When building RPCs for digital camera aerial photography and pushbroom sensor imagery, you will need to enter various required parameters such as principal points, focal lengths and pixel sizes, and incidence angles. This section provides guidelines on determining these values.
Principal Point Coordinates
Principal point coordinates are often set to [0.0, 0.0], which assumes that the principal point is the center of the image for a frame central projection, and the center of each scan line for a line central projection. A laboratory calibration report should provide the principal point coordinates.
Focal Length and Pixel Size
Focal length is the orthogonal distance from the perspective center to the image focal plane. Pixel sizes correspond to the CCD cells (detectors of the camera that captured the images). Typically, aerial digital cameras and satellite pushbroom sensors have square pixels, which means that the pixel size is the same in the x and y dimension.
Focal length and pixel sizes are usually available through the data provider or camera calibration report. You can often evaluate the correctness of the input pixel size based on its relationship with flight height, ground resolution, and focal length:
Where:
f is the focal length
H is the aircraft or satellite altitude
PS is the pixel size on the camera lens
GSD is the ground sample distance or ground resolution.
The following table lists focal lengths and pixel sizes for some aerial cameras and satellite pushbroom sensors:
|
Sensor Name |
Focal Length (mm)
|
Image Pixel Size (mm)
|
|
ADS40 |
62.77 |
(0.0065, 0.0065)
|
|
ALOS AVNIR-2 |
800.0 |
(0.0115, 0.0115)
|
|
ALOS PRISM |
1939.0 |
(0.007, 0.007) |
|
ASTER |
329.0 (Bands 1, 2, 3N)
376.3 (Band 3B) |
(0.007, 0.007)
Bands 1, 2, 3N, 3B
|
|
Cartosat-1 |
1945.0 |
(0.007, 0.007) |
|
Deimos-1 |
155.9 |
(0.005, 0.005) |
|
EROS-A1 |
3500.0 |
(0.013, 0.013) |
|
FORMOSAT-2 |
2896.0 |
(0.0065, 0.0065) Pan
|
|
GeoEye-1 |
13300.0 |
(0.008, 0.008) Pan
(0.032, 0.032) Multispectral
|
|
Göktürk-2 |
985.0 |
|
|
IKONOS-2 |
10000.0 |
(0.012, 0.012) Pan
|
|
IRS-1C |
982.0 |
(0.007, 0.007) Pan
|
|
IRS-1D |
974.8 |
(0.007, 0.007) Pan
|
|
KOMPSAT-2 |
9000.0 Pan
2250.0 Multispectral
|
(0.013, 0.013) |
|
KOMPSAT-3 |
8562.0 |
(0.00875, 0.00875) Pan
(0.0175, 0.0175) Multispectral
|
|
Kodak DCS420 |
28.0 |
(0.009, 0.009) |
|
Landsat 8 OLI |
886.0 |
(0.018, 0.018) Pan
(0.036, 0.036), Multispectral
|
|
MOMS-02 |
660.0 |
(0.01, 0.01) |
|
Pleiades-HR |
12905.0 |
(0.013, 0.013) Pan
(0.052, 0.052) Multispectral
|
|
Proba-V (VGT-P) |
109.6 |
(0.013, 0.013) VNIR
|
|
QuickBird |
8836.2 |
(0.013745, 0.013745)
|
|
RapidEye |
637.0 |
(0.0065, 0.0065)
|
|
ResourceSat AWiFS
|
139.5 |
(0.010, 0.007) |
|
ResourceSat LISS-IV
|
982.0 |
(0.007, 0.007) |
|
ResourceSat LISS-III
|
347.5 |
(0.010, 0.007) VNIR
(0.013, 0.013) SWIR
|
|
SkySat-1 |
3600.0 |
(0.0065, 0.0065)
|
|
SPOT-1 through -4
|
1082.0 |
(0.013, 0.013) Pan
|
|
SPOT-5 HRS |
580.0 |
(0.0065, 0.0065) Pan
|
|
SPOT-6 and -7 |
3760.0 |
(0.008, 0.008) Pan
(0.033, 0.033) Multispectral
|
|
SSOT |
5131.0 |
|
|
STARLABO TLS |
60.0 |
(0.007, 0.007) |
|
UK-DMC-2 |
155.9 |
(0.005, 0.005) |
|
Vexcel UltraCamD
|
101.4 |
(0.009, 0.009) Pan
|
|
VNRedSat-1 |
5131.0 |
(0.012, 0.012) Pan
(0.048, 0.048) Multispectral
|
|
WorldView-2 |
13311.0 |
(0.008, 0.008) |
|
Z/I Imaging DMC |
120.0 |
(0.012, 0.012) |
|
ZY-1-02C |
1010.0 |
(0.0065, 0.0065)
|
Along Track and Across Track Incidence Angles
These parameters only apply to pushbroom sensors, and they are different for each sensor. Data from some pushbroom sensors have fixed incidence angles across all images, while other data have different incidence angles for each image. These angles are only used to set the initial values for the exterior orientation parameters and do not have to be exact.
The along track incidence angle is the angle (in degrees) between the vertical position of the satellite and its forward or backward viewing direction. If viewed from the ground point corresponding to the scene center, the along track incidence angle has a positive value if the viewing direction is northward.
The across track incidence angle is the angle (in degrees) between the vertical position of the satellite and its side-viewing direction when the sensor is scanning along the side. If viewed from the ground point corresponding to the scene center, the across track incidence angle has a positive value if the viewing direction is eastward.
Following are some guidelines for determining the incidence angles for different pushbroom sensors.
ASTER
You can set both angles to 0.0 degrees. However, for Band 3B, you should set the along track incidence angle to -27.6 degrees (descending orbit) or 27 degrees (ascending orbit), and the across track incidence angle to 0.0 degrees.
Note: ASTER and SPOT data files contain RPC information, which you can retain before orthorectification or DEM extraction and avoid using Build RPCs altogether.
IKONOS
The *_metadata.txt file associated with an IKONOS dataset lists Nominal Collection Elevation Angle and Nominal GSD (Cross Scan and Along Scan) for each source image. Use these values to compute the approximate along track and across track incidence angles with the following equations:
Where:
You should set the signs of the incidence angles according to the actual pointing direction, which you can determine from the Nominal Collection Azimuth value in the *_metadata.txt file.
IRS-1C/1D
Set the along track incidence angle to 0.0 degrees. Set the across track incidence angle according to the Input view angle (Deg) value in the leader file.
KOMPSAT-2
Set the approximate incidence angles using the AUX_IMAGE_SATELLITE_INCIDENCE_DEG metadata field in the associated ephemeris data file (.eph).
QuickBird
Set the approximate incidence angles (and signs) using the inTrackViewAngle and crossTrackViewAngle values in the associated *.IMD file.
RapidEye
Set the across track incidence angle to the spaceCraftViewAngle value in the associated *_metadata.xml file. Set the along track incidence angle to 0.0 degrees.
SPOT
Incidence angles are available in the leader file (CAP format) or XML metadata file (DIMAP format).
For SPOT-1 through SPOT-4 data, you can set the along track incidence angle to 0 because this type of viewing is not allowed. For SPOT-5 data, the XML metadata lists the along track incidence angle in the <INCIDENCE_ANGLE> tag. Use the <VIEW_ANGLE> tag to set the across track incidence angle.
For CAP-format data, the incidence angle is in the byte offset 453-468 within the header record. You can use a simple text editor to view the header record. The format for the incidence angle is <X>AA.A, for example, L12.7 or R18.1. If the prefix is L, set the angle to a negative value. If the prefix is R, set the angle to a positive value.
WorldView-1 and WorldView-2
Set the approximate incidence angles (and signs) using the mean InTrackViewAngle and meanCrossTrackViewAngle values in the associated *.IMD file.