Reguarding the plugin Filters>Edge-Detect>Edges... naming convention for it's
algorithm parameter. The first algorithm is named 'Sobel' and is a Sobel
gradient edge detector.

http://homepages.inf.ed.ac.uk/rbf/HIPR2/sobel.htm

A variation of this is the prewitt gradient edge detector. The difference is the
choice of convolution kernel.

sobel kernel
[  1  2  1;
   0  0  0 ;
  -1 -2 -1 ]

prewitt kernel
[  1  1  1;
   0  0  0 ;
  -1 -1 -1 ]

That is..
k=1 for prewitt and k=2 for sobel

[  1  k  1;
   0  0  0 ;
  -1 -k -1 ]

The second algorithm is name 'Prewitt' and is a compass edge detector with a
prewitt kernel and not a prewitt gradient edge detector. This is not obvious
unless one look at the source code. The two algorithms using the same kernel are
quite different too.

http://homepages.inf.ed.ac.uk/rbf/HIPR2/prewitt.htm


I'd suggest to change the algorithm name to something less confusing like
'Compass Prewitt'.