[GRASS-user] NED Shaded Relief, Grass 6.4

[Michael Barton]

On Apr 10, 2009, at 11:22 PM, <grass-user-request at lists.osgeo.org>  
wrote:

> From: Markus Neteler <neteler at osgeo.org>
> Date: April 10, 2009 11:12:40 PM GMT-07:00
> To: Tripp Lowe <tripplowe at gmail.com>
> Cc: GRASS user list <grass-user at lists.osgeo.org>
> Subject: Re: [GRASS-user] NED Shaded Relief, Grass 6.4
>
>
> Tripp,
>
> On Sat, Apr 11, 2009 at 8:05 AM, Tripp Lowe <tripplowe at gmail.com>  
> wrote:
>> Hello Folks,
>>
>> I am trying to create a shaded relief map based on a 30-meter NED  
>> file using
>> r.shaded.relief.  The results are very 'blocky' (or stair- 
>> stepped).  Running
>> a 5-cell average neighborhood across the DEM did not help. Can  
>> somebody tell
>> me the correct procedures I should follow to create the relief map  
>> that is
>> smooth?
>>
>> I've put a screenshot of my initial shaded relief map up here to  
>> show you
>> what I'm talking about...
>> http://www.rlogis.com/GMaps/test/grass64_shadrel.jpg
>
> did you run
> g.region rast=your_ned_map -p
>
> before
> r.shaded.relief
> ? I suspect that your current region is of lower resolution right now.
>
> Markus
>
>

Additionally, NED data are not very good in this respect. In many  
(all?) cases, they are DEM's interpolated from contour maps, which  
were hand or machine drawn from aerial photo data with surveyed  
reference points. They tend to have inherent stair steps. So even if  
you get rid of the blocks, it still probably won't be the kind of  
shaded relief that you want. For North America, the 30m SRTM level 3  
or 4 data are much better.

Michael

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