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Orthorectifying UltraCam and DMC aerial imagery

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Vexcel’s UltraCam digital camera system has a focal distance of approximately 100mm and offers a base panchromatic (black and white) resolution of 11,500 x 7,500 pixels, along with four color channels: red, green, blue (RGB) and near-infrared. Forward motion compensation can support pixels as small as three centimeters on the ground.

The UltraCam features imagery with a better than 12-bit per pixel dynamic range. This improves the radiometry for better matching accuracy, allowing for more flying days in marginal weather as well as better interpretability, better stereo, and more success in automated procedures.

PCI Geomatica supports UltraCam L3 type of images. This tutorial will demonstrate the processing steps of UltraCam L3 images in Geomatica OrthoEngine.

Note: Processing steps for DMC are same as UltraCam except different Camera Calibration parameters.

Geomatica OrthoEngine

1. Initial Project Setup

Start OrthoEngine and click ‘New’ on the File menu to start a new project. Give your project a ‘Filename’, ‘Name’ and ‘Description’. Select ‘Aerial Photography’ as the Math Modeling Method. Under Options, select ‘Camera Type’ as ‘Digital / Video’ and ‘Exterior Orientation’ as ‘User Input’. Exterior orientation is selected as User Input, if GPS/INS information is available from digital camera. Otherwise select ‘Compute from GCPs and   Tie   Points’   and   the   math   model   will   be   calculated   from   user   entered   GCPs   and   TPs.

After accepting this panel you will be prompted to set up the projection information for the output files and the output pixel spacing. Enter the appropriate projection information for your project.

2. Camera Calibration

OrthoEngine will automatically prompt you to enter ‘Digital Camera Calibration Information’ after projection information in above step.
 
Camera Calibration information is generally provided with your digital aerial dataset. Check support files of your UltraCam dataset and enter appropriate camera calibration information in OrthoEngine.

Principal point offset value can be calculated as:

Principal Point of Symmetry (POS) + Principal Point of Autocollamation (PPA) – Indicated Principal Point (IPP)

Principal point offset for DMC camera is usually zero.

NOTE: DMC digital camera airphotos can be processed using exact same steps as UltraCam  except ‘Camera Calibration’ information. Camera calibration parameter of DMC and UltraCam are distinct.

Below images show demo Camera Calibration panels for UltraCam and DMC digital cameras.

 

3. Data Input

Go  to  ‘Data  Input’  processing  step  and  click ‘Open a new or existing image’ button. Add your airphotos using ‘New Image’ option in ‘Open Image’ panel.

 

4. Exterior Orientation

Exterior  orientation  represents  a  transformation  from  ground  co-ordinate  system  to  image  co-ordinate system. In other words, exterior orientation is the position and orientation of the camera when the image was
 
taken. Most photogrammetric cameras are equipped with onboard GPS and Inertial Navigation System (INS). These systems collect the exterior orientation directly on the plane and ship this information with digital dataset.

Exterior Orientation can be either imported using GPS/INS data or can be computed from GCPs and Tie Points. If you are using GCPs and TPs to compute exterior orientation, then adding estimated or observed exterior orientation to your project can reduce the number of GCPs required. It also helps to automate the tie point collection and it decreases the time needed to setup the project because it provides an approximate location of the images.

In this project we will import GPS/INS data from an external file. In ‘Data Input’ processing step, click on ‘Import exterior orientation data from text file’ button to pop-up the import window. Select appropriate ‘File Format’ according to the format of your GPS/INS file and import the text file into the orthoengine project. Use ‘Manually edit exterior orientation data’ window, in case you need to edit the EO information before processing the images.

 

5. Collect GCPs and Tie Points

This step is valid only if exterior orientation is selected as ‘Compute from GCPs and Tie Points’ during initial project setup.

Select the ‘GCP/TP Collection’ processing step. GCP collection can be done using various options viz. ‘Manual Entry’, ‘Geocoded Images/Vectors’, ‘Chip Database’ or a ‘Text File’.

For Aerial photographs 3 GCPs per image are recommended for highest accuracy. Tie Points are usually collected in a typical Von Gruber pattern of 3X3 points per image. User should try to add good quality GCPs and TPs. Features that are closer to ground should be selected as TPs instead of features like tree tops, shadows etc. Tie points that join multiple images together produces a more accurate math model. Also, if available, enter the elevation value of the tie points in your project. Tie points with an elevation value help to control elevation errors and improve the accuracy of geometric model.

After GCPs and TPs collection, click ‘Compute Model’ button under ‘Model Calculations’ processing step. This will compute the math model for your Airphoto OrthoEngine project.

 

6. Generating Orthos

The next step is to set up your ‘Ortho Image Production’. Proceed to the ‘Ortho Generation’ processing step and select the file(s) to be orthorectified. Choose the DEM file to be used in the processing, and other processing parameters. Click on ‘Generate Orthos’ to create the final orthorectified airphoto.

 

The Image below shows individual orthorectified airphotos.

 

7. Mosaicking

The final step is to generate Mosaic of your orthorectified airphotos. OrthoEngine provides an option of manual as well as automatic mosaicking. Based on the size of dataset, manual mosaicking could be very difficult and time consuming. In this project we will perform automatic mosaicking of above two ortho airphotos.
 
Use ‘Define mosaic area’ to create a blank mosaic file. Enter a mosaic file name and select the footprint of both airphotos in define mosaic panel. Click ok to close the define mosaic window.

Activate ‘Automatic Mosaicing’ panel from ‘Automatic Mosaic Generation’ button on Mosaic processing step in Orthoengine. Set the mosaicing parameters appropriately and ‘Generate Mosaic’. An automatic cut line generation and color balancing is performed internally before mosaicking the images.

Images below show a snapshot of ‘Define Mosaic’ panel, ‘Automatic Mosaicing’ panel and the final mosaiced airphoto.

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