Re: GRDGRADIENT

OK, I downloaded the SA slab from http://earthquake.usgs.gov/data/slab/

cut a sub region, tested with GMT and Mirone, and had to dig in my code 
to find out why grdgradient in Mirone is giving the right result but not 
on command line. It turns out that (and I don't remember why) one has to 
multiply z by -1 before computing the "aspect" (that is what you are 
looking for).
Note that you don't want the slope, so don't use -S . The following 
gives the correct result

grdgradient sam_slab.grd=nf/-1 -D -Gaspect.grd -M

Joaquim




> Hi,
>
> I used grdgradient to calculate the strike of downdip direction at 
> every grid point of the subducting slab:
>
>
> *grdgradient slabsurf2.grd -Dn -Sslabgradient.grd -M -V*
>
>
> I am getting the following results:
>
>
> -69.9441860465  -17.0522875817  35.6361160278
> -69.8697674419  -17.0522875817  36.5936279297
> -69.7953488372  -17.0522875817  37.6132316589
> -69.7209302326  -17.0522875817  38.6617622375
> -69.6465116279  -17.0522875817  34.4591636658
> -69.5720930233  -17.0522875817  31.943397522
> -69.4976744186  -17.0522875817  31.6246490479
> -69.423255814   -17.0522875817  30.8437995911
>
>
> I roughly know that the strike of downdip direction should vary in the 
> range of 30degree-120 degree in this region. Do I need to use some 
> more functions to calculate the reasonable strike direction. I 
> urgently need to calculate this and would be extremely grateful for 
> any suggestions.
>
>
> Thanks
>
> Abhash kumar
>
> University of North Carolina at Chapel Hill
>
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