Best,
> Edgar Pimiento Chamorro
>e_pimiento at yahoo.com
>
>
>
>
>
________________________________
From: Bulent Arikan <bulent.arikan at gmail.com>
>To: grass-user at lists.osgeo.org
>Sent: Mon, March 21, 2011 3:03:35 AM
>Subject: [GRASS-user] interpolating images with coastal areas
>
>
>Dear List,
>
>
>I am working on interpolating 30m ASTER DEMs to 10m resolution. I am using GRASS 
>6.4.1 svn on Mac OS 10.6.6. Since the imagery covers a large area, I crop the 
>30m DEM into individual tiles in order to make the project more manageable. The 
>interpolation process (using v.surf.rst) has been working fine and the values in 
>resultant 10m DEMs have been quite accurate, especially for inlands. However, I 
>have some problems with interpolating tiles where coastline is present. In these 
>tiles, the range of values in 10m DEMs go below zero. In some cases I have 
>values such as -21 meters whereas the original –30m– tiles have only zero. 
>Obviously, I am concerned about the accuracy of the end product. 
>
>
>Do I get high negative values because of the cells with zero value?
>
>
>Should I zoom further in and avoid having zero-value cells? (which is a problem 
>with coastlines where they are not straight)
>
>
>Does any one have a suggestion about how to deal with this problem?  
>
>
>* I sample the original imagery at 50% in r.random,
>
>
>* I use the default parameter values in v.surf.rst (tension= 40.) and use the 
>30m DEM as MASKMAP,
>
>
>* I align region to resolution, using g.region.
>
>
>Thanks!
>
>-- 
>BÜLENT 
>
>
>


-- 
BÜLENT ARIKAN, PhD
Postdoctoral Scholar
Center for Social Dynamics and Complexity &
School of Human Evolution and Social Change
Arizona State University
Tempe - AZ
85287-2402



      
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