Re: Detemining gradient at individual points

If the grid itself is in in dimensions of lat and lon then you need to supply -fg or -M, depending on GMT version, to convert degrees to meters.  Also if the depth to the slab is in km then you need to account for that before taking atan since your slope would be in km/m.  So perhaps something like

grdgradient slabsurf2.grd -Dn -Sgradient.grd -V

grdmath gradient.grd 1000 MUL ATAN = dips.grd

On Feb 12, 2014, at 4:28 PM, Mr Abhash Kumar <[email protected]> wrote:

> Hi,
> I am grateful for your concerned reply Dr. Wessel. I actaully have the slab grid file and latitude and longitude of few points. I need to find the dip of the slab at exactly those points. I am using the grdgradient command in the following manner:
> 
> grdgradient slabsurf2.grd -Dn -Sgradient.grd -V
> 
> I don't know where in the above command line, I need to put the latitude and longitudes of those points, where I actually want to calculate the dip of the slab. Should I make a separate text file having coordinates of those points and use that text file in the above command somewhere. I would be extremely grateful for any suggestions.
> 
> Thanks
> Abhash kumar
> University of North Carolina at Chapel Hill
> From: World-Wide GMT Usage and Help Mailing List <[email protected]> on behalf of Paul Wessel <[email protected]>
> Sent: Wednesday, February 12, 2014 8:55 PM
> To: [email protected]
> Subject: Re: [GMT-HELP] Detemining gradient at individual points
>  
> Hi Abhash-
> 
> That is a good question.  However, grdgradient with -D and -S can also return the direction of the gradient or the strike (-Dn), as well as the absolute value of the gradient.  If your units are correct (all meters or geographic with -fg (or -M in GMT4)) then take atan of the gradient and that should give you dip, no?
> 
> -p
> 
> On Feb 12, 2014, at 3:37 PM, Mr Abhash Kumar <[email protected]> wrote:
> 
>> Hi,
>> I have a 2-D grid file for the subducting slab surface. I need to find dip of the slab at a particular latitude and longitude. I looked at the man page of grdgradient and it can only calculate the gradient along a particular azimuth. I would be extremely grateful for any suggestions in this regards.
>> 
>> Thanks
>> Abhash kumar
>> PHD candidate
>> University of North Carolina at Chapel Hill
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