[GRASS-user] rectifying digital images

[Markus Metz]

Hi Maning,

if I understand you right, the purpose is to obtain a georeferenced
mosaic of the aerial photographs. There are several ways to get to there.
The easiest is to georeference each aerial photograph against your
Quickbird imagery. This is straightforward and well documented in the
GRASS help. Then you mosaic the photographs together, either with the
GRASS script or, if you're not happy with the results of i.image.mosaic,
use r.mapcalc and get inspiration from i.image.mosaic. This should
already give you a fairly good result.

if the camera was tilted against the ground which is apparently the case
with your aerial photographs, steep slopes and large differences in
elevation within an image might cause distortions which are not removed
by simple georectification, unless you use a brute force approach with
the orthorectified Quickbird imagery, i.e. at least 100 control points
per image and the strongest rubbersheeting method available.

Orthorectification is apparently only supported for analogue aerial
photography in GRASS, because you need to know points of symmetry and
the location of fiducial marks. Nowadays orthorectification of aerial
photography taken with a digital camera requires you to know not more
than the CCD resolution, focal length and flying height above ground,
but this applies only to some commercial software packages. It helps to
have a camera calibration data sheet, but you can get away without it.
Anyway, digital cameras are apparently not supported in i.ortho.photo.

I heard of some tools that can add fiducial marks to digital
photographs, but don't know where to get them and how they work, and I
would advise against patching like this.

Not sure if that helps, but good luck,

Markus
 

maning sambale wrote:
> Hi,
>
> I have a lot of aerial photos I need rectify.  The data came from an
> incomplete project intending to acquire geospatial data for landslide
> risk assessment.
> The project ended prematurely.
>
> Here are the details:
> Images were acquired using this method.
>
> Two GPS receivers were used, one attached to the laptop to facilitate
> the navigation of the
> airplane and the other to the camera to determine the geographic location of the
> center of the photographs. The plane flew along the flight lines
> indicated in the plan.
> The photographs were taken along the flight line using a professional
> Kodak N14 digital
> SLR camera, which has a resolution of 14 megapixels. The camera was
> fastened to a
> mount, made from plywood and customized to accommodate the camera and the laptop
> computer. The seat next to the pilot was detached to make room for the mount. A
> camera door, designed specially for this purpose, replaced the normal Cessna 172
> passenger door.
>
> What do I have now:
> over 800 photos
> GPS point location for each photo
> Quickbird image of the whole study area
>
> According to their report:
> There is difficulty in mosaicking the photographs. The
> original plan was to stitch the photographs together to create a
> composite image.
> But the tilt in the photographs due to the movement of the aircraft
> requires that
> certain adjustments be made.
>
> Any ideas on how to resolve this.  As far as I know, you need certain
> camera parameters to orthorectify aerial photos
> CAMERA NAME:               camera name______
> 	CAMERA IDENTIFICATION:     identification___
> 	CALIBRATED FOCAL LENGTH mm.:_________________
> 	POINT OF SYMMETRY (X)   mm.:_________________
> 	POINT OF SYMMETRY (Y)   mm.:_________________
> 	MAXIMUM NUMBER OF FIDUCIALS:_________________
>
>
> How do I get this?  Is it OK to rectify the imges from the quickbird
> satellite data?
>
> cheers,
>
> maning
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