Hamish wrote:
> If a georeferencing points file and target are set up it is possible to
> use g.transform to say how good the fit is.
>
> I'd like to go one further: I'd like to pass g.transform a handful of x,y
> pixels from the the same neighborhood as the GCPs and get an approximate
> projected easting and northing back.
>
> If possible at all, I guess we have to write something new for
> g.transform; any good ideas on how to do this?
CRS_compute_georef_equations() calculates the forward and reverse
transformation coefficients, while CRS_georef() applies the
transformation to a single point.
E12/N12 are the coefficients for the forward transformation, while
E21/N21 are the coefficients for the reverse transformation.
CRS_compute_georef_equations() computes both pairs of arrays, while
CRS_georef() expects one pair to be supplied, depending upon whether
you want the forward or reverse transformation.
Either the E12/N12/E2/N21 arrays need to be moved out of
compute_transformation() and into global variables, or the code to
transform the points needs to be added to compute_transformation().
The transformations are:
order=1:
e = [E0 E1][1].[1]
[E2 0][e] [n]
n = [N0 N1][1].[1]
[N2 0][e] [n]
order=2:
e = [E0 E1 E3][1 ] [1 ]
[E2 E4 0][e ].[n ]
[E5 0 0][e²] [n²]
n = [N0 N1 N3][1 ] [1 ]
[N2 N4 0][e ].[n ]
[N5 0 0][e²] [n²]
order=3:
e = [E0 E1 E3 E6][1 ] [1 ]
[E2 E4 E7 0][e ].[n ]
[E5 E8 0 0][e²] [n²]
[E9 0 0 0][e³] [n³]
n = [N0 N1 N3 N6][1 ] [1 ]
[N2 N4 N7 0][e ].[n ]
[N5 N8 0 0][e²] [n²]
[N9 0 0 0][e³] [n³]
["." = dot-product, (AE).N = N'EA.]
IOW, order=1 and order=2 are equivalent to order=3 with the higher
coefficients equal to zero.
--
Glynn Clements <[email protected]>
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