Re: Clalculating dip

Abhash, the dips returned are correct, but they are in radians - not
degrees. (ATAN returns a radian value not a degree value). Multiply
them by 57.296 to get them in degrees (180/pi). E.g.

grdmath slabgradient.grd 1000 MUL ATAN 57.295779 MUL = dip.grd

-ashton

On Fri, Feb 14, 2014 at 7:27 PM, Mr Abhash Kumar <[email protected]> wrote:
> Hi,
>
> I have 2D grid file for the subducting slab in terms of latitude, longitude
> in degrees and depth in km. 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 the 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. I use the following
> commands as per previous suggestions from the GMT-HELP:
>
>
>  grdgradient slabsurf2.grd -Dn -Sslabgradient.grd -M -V
>
>
> grdmath slabgradient.grd 1000 MUL ATAN = dip.grd
>
>
> I used grd2xyz to look at the dip at various grid points as follows:
>
>
> grd2xyz dip.grd > dip.xyz
>
>
> I checked the dip.xyz file and it has dip in the third column as:
>
>
> -71.4326        -16.0458        0.44211
> -71.3581        -16.0458        0.425007
> -71.2837        -16.0458        0.428402
> -71.2093        -16.0458        0.44172
> -71.1349        -16.0458        0.460254
> -71.0605        -16.0458        0.478551
>
> I was expecting the dip to be in the range of 20-30 degree since the
> subduction is normal and slab is roughly subducting at 30 degree from the
> horizontal. If I take inverse tan of the last column than the dip angle
> falls in the expected range. I would be extremely grateful for any
> suggestions in this regard.
>
>
> Abhash kumar
>
> University of North Carolina at Chapel Hill
>
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