# [GRASS-user] i.ortho.photo questions -testing in 6.4.2RC1

Shane Litherland litherland-farm at bigpond.com
Mon Oct 17 00:56:43 EDT 2011

```Hi again all,

I'm trying i.ortho.photo from wxGUI to assess an earlier problem. So far
I have hit a similar error message when zooming after a few GCP's are
present, but I am wondering if I have done some things wrong in earlier
steps.

For entering camera position at step 6 for omega/phi/kappa, I read the
help/instructions in GRASS, and based on having the camera pointing
straight down from the plane, I put the angles for both of these at 0.
The instructions state that I don't need this step when using vertical
images anyway, but for the sake of me understanding how this whole
process works I wanted to enter the values (and can just leave 'use at
run-time' set to 0).
Should I actually have used 90? My interpretation of the instructions
may have been wrong in assuming zero is when camera view is
perpendicular to ground in the x and y planes. Is camera angle for these
values measured from horizontal?
Then, if there is roll or pitch, do either of these have a
positive/negative or clockwise/counterclockwise format? the instructions
explain yaw is measure in degrees from north, clockwise being positive
(but not full 360; goes to +180 or -180)?

Another key thing I may have shot myself in the foot with, is editing
original image. It was too big to use in entirety, so I cropped out the
section I wanted in the middle. I also increased resolution of this
cropped bit to give a clearer picture. So whilst dimensions have
changed, it is not the extent of image that is indicated by original
camera specs e.g. focal length, width/height, pixel dimensions will be
different.
Does this mean the use of orth-photo rectification is voided for me? Or
can I employ some trigonometry and basic maths to calculate parameters
for the cropped image from the parameters of the original image and then
feed those into i.ortho.photo?

Comments appreciated.

Regards,
Shane.

PS, on similar note, another poster asked about using ortho-photo
rectification of satellite images, to which the advice was 'you can't'.
I would have thought a satellite image is akin to an aerial image taken
just a little higher off the ground...?? or do sat images span too much
curvature of the earth to rectify well in i.ortho.photo? or do they lack
the parameters required such as camera angles, focal lenghts/points,
etc?

```

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