[GRASS-dev] i.points, i.vpoints now use G_parser()

[Glynn Clements]

Michael Barton wrote:

> For total RMS, do I simply combine forward and backward (i.e., like I have
> twice as many points)? Or is it better to display separate forward and
> backward RMS values? Same with displaying 'g' for individual points--just
> display forward or display the average of forward and backward?

Forward and backward errors will be in different coordinate systems,
so it isn't meaningful to compute an average (arithmetic mean).

For a projection which consists solely of translation, rotation and
uniform scaling, the forward and backward errors will be directly
proportional.

For projections with significant levels of distortion, the forward and
backward errors will differ depending upon the scale factor in the
direction of the displacement.

E.g. if you had an image of the whole earth, misplacing a GCP near the
horizon (where the surface is tangential to the view) would result in
a large forward error but a small backward error, while misplacing a
GCP in the centre (where the surface is perpendicular to the view)
would result in the forward and backward errors being roughly
proportional.

Also, bear in mind that rasters are projected using the reverse
transformation, while vectors use the forward transformation.

-- 
Glynn Clements <glynn at gclements.plus.com>