Ratios
Abstract
Stereoscopic vision is fundamental in the task of photogrammetric restitution (stereo compilation) in which, by inserting a floating mark in the 3D observation of pairs of images, it is possible to draw the elements of the terrain in space and obtain cartography of a part of the land cover from aerial images. Initially with film photographs, which were later scanned, and finally with large format digital cameras that began in the 2000s, photogrammetry has undergone a series of technological revolutions up to the present time. In this chapter, after a brief exposition of the basic principles of photogrammetric restitution, a review of current large-format digital cameras and their main implications in restitution is made, which, despite the advances and other similar semi-automatic products (DTM, orthophoto) is still manual and must be operated by a person with the implications that this entails in stereoscopic vision.
Keywords
- photogrammetry
- aerial digital cameras
- stereoscopic precision
- Ground Sample Data
- photogrammetric restitution
1. Introduction
Photogrammetry (the art and science of determining the position and shape of objects from photographs [1]) has been used since the early 20th century as an efficient method of generating mapping of large areas of the territory, from images obtained with cameras on board an aircraft.
The “General Method of Photogrammetry” describes the stereoscopic processing of images: acquisition of a pair of images that verify artificial stereoscopy conditions; orientation of the images to each other; and virtual three-dimensional exploration of the stereoscopic space generated and cartographic capture of points:
Acquisition of a pair of images that verify artificial stereoscopy conditions. The process of artificial stereoscopic vision is based on generating an image for the left eye and an image for the right eye (as in natural stereoscopic vision). In photogrammetry, images are separated from each other by a certain distance (called base, b) and the image axes are normal to this base and parallel to each other. This configuration is known as a normal case.
Orientation of the images to each other. This achieves a stereoscopic model that is also metric (relative to an external reference system).
Virtual three-dimensional exploration of the stereoscopic space generated and cartographic capture of points. In 1892 Stolze invented the floating mark, which allows metric three-dimensional exploration. Two marks located in photography paths are perceived as a single point located in space. If the observer can move the marks on the images while receiving a stereoscopic perception of them, they can “pose the floating mark” on the surface of the object. This way of obtaining 3D coordinates is known as photogrammetric restitution or stereo compilation (Figure 1).
The
The precision of the measure on the image plane,
Moreover, the product of pixel size for image scale provides the pixel size in the ground,
Thus, the precision observed in
From this expression it follows that the higher
It is important to note that
The precision in
The measurement precision of the horizontal parallax can be replaced by the measurement precision in the image plane,
The precision of the measure in the image plane,
Moreover, the product of pixel size for image scale provides the pixel size in the ground,
As can be seen, precision in
The value
Camera | Width (mm) | |||
---|---|---|---|---|
Analogue | 150 | 220 | 88 | 1,70 |
DMC | 120 | 95 | 38 | 3,16 |
UltraCamD | 100 | 67,5 | 27 | 3,70 |
DMC III | 92 | 56,9 | 22.8 | 4,04 |
UltraCam Eagle M3 f80 | 80 | 68 | 2722 | 2,94 |
UltraCam Eagle M3 f210 | 210 | 68 | 27.2 | 7,72 |
2. Aerial digital cameras
2.1 Technological evolution: film to digital
Photogrammetry has undergone three digital revolutions. The first took place in the 1980s with the digitization of the mathematical model that resulted in analytical stereoplotters (process input, frames, remained analogue). Analogue stereoplotters that solved the mathematical model by mechanical analogy disappeared.
The second occurred in the 1990s with digitization, using powerful and accurate photogrammetric scanners, analogue images from the aircraft that resulted in digital stations. This revolution was possible when personal computers had sufficient capacity to efficiently handle digital images, and represented the extinction of analytical stereoplotters and, with them, the stereoplotters themselves; that is, of the specific physical machines used for photogrammetry.
The third revolution took place in the first decade of the 21st century thanks to the development of digital cameras that began to compete with the large format (230 x 230 mm) of analogue cameras. These cameras were already used in terrestrial photogrammetry, where the small and medium formats were enough to develop projects. The need to cover large areas of land in aerial photogrammetry made the large size of the camera the only possible solution between the two photogrammetric requirements: accuracy (requiring long focal length), and performance in object coverage (requiring either short focal or large focal planes). This phase involves the disappearance of analogue cameras and, consequently, films. Therefore it will also represent the disappearance of photogrammetric scanners (Table 2).
It consists of | Appears | Disappears | |
---|---|---|---|
Digitizing the mathematical model | Analytical stereoplotters | Analogue stereoplotters | |
Post-flight scanning of the image | Digital station Photogrammetric scanner | Analytical stereoplotters | |
In-flight scanning of the image | Digital aerial camera | Analogue aerial camera Aerial films Photogrammetric scanner |
The main advantages of digital versus analogue images are [2]: the ability to establish an entirely digital workflow (suppressing the scanners), a considerable improvement in radiometric quality as well as the possibility of simultaneously acquiring panchromatic images in the different color bands and in the near infrared; as well as the ability to generate real-time mapping [3]. The color depth of digital cameras (12-bit) should allow flights to be carried out in poor lighting conditions [4].
A first approach to digital cameras for photogrammetric use allows them to be classified into two large groups: frame and pushbroom. The first can be classified according to the size of the sensor [5]: small format cameras (up to 16 megapixels); medium format cameras (from 16 to 50 megapixels); and large format cameras (50 megapixels or higher). More recently, medium format can be located at 80–100 megapixels [6], and large format larger than 100 megapixels.
2.2 Cameras in the photogrammetric mapping sector
The two large manufacturers of analogue cameras for aerial photogrammetry took the first steps towards digital cameras around 2000, each opting for a different technology. Leica, who manufactured the RC30 model, used a pushroom camera, ADS40, based on space sensors (HSRC, WAAC) from the German institute DLR. Meanwhile, Z/I Imaging moved from the RMK-TOP analogue model to the DMC modular camera in 2000 (synchronous mode).
Following the launch in 2000 of ADS40 and DMC, Vexcel, manufacturer of scanners for photogrammetric use, offered the UltraCamD digital camera to the market in 2003 (syntopic mode).
Currently there are also other large format cameras and even some medium format ones competing for the same sector, but it is not our intention in this chapter to give a review of the current offer of cameras for mapping, only to study the characteristics related to stereoscopic accuracy. To do this, the current models of Leica, Z/I Imaging (now both in Hexagon Group) and Vexcel are considered representative: DMC III and UltraCam Eagle M3. In addition, they allow working with interchangeable lenses, which adds versatility so as to be able to use the most appropriate focal length for each use: short for mapping purposes, and long for orthophotos generation.
The sensors of large format digital cameras are clearly smaller in size than conventional cameras (Width in Table 3). To find an easily interpretable equivalence, we could say that, in order for digital cameras to compete with analogue, it must be assumed that there is an equivalence between 20 μm, the size of the scanned pixel, and the 10 μm average pixel size in a digital camera (Table 3).
Camera | Width (mm) | Pixel size (μm) | Focal lenght (mm) | Angular resolution (″) | AGL (m) for GSD = 0,10 m |
---|---|---|---|---|---|
Analogue | 220 | 20 | 150 | 28 | 750 |
Analogue | 220 | 15 | 150 | 21 | 1000 |
DMC | 168 | 12 | 120 | 21 | 1000 |
UltraCamD | 103,5 | 9 | 100 | 19 | 1111 |
DMC III | 56,9 | 3,9 | 92 | 9 | 2359 |
UltraCam Eagle M3 f80 | 68 | 4 | 80 | 10 | 2000 |
UltraCam Eagle M3 f210 | 68 | 4 | 210 | 4 | 5250 |
Initially, the equivalence between 20 μm scanned and 10 μm digital is assumed by the manufacturers of these cameras because they think that the digital pixel is of higher quality than the scanned pixel [7]. According to these same authors, different experiments indicate that automatic matching is 2,5 times better in the case of a digital image.
Angular resolution refers to the angle subtended by a pixel from the point of view. The initial digital cameras can be considered equivalent to the 15 μm scan; however, the current ones are much better (less than 10″).
Figure 2 provides a comparison of a reticle photographed and scanned at a resolution of 5, 10 and 15 μm (a, b and c) while on the right (d) the same grid obtained directly by a digital camera appears.
3. Stereoscopic performance
Another issue in favor of digital cameras is their increased ability to get more images [7]. This gives them an advantage in terms of the possibility of achieving greater redundancy in the aerotriangulation process, and obtaining greater overlaps that decrease the overlap in the direction of flight.
The following sections provide a theoretical analysis of the stereoscopic performance of the DMC and UltraCam compared to the performance of analogue cameras.
3.1 Frame rate
Considering an airplane speed of 75 m/s and a
Where
This means that images with GSD from 25 mm to 90% cannot be achieved, because the plane cannot fly that slowly (750 pixels * 25 mm = 18,75 m). These 25 mm imply a flight height 277,78 m:
where f is the focal length (100 mm), sx the pixel size (9 μm) and GSD is Ground Sample Distance (25 mm).
The following Tables 4–6 show the resulting minimum
RL (%) | GSD min (m) | H (m) |
---|---|---|
60 | 0,025 | 277,778 |
70 | 0,033 | 366,667 |
80 | 0,050 | 555,56 |
90 | 0,10 | 1111,111 |
RL (%) | GSD min (m) | H (m) |
---|---|---|
60 | 0,020 | 276,451 |
70 | 0,027 | 368,601 |
80 | 0,040 | 552,902 |
90 | 0,080 | 1105,803 |
RL (%) | GSD min (m) | H (m) |
---|---|---|
60 | 0,049 | 488,281 |
70 | 0,065 | 651,042 |
80 | 0,098 | 976,563 |
90 | 0,195 | 1953,125 |
3.2 Coverage
A fundamental part of any photogrammetric flight project is determining the direction and number of passes, number of total and past photographs, among other data. All this is determined from a series of initial conditions that establish the work area, which define the scale of the photograph to be obtained, etc…
First, a flight made with analogue camera is analyzed, for example, with the following characteristics:
Scale of photography: 1:20.000
Scanning photo size: 15 μm.
Useful photo size: 220x220 mm.
This provides a GSD of:
considering that each frame has a useful format of 220x220 mm, due to the space reserved in the frame for marginal information, the surface contained per frame is:
Now, with that same GSD, the resulting area for the
Comparing both surfaces,
This means that approximately the 2.5-frame area of
3.3 Number of frames
Considering first longitudinally, and with the overlap of 60%, each new frame adds one side of 40% more to the strip. That is, the covered length,
To relate the number of photos to both flights to cover the same
Similar reasoning can be followed for cross-sectional overlap:
which in the example provides the following relationship:
If we now want to know the relationship between the total number of photographs between the two flights:
corresponding to the amount found in Eq. (15).
3.4 Conclusions on coverage
The relationship between the number of frames taken with analog camera and
With the digital camera it is enough to multiply the number of pixels in height and width of the image by the GSD to obtain the actual magnitudes of the terrain to be covered with each frame In addition, it is clear that said GSD, the pixel size in the CCD, and the focal length will determine the flight height and scale of the photograph (Table 7).
Camera | Useful width | Useful height | Coverage | Ratio |
---|---|---|---|---|
220 mm | 220 mm | 1.936 Ha | — | |
7.500 pixels | 11.500 pixels | 776,25 Ha | 2,49 | |
9.420 pixels | 14.430 pixels | 1.223,38 Ha | 1,58 | |
7.680 pixels | 13.824 pixels | 955,51 Ha | 2,03 |
While with the analog camera it is necessary to determine either the frame scale or the scanning pixel (variable depending on the scanner). Depending on one parameter, the other parameter is obtained. However, it is true that it is scanned at 15–20 μm., therefore, usually this pixel size determines the frame scale.
Acknowledgments
Department of Cartographic and Land Engineering, University of Salamanca.
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