Re: Detemining gradient at individual points

Hi Abhash,

follow Paul advice, and then use grdtrack to sample the grid at your  
locations.

Eduardo.-

Quoting Mr Abhash Kumar <[email protected]>:

> Hi,
> I do have slab grid file in terms of latitude, longitude and depth   
> in km. I don't want to determine gradient at every single latitude   
> and longitude, which I have in the slab grid file. My slab grid file  
>  (slabsurf2.grd) has 345639 data points in it and extended between   
> 10-18 degree south and 66-78 degree west. I want to determine   
> gradient at some user defined latitude and longitude (example:   
> 74.567 degree west, 12.345 degree south), which falls with in the   
> range of extension of the slab but doesn't coincide exactly with any  
>  specific point present in the slab grid file. Is there any way to   
> determine gradient at a user specified location.
>
> 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 9:50 PM
> To: [email protected]
> Subject: Re: [GMT-HELP] 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]<mailto:[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]<mailto:[email protected]>> on   
> behalf of Paul Wessel <[email protected]<mailto:[email protected]>>
> Sent: Wednesday, February 12, 2014 8:55 PM
> To: [email protected]<mailto:[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]<mailto:[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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-- 
Eduardo A. Suarez
Facultad de Ciencias Astronómicas y Geofísicas - UNLP
FCAG: (0221)-4236593 int. 172/Cel: (0221)-15-4557542/Casa: (0221)-4526589


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