[GRASSLIST:6848] Re: i.rectify polynomial order

[Hamish]

> Im now using i.rectify to gereference some maps, but i dont understand
> the difference between using a 1,2 or 3 grade polynomial. I see that
> if  i want to use the 3 grade polynomial i have to mark 10 points (or
> more),  is a 3 grade polynomial better (in precicion and accuracy)
> than a 2 or  1-grade polynomial? If i use a a 1 grade polynomial is
> not the same if i  use a 3 grade  with the same number of point? where
> is the difference?


In the 1-d sense, it is the difference between fitting a regression
line, fitting a simple curve, and fitting a complex curve to the data.

Depending on your map projections and data, simple rotation and scaling
may be fine or may give very poor results. If so, you need to use a
higher order polynomial fit.


i.rectify 3rd order has problems, see GRASS bug #3166
  https://intevation.de/rt/webrt?serial_num=3166

So I recommend using 2nd order.



Hamish