**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**).

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.


#### **Table 3.** *Data of large format digital cameras.*

#### **Figure 2.**

*A reticle photographed and scanned at a resolution of 5, 10 and 15 μm (a, b and c). The same grid obtained directly by a digital camera (d) [8].*

## **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 *frame rate* of 1 second, the movement of the aircraft between two shots is 75 metros. The image size, in the case of the UltracamD camera, is 7.500 pixels in the direction of flight (*along track*). If the longitudinal overlap is 60%, this means that the base is 3.000 pixels. In this way, the minimum GSD with stereoscopic overlap that can be obtained with 60% longitudinal overlap is 25 mm.

$$\text{GSD}\_{\text{minimum}} = \frac{\text{Desplaz}}{\text{Px} \* (1 - R\_L)} \tag{10}$$

Where *Desplaz* is the displacement of the aircraft between two shots (75 m), *Px is* the number of pixels of the sensor in the direction of flight (7.500), *RL* is the longitudinal overlap (60%).

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:

$$H = \frac{c}{\mathbf{sx}} \ast \mathbf{GSD} \tag{11}$$

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 *stereoscopic* GSD sizes for different longitudinal overlaps, for *UltraCamD, UltraCamX,* and *DMC cameras*.

### *Stereoscopic Precision of the Large Format Digital Cameras DOI: http://dx.doi.org/10.5772/intechopen.97125*


*It has been considered an aircraft speed of 75 m/s and a frame rate of 1 second (base on the ground of 75 meters).*

#### **Table 4.**

*Minimum stereoscopic GSD sizes and their corresponding flight height (H) for the UltraCamD camera (Px = 7.500; sx = 9 μm; f = 100 mm), given the desired longitudinal overlap (RL).*


#### **Table 5.**

*Minimum stereoscopic GSD sizes and their corresponding flight height (H) for the UltraCamX camera (P = 9.420; sx = 7.2 μm; f = 100 mm), given the desired longitudinal overlap (RL).*


#### **Table 6.**

*Minimum stereoscopic GSD sizes and their corresponding flight height (H) for the DMC camera (P = 7.680; sx = 12 μm; f = 120 mm), given the desired longitudinal overlap (RL).*

#### **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:


This provides a GSD of:

$$25\mu m \* 20.000 = \text{300 mm} \tag{12}$$

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:

$$\begin{aligned} \text{1220 mm} & \ast 20.000 = 4.400 \text{ m} \\ 4.400 \ast 4.400 &= 1.936 \text{ Hz} \end{aligned} \tag{13}$$

Now, with that same GSD, the resulting area for the *UltraCamD image* is:

$$\begin{aligned} &\text{11.500} \text{ pixels} \ast \text{300 mm} = \text{3.450 m} \\ &\text{17.500} \text{ pixels} \ast \text{300 mm} = \text{2.250 m} \\ &\text{3.450} \ast \text{2.250} = \text{776, 25} \text{Ha} \end{aligned} \tag{14}$$

Comparing both surfaces,

$$\frac{1.936}{776,25} = 2,49\tag{15}$$

This means that approximately the 2.5-frame area of *UltraCamD is required* to cover the same surface as an analogue image. However, in the particular case of a frame, this is not true due to the different shapes of these (analog and rectangular square in digital camera). However, if the approach is generic, i.e., to cover a certain area for a project, that relationship can actually be valid.

#### **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, *L,* by *n* frames of a certain width, is determined by:

$$\begin{aligned} L &= \text{width} \ast \mathbf{n} \ast 4 \mathbf{0} \%\\ L &= \text{width} \ast \mathbf{n} \ast (\mathbf{100} - R\_L) \end{aligned} \tag{16}$$

To relate the number of photos to both flights to cover the same *length L*, with the same longitudinal overlap, this ratio is determined by the relationship between the widths of the frames, calculated above (Eqs (13) and (14)):

$$\frac{4.400}{2.250} = 1,956\tag{17}$$

Similar reasoning can be followed for cross-sectional overlap:

$$L = \text{high} \ast \mathbf{n} \ast (\mathbf{100} - R\_T) \tag{18}$$

which in the example provides the following relationship:

$$\frac{4.400}{3.450} = 1,275\tag{19}$$

If we now want to know the relationship between the total number of photographs between the two flights:

*Stereoscopic Precision of the Large Format Digital Cameras DOI: http://dx.doi.org/10.5772/intechopen.97125*


**Table 7.**

*Surfaces contained in a frame for different cameras, considering in all cases a GSD, where ratio expresses the ratio between the coverage of the analog camera and the coverage of the digital camera 300 mm.*

$$1,956 \ast 1,275 = 2,49\tag{20}$$

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 *UltraCamD* digital camera depends on the size of the pixel in the field, GSD. That is, by imposing a certain size of GSD, the relationship between the number of frames in both cameras is determined at the same time.

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**).

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.

## **Acknowledgements**

Department of Cartographic and Land Engineering, University of Salamanca.
