# [ncl-talk] Computing Aspect

Dennis Shea shea at ucar.edu
Tue Mar 13 22:51:56 MDT 2018

```Hi Mike,

Given your:

; a b c

; d e f

; g h i

dzdx = ( (c + 2*f + i) - (a + 2*d + g))/dx

----

dzdy = ( (a + 2*b + c) - (g + 2*h + i))/dy

or, maybe, not sure of sign convention

dzdy = ( (g + 2*h + i) - (a + 2*b + c))/dy

----

Your code may need a minus sign before the 2nd argument:

aspect = 57.29578*atan2(dzdy,-dzdx)   ; <=== minus sign ???

if (aspect.lt.0) then

aspect = 90 - aspect

end if

------------------

if (aspect(jj,ii).le.0.) then

aspect(jj,ii)=90.-aspect(jj,ii)

end if

--

If you use a minus sign, maybe you do not need other 'if' blocks?

On Mon, Mar 12, 2018 at 3:27 PM, Michael Notaro <mnotaro at wisc.edu> wrote:

> I used the following to compute terrain aspect based
>
> on elevation.  But it seems the values are possibly not
>
> quite right, since they disagree with another aspect
>
> dataset I have.  Does anyone see something wrong below?
>
> Michael
>
>
> ny=dims(0)
>
> nx=dims(1)
>
> aspect=new((/ny,nx/),"float")
>
> aspect at _FillValue=-999.
>
> aspect=-999.
>
>
> ; a b c
>
> ; d e f
>
> ; g h i
>
>
> do ii=1,nx-2
>
>   do jj=1,ny-2
>
>     if (.not.ismissing(elev(jj,ii))) then
>
>       a=elevation(jj+1,ii-1)
>
>       b=elevation(jj+1,ii)
>
>       c=elevation(jj+1,ii+1)
>
>       d=elevation(jj,ii-1)
>
>       e=elevation(jj,ii)
>
>       f=elevation(jj,ii+1)
>
>       g=elevation(jj-1,ii-1)
>
>       h=elevation(jj-1,ii)
>
>       i=elevation(jj-1,ii+1)
>
>       dzdx=((a+2.*d+g)-(c+2.*f+i))/(8.*800.) ; 800 m PRISM
>
>       dzdy=((g+2.*h+i)-(a+2.*b+c))/(8.*800.)
>
>       aspect(jj,ii)=57.29578*atan2(dzdy,dzdx)
>
>       if (aspect(jj,ii).le.0.) then
>
>         aspect(jj,ii)=90.-aspect(jj,ii)
>
>       else if (aspect(jj,ii).gt.90.) then
>
>         aspect(jj,ii)=360.-aspect(jj,ii)+90.
>
>       else
>
>         aspect(jj,ii)=90.-aspect(jj,ii)
>
>       end if
>
>       end if
>
>     end if
>
>   end do
>
> end do
>
>
> Michael Notaro
> Associate Director
> Nelson Institute Center for Climatic Research
> Phone: (608) 261-1503
> Email: mnotaro at wisc.edu
>
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