Image and Data Acquisition

Producer Field Guide

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Producer Field Guide
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During photographic or image collection, overlapping images are exposed along a direction of flight. Most photogrammetric applications involve the use of overlapping images. In using more than one image, the geometry associated with the camera/sensor, image, and ground can be defined to greater accuracies and precision.

During the collection of imagery, each point in the flight path at which the camera exposes the film, or the sensor captures the imagery, is called an exposure station, as shown in the figure below.

Exposure Stations Along a Flight Path

photog_exposure_stations_flight_plan

Each photograph or image that is exposed has a corresponding image scale associated with it. The image scale expresses the average ratio between a distance in the image and the same distance on the ground. It is computed as focal length divided by the flying height above the mean ground elevation. For example, with a flying height of 1000 m and a focal length of 15 cm, the image scale (SI) would be 1:6667.

Flying height above ground is used, versus the altitude above sea level.

A strip of photographs consists of images captured along a flight line, normally with an overlap of 60%. All photos in the strip are assumed to be taken at approximately the same flying height and with a constant distance between exposure stations. Camera tilt relative to the vertical is assumed to be minimal.

The photographs from several flight paths can be combined to form a block of photographs. A block of photographs consists of a number of parallel strips, normally with a sidelap of 20-30%. Block triangulation techniques are used to transform all of the images in a block and ground points into a homologous coordinate system.

A regular block of photos is a rectangular block in which the number of photos in each strip is the same. The figure below shows a block of 5 × 2 photographs.

A Regular Rectangular Block of Aerial Photos

photog_rectangular_photo_block

Photogrammetric Scanners

Photogrammetric quality scanners are special devices capable of high image quality and excellent positional accuracy. Use of this type of scanner results in geometric accuracies similar to traditional analog and analytical photogrammetric instruments. These scanners are necessary for digital photogrammetric applications that have high accuracy requirements.

These units usually scan only film because film is superior to paper, both in terms of image detail and geometry. These units usually have a Root Mean Square Error (RMSE) positional accuracy of 4 microns or less, and are capable of scanning at a maximum resolution of 5 to 10 microns (5 microns is equivalent to approximately 5,000 pixels per inch).

See RMS Error in Rectification for more information about Root Mean Square Error.

The required pixel resolution varies depending on the application. Aerial triangulation and feature collection applications often scan in the 10- to 15-micron range. Orthophoto applications often use 15- to 30-micron pixels. Color film is less sharp than panchromatic, therefore color ortho applications often use 20- to 40-micron pixels.

Desktop Scanners

Desktop scanners are general purpose devices. They lack the image detail and geometric accuracy of photogrammetric quality units, but they are much less expensive. When using a desktop scanner, you should make sure that the active area is at least 9 × 9 inches (that is, A3 type scanners), enabling you to capture the entire photo frame.

Desktop scanners are appropriate for less rigorous uses, such as digital photogrammetry in support of GIS or remote sensing applications. Calibrating these units improves geometric accuracy, but the results are still inferior to photogrammetric units. The image correlation techniques that are necessary for automatic tie point collection and elevation extraction are often sensitive to scan quality. Therefore, errors can be introduced into the photogrammetric solution that are attributable to scanning errors. IMAGINE Photogrammetry Project Manager accounts for systematic errors attributed to scanning errors.

Scanning Resolutions

One of the primary factors contributing to the overall accuracy of block triangulation and orthorectification is the resolution of the imagery being used. Image resolution is commonly determined by the scanning resolution (if film photography is being used), or by the pixel resolution of the sensor. In order to optimize the attainable accuracy of a solution, the scanning resolution must be considered. The appropriate scanning resolution is determined by balancing the accuracy requirements versus the size of the mapping project and the time required to process the project. Scanning Resolutions Table lists the scanning resolutions associated with various scales of photography and image file size.

Scanning Resolutions Table

12 microns
(2117 dpi1)

16 microns
(1588 dpi)

25 microns
(1016 dpi)

50 microns
(508 dpi)

85 microns
(300 dpi)

Photo Scale

1 to

Ground Coverage (meters)

Ground Coverage (meters)

Ground Coverage (meters)

Ground Coverage (meters)

Ground Coverage (meters)

1800

0.0216

0.0288

0.045

0.09

0.153

2400

0.0288

0.0384

0.06

0.12

0.204

3000

0.036

0.048

0.075

0.15

0.255

3600

0.0432

0.0576

0.09

0.18

0.306

4200

0.0504

0.0672

0.105

0.21

0.357

4800

0.0576

0.0768

0.12

0.24

0.408

5400

0.0648

0.0864

0.135

0.27

0.459

6000

0.072

0.096

0.15

0.3

0.51

6600

0.0792

0.1056

0.165

0.33

0.561

7200

0.0864

0.1152

0.18

0.36

0.612

7800

0.0936

0.1248

0.195

0.39

0.663

8400

0.1008

0.1344

0.21

0.42

0.714

9000

0.108

0.144

0.225

0.45

0.765

9600

0.1152

0.1536

0.24

0.48

0.816

10800

0.1296

0.1728

0.27

0.54

0.918

12000

0.144

0.192

0.3

0.6

1.02

15000

0.18

0.24

0.375

0.75

1.275

18000

0.216

0.288

0.45

0.9

1.53

24000

0.288

0.384

0.6

1.2

2.04

30000

0.36

0.48

0.75

1.5

2.55

40000

0.48

0.64

1

2

3.4

50000

0.6

0.8

1.25

2.5

4.25

60000

0.72

0.96

1.5

3

5.1

B/W File Size (MB)

363

204

84

21

7

Color File Size (MB)

1089

612

252

63

21

1 dots per inch

The ground coverage column refers to the ground coverage per pixel. Thus, a 1:40000 scale photograph scanned at 25 microns [1016 dots per inch (dpi)] has a ground coverage per pixel of 1 m × 1 m. The resulting file size is approximately 85 MB, assuming a square 9 × 9 inch photograph.

Coordinate Systems

Conceptually, photogrammetry involves establishing the relationship between the camera or sensor used to capture imagery, the imagery itself, and the ground. In order to understand and define this relationship, each of the three variables associated with the relationship must be defined with respect to a coordinate space and coordinate system.

Pixel Coordinate System

File coordinates of a digital image are defined in a pixel coordinate system. A pixel coordinate system is usually a coordinate system with its origin in the upper-left corner of the image, the x-axis pointing to the right, the y-axis pointing downward, and the unit in pixels, as shown by axis c and r in the figure below. These file coordinates (c, r) can also be thought of as the pixel column and row number.

Pixel coordinate system is referenced in this section as:

pixel coordinates (c, r)

Pixel Coordinates and Image Coordinates

photog_coord_system_pixel

Image Coordinate System

An image coordinate system or an image plane coordinate system is usually defined as a two-dimensional coordinate system occurring on the image plane with its origin at the image center, normally at the principal point or at the intersection of the fiducial marks as illustrated by axis x and y in the figure above. Image coordinates are used to describe positions on the film plane. Image coordinate units are usually millimeters or microns.

Image coordinate system is referenced in this section as:

image coordinates (x, y)

Image Space Coordinate System

An image space coordinate system is identical to image coordinates, except that it adds a third axis (z). The origin of the image space coordinate system is defined at the perspective center S as shown in the figure below. Its x-axis and y-axis are parallel to the x-axis and y-axis in the image plane coordinate system. The z-axis is the optical axis, therefore the z value of an image point in the image space coordinate system is usually equal to -f (focal length). Image space coordinates are used to describe positions inside the camera and usually use units in millimeters or microns.

Image space coordinate system is referenced in this section as:

image space coordinates (x, y, z)

Image Space and Ground Space Coordinate System

photog_coord_system_image_space

Ground Coordinate System

A ground coordinate system is usually defined as a three-dimensional coordinate system that utilizes a known map projection. Ground coordinates (X,Y,Z) are usually expressed in feet or meters. The Z value is elevation above mean sea level for a given vertical datum.

Ground coordinate system is referenced in this section as:

ground coordinates (X,Y,Z)

Geocentric and Topocentric Coordinate System

Most photogrammetric applications account for the Earth’s curvature in their calculations. This is done by adding a correction value or by computing geometry in a coordinate system which includes curvature. Two such systems are geocentric and topocentric coordinates.

A geocentric coordinate system has its origin at the center of the Earth ellipsoid. The Z-axis equals the rotational axis of the Earth, and the X-axis passes through the Greenwich meridian. The Y-axis is perpendicular to both the Z-axis and X-axis, so as to create a three-dimensional coordinate system that follows the right hand rule.

A topocentric coordinate system has its origin at the center of the image projected on the Earth ellipsoid. The three perpendicular coordinate axes are defined on a tangential plane at this center point. The plane is called the reference plane or the local datum. The x-axis is oriented eastward, the y-axis northward, and the z-axis is vertical to the reference plane (up).

For simplicity of presentation, the remainder of this chapter does not explicitly reference geocentric or topocentric coordinates. Basic photogrammetric principles can be presented without adding this additional level of complexity.

Terrestrial Photography

Photogrammetric applications associated with terrestrial or ground-based images utilize slightly different image and ground space coordinate systems. The figure below illustrates the two coordinate systems associated with image space and ground space.

Terrestrial Photography

photog_terrestrial_photography

The image and ground space coordinate systems are right-handed coordinate systems. Most terrestrial applications use a ground space coordinate system that was defined using a localized Cartesian coordinate system.

The image space coordinate system directs the z-axis toward the imaged object and the y-axis directed North up. The image x-axis is similar to that used in aerial applications.

The XL, YL, and ZL coordinates define the position of the perspective center as it existed at the time of image capture.

The ground coordinates of ground point A (XA, YA, and ZA) are defined within the ground space coordinate system (XG, YG, and ZG). With this definition, the rotation angles greek_omega_symbol, greek_phi_symbol, and greek_kappa_symbol are still defined as in the aerial photography conventions.

In IMAGINE Photogrammetry Project Manager, you can also use the ground (X, Y, Z) coordinate system to directly define GCPs. Thus, GCPs do not need to be transformed. Then the definition of rotation angles greek_omega_prime_symbol, greek_phi_prime_symbol, and greek_kappa_prime_symbol are different, as shown in the figure above.