Merge pull request #20 from hawahee/master

Thanks @hawahee 
and sorry for the late merge!
curvature: Fix inaccuracies in the implemented Evans method
Cheers, Wolfgang

@GRIDobj/curvature.m

@@ -124,13 +124,19 @@
  correctedges = true;
  shape = 'valid';
 end
-
-% First-order partial derivatives:
-[fx,fy] = gradient(dem,cs);
 
 if correctedges
  dem = padarray(dem,[1 1],'symmetric');
 end
+
+% First-order partial derivatives:
+% kernel for dz/dx
+kernel = [-1 0 1; -1 0 1; -1 0 1]./(6*cs);
+fx = conv2(dem,kernel,shape);
+% kernel for dz/dy
+kernel = [1 1 1; 0 0 0; -1 -1 -1]./(6*cs);
+fy = conv2(dem,kernel,shape);
+
 % Second order derivatives according to Evans method (see Olaya 2009)
 %
 % z1 z2 z3
@@ -159,9 +165,9 @@
  case 'profc'
  curv = - (fx.^2 .* fxx + 2*fx.*fy.*fxy + fy.^2.*fyy)./((fx.^2 + fy.^2).*(1 + fx.^2 + fy.^2).^(3/2));
  case 'tangc'
- curv = - (fy.^2 .* fxx + 2*fx.*fy.*fxy + fx.^2.*fyy)./((fx.^2 + fy.^2).*(1 + fx.^2 + fy.^2).^(1/2));
+ curv = - (fy.^2 .* fxx - 2*fx.*fy.*fxy + fx.^2.*fyy)./((fx.^2 + fy.^2).*(1 + fx.^2 + fy.^2).^(1/2));
  case 'planc'
- curv = - (fy.^2 .* fxx + 2*fx.*fy.*fxy + fx.^2.*fyy)./((fx.^2 + fy.^2).^(3/2));
+ curv = - (fy.^2 .* fxx - 2*fx.*fy.*fxy + fx.^2.*fyy)./((fx.^2 + fy.^2).^(3/2));
  case 'meanc'
  curv = - ((1+fy.^2).*fxx - 2.*fxy.*fx.*fy + (1+fx.^2).*fyy)./ ...
  (2.* (fx.^2+fy.^2+1).^(3/2));