# Aerial Photographs

PCI Geomatics -

#### Can I process UAV imagery with PCI Geomatics software?

A. If you happen to know the focal length, chip size and distortion of your camera and images you can run an aerial photography project in OrthoEngine.

If your images have already been processed in another software but have not been accurately aligned you can import your data into STAX. You can align the images, run vegetation analysis and generate reports. Please see the STAX website for additional information - http://stax.pcigeomatics.com/

#### How do I orthorectify my historical aerial photos if they do not have fiducial marks and I do not know anything about the camera the photos were taken with (i.e. the focal length)?

If you have fiducials but NO coordinates:

• Measure between each set of fiducials along the edge of the photo, in mm. Use the "Compute from Length" button and enter the four lengths.

If you have NO fiducials but do you have a cross marking of the photo center:

• Measure the distance from photo center to each edge, in mm. Use the corner pixels of the photo as fiducials and enter the following coordinates for the camera calibration information as fiducial coordinates:
Top Left: X=-X1 Y=Y1
Top Right: X=X2 Y=Y1
Bottom Right: X=X2 Y=-Y2
Bottom Left: X=-X1 Y=-Y2

If you have NO fiducials, and NO photo center:

• Measure the length of each side of the photo using "Compute from Length" in the Camera Calibration panel - "Fiducial Coordinates" section. Enter your measurements, in mm. Use the image corners as the fiducials.

Focal length of the camera is unknown:

If the focal length of the camera is unknown, there are a few different things you can try:

• Most older cameras were constructed with 6", 8" or 12" focal lengths. A first approach would be to try each of these. If you do not have the right focal length, the resulting flying height will be way off, and you will have remaining distortions in the imagery.
• Publications such as the "ASPRS Manual of Photogrammetry" list photogrammetric cameras through history, with frame sizes and nominal focal lengths. If you have an unusual frame size i.e., not 9"x 9", this would be a valuable tool.
Note: The focal length estimates need only be performed for one photo. After that, you can simply enter the same data for the remainder of the photos.

#### What should the order of my workflow be for my air photo project?

1. Create the project and import the imagery.
2. Import the GPS/INS data for all available photos.
3. Enter GCPs, if desired.
4. Enter at LEAST Tie Points (manually) and GCPs if possible, for any photos which do not have GPS/INS. The auto tie point can only operate if there is an initial approximation for where each photo is. Otherwise, it won't know what photos to match with.
5. Run auto tie point.

#### Do all my photos have to be rotated the same way for auto fiducial collection to run properly?

The auto fiducial collection will run successfully for any combination of orientations. However, the calibration edge has to be set manually. There is no automatic way to do this, since the digital image can be in any orientation. So, you have two options:

1. Run the automatic fiducial collection on the raw images, and then manually change the calibration edge for the affected photos.
2. Rotate the images to a common orientation using the batch rotate function before running the auto fiducial collection. The calibration edge does not need to be adjusted in this case.
The first option is generally the best as it is easiest to work with photos if they are all scanned with left to right overlap, with the up axis of the digital images all pointing the same way (i.e all North-up, or all East-Up, or all 45-degrees-Up.) That makes stereo viewing possible, makes epipolar generation easier, and makes the GCP collection more intuitive.