CC BY
153
ПЛАНИРОВАНИЕ АЭРОСЪЁМКИ В ЭПОХУ ЦИФРОВОЙ ФОТОГРАФИИ ИЛИ ОТ ПОПЕРЕЧЕНОГО ПЕРЕКРЫТИЯ К УКЛОНУ ЗДАНИЙ
Юрий Григорьевич Райзман
Компания Visionmap, ул. Менахем Бегин, 7, Рамат-Ган, 52681, Израиль, зам. директора по науке, тел. +972 3 6091042, e-mail: yuri@visionmap.com
В статье приводится новый подход к планированию аэросъёмки в эпоху цифровой фотографии.
Ключевые слова: аэросъёмка, ортофотоплан, цифровая камера.
FLIGHT PLANNING FOR THE DIGITAL ERA Yuri Raizman
Visionmap Ltd, 7 Menachem Begin Rd. Ramat-Gan. 52681, Israel, VP EMEA & Chief Scientist, tel. +972 3 6091042, e-mail: yuri@visionmap.com
The article describes a new approach for flight planning for the digital era.
Key words: aerial survey, orthophoto, digital camera.
In the era of analog aerial surveys, which spanned almost the entire twentieth century, aerial cameras were standardized, as were the sizes of their images1. Parameters such as forward and side overlap between images and strips, and the focal length of the camera were used to ensure proper aerial survey execution and assure the quality of orthophoto production.
With the introduction and growth of digital aerial surveying in the past decade, however, the situation has changed. A large variety of airborne cameras with different frame sizes, CCD resolutions and focal lengths of lenses is available in the aerial survey market. As such, a new approach is needed to specify standards and ensure quality.
This article presents an innovative universal approach to aerial survey planning and orthophoto production that is relevant and appropriate for the digital photogrammetry era.
Fig. 1 shows a typical aerial survey flight planning diagram.
1 Standard images are 24 x24 cm or 18 x 18 cm. Focal lengths for 24 x 24 cm images are 150, 210 and 300 mm. Focal lengths for 18 x 18 cm images are 70, 100, 140, 200, 250 and 350 mm.
Variables in the diagram are as follows:
FOV Camera field of view across the flight line, corresponding to W distance on the ground.
2a Permissible orthophoto angle across the flight line, corresponding to Wo distance on the
ground. Only this part of the image is used for orthophoto production.
D Distance between flight lines
Q Side overlap
2X, (not Permissible orthophoto angle along flight line. This angle is specified by forward
shown) overlap. From the orthophoto quality point of view, 2X must be equal to or less than 2a.
In practice, 2X is generally less than 2a.
This scheme, in principle, is valid for all types of cameras, with only parameter values varying with the camera type.
Fig. 2 presents the impact of permissible orthophoto angle on building leaning and occlusion area on orthophoto.
<4----------►
Figure 2. Building leaning and occlusion area on orthophoto
In this diagram:
FOV Field of view, generally, 27° - 110°
2a Permissible orthophoto angle
L1, L2 Occlusion
BL (Building leaning) = tg(a)*100%; 2a2 > 2a1; L2 > L1;
As the diagram illustrates, a larger orthophoto angle results in greater building
leaning and more occluded area on the edge of the image. In fact, the permissible
angle and consequently, the building leaning are the main geometric parameters defining the metric quality and readability of the orthophoto.
In the following discussion, we will take the recommended values for side and forward overlap for analog cameras as a given. Based on experience and standard procedures, cameras with a focal length of 300 mm are recommended for urban areas, while for rural areas, recommended focal length is 150 mm. The forward overlap P in both cases is expected to be 60%. Side overlap Q is set to 25-30% for rural areas and 35-40% for urban areas. The orthophoto is constructed from a part of the image (Wo) that is bounded by cut-lines that pass around the middle of the overlap area.
When comparing between digital cameras, it is preferable to utilize pixels, rather than millimeters or centimeters as the standard unit of measure. A standard image from an analog camera that is scanned with a resolution of 15 microns will have an
equivalent frame size of 16,000 by 16,000 pixels. A focal length of 10,000 pixels corresponds to the 150 mm length for rural areas and 20,000 pixels to the 300 mm focal length for urban areas. The relationship between the permissible orthophoto angle and the overlap is calculated as follows:
[1] 2a = 2 * atan (Lq *
[2] 2A = 2 * atan (¿p *
Where:
2a, 2A, - permissible orthophoto angles across and along flight lines, respectively
Q, P - side and forward overlaps
Lq, Lp - frame size in pixels across and along the flight line
F - focal length in pixels
Using these formulas, we have calculated the values for permissible orthophoto angle and building leaning for different cameras based on image parameters from the cameras and commonly used side and forward overlap values. The results are shown in Table 1:
Table 1. Orthophoto angle and building leaning for different cameras
Camera A3 UC EAGLE UC- Xp UC-Xp wa DMCII 250 ADS 80 RC-30 / RMK-TOP
Focal length (mm) 300 210 80 100 70 112 62.77 150 300
Pixel Size (^) 9 5.2 5.2 6 6 5.6 6.5 15 15
Focal length (pix) 33,333 40,385 15,385 16,667 11,667 20,000 9,657 10,000 20,000
Frame size cross track (Pix) 62,000 20,010 20,010 17,310 17,310 17,216 12,000 16,000 16,000
Frame size along track (pix) 8,000 13,080 13,080 11,310 11,310 14,656 7,530 16,000 16,000
Frame area (Mpix) 496 262 262 196 196 252 90 256 256
FOV across track 110° 27.8° 66.1° 54.9° 73.1° 46.6° 63.7° 77.3° 43.6°
FOV along track 13.4° 18.4° 46.1° 37.5° 51.7° 40.2° 42.6° 77.3° 43.6°
Forward overlap 60% 60% 60% 60% 60% 60% 60% 60% 60%
Permissible angle (2X) 5° 7° 19° 15° 22° 17° 18° 35° 18°
Building leaning at X 5% 6% 17% 14% 19% 15% 16% 32% 16%
Side overlap 40% 40% 40% 40% 40% 40% 40% 40% 40%
Permissible angle (2a) 80° 17° 43° 35° 4 00 o 29° 41° 51° ° 2
Building leaning at a 84% 15% 39% 31% 45% 26% 37% 48% 24%
Side overlap 25% 25% 25% 25% 25% 25% 25% 25% 25%
Permissible angle (2a) 93° 21° 52° 43° 5 00 o ° 3 50° 62° 33°
Building leaning at a 105% 19% 49% 39% 56% 32% 47% 60% 30%
The table indicates that for the same forward and side overlap values, the permissible orthophoto angles and consequently, building leaning and occluded areas vary greatly among the cameras.
For a standard 60% forward overlap, for instance, building leaning ranges from 5% to 32%. For side overlap of 40%, building leaning ranges between 15% - 84% among the various cameras, with an even greater range of 19% to 105% building leaning for 25% side overlap.
Smaller building leaning indicates that the “usual” orthophoto approaches more closely the "true" orthophoto, resulting in better readability and superior metric quality of the product.
In Table 2, we calculate the side overlaps for the same set of digital cameras using building leaning values of 60% and 24%, which correspond to 25% and 40% side overlap for analog cameras RC-30 and RMK-TOP in the table above.
For building leaning of 24%, the side overlap values for digital cameras vary between 3% and 80%. An aerial survey with building leaning of 60%, which is typical for an open flat-plain area, can be conducted by only three of the cameras, the VisionMap A3 , RC-30 and RMK-TOP with F = 150 mm. Due to their relatively small fields of view, the other cameras do not allow aerial survey with these parameters.
Table 2. Forward and side overlap according to building leaning
Camera A3 UC EAGLE UC- Xp UC-Xp wa DMCII 250 ADS 80 RC-30/ RMK-TOP
Focal length (mm) 300 210 80 100 70 112 62.77 150 300
FOV across track 110° 27.8° 66.1° 54.9° 73.1° 46.6° 63.7° 77.3° 43.6°
Required building leaning (urban area) 24% 24% 24% 24% 24% 24% 24% 24% 24%
Permissible angle (2a) 27 27 27 27 27 27 27 27 27
Side overlap 80% 3% 63% 54% 68% 44% 61% 70% 40%
Required building leaning (rural area) 60% 60% 60% 60% 60% 60% 60% 60% 60%
Permissible angle (2a) 62 62 62 62 62 62 62 62 62
Side overlap 60% -142% 8% -16% 19% -39% 3% 25% -50%
For each of the cameras we assessed, we created a flight plan for an open area using forward overlap of not less than 60%, side overlap not less than 25%, ground resolution of 25 cm, permissible orthophoto angle of not more than 60° and aircraft ground speed of 250 knots:
Table 3. Flight plan calculation
Camera A3 UC EAGLE DMC II 250 UC-Xp wa UC- Xp RC-30/ RMK-TOP ADS8 0
Focal length (mm) 300 210 80 112 70 100 300 150 62.77
FOV across track 110° 27.8° 66.1° 46.6° 73.1° 54.9° 43.6° 77.3° 63.7°
FPS (frame/sec) 7.40 0.56 0.56 0.59 0.50 0.74 - - -
GSD (m) 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00
Flight altitude (m) 8,333 10,096 3,846 5,000 2,917 4,167 5,000 2,500 2,414
Forward overlap 60% 93% 93% 94% 91% 94% 60% 60% 60%
Side overlap 59% 25% 25% 25% 25% 26% 26% 28% 25%
Permissible angle (2a) 60° 21° 52° 36° 58° 42° 33° 60° 50°
Frame area (sq. km) 63.51 16.36 16.36 15.77 12.24 12.24 16.00 16.00 5.65
Distance between Flight Lines (m) 9,623 3,742 3,752 3,249 3,233 3,199 2,962 2,887 2,251
The elements of Table 3 are calculated as follows:
1. Forward overlap at an affordable frame rate (FPS) and the specified aircraft speed
2. Side overlap and permissible orthophoto angle are linked and depend on the field of view (see Figure 1). Side overlap cannot be less than 25%, while the permissible angle of aerial photography cannot be greater than 60°.
As highlighted in red in the table, only three cameras (A3, RC-30 and RMK-TOP) achieve the maximum permissible orthophoto angle of 60°. A fourth camera, UC-XP, came close to reaching this angle.
By placing the same geometric constraints on all aerial surveys, we can accurately compare between existing cameras on the market. The following graph summarizes the distance between flight lines from Table. 3.
Distance between flight lines (km)
i A3
EAGLE 210 EAGLE 80
DMCII-250 UC-Xp wa
UC-Xp
w RC-30 300
01
01
01
-Q
RC-30 150
ADS80
Conclusions
Using parameters such as forward and side overlap to specify orthophoto production is no longer relevant for aerial surveys to be carried out by today's digital or analog cameras. Due to the wide range of frame sizes, focal lengths and CCD resolutions, quality and readability are not determined by overlap. By continuing to use these parameters to specify orthophoto production, we condemn ourselves to measurements that are inaccurate, and to pour quality and readability of the orthophoto.
To ensure that orthophotos of the same quality will be obtained from various suppliers, permissible orthophoto angle or building leaning parameters should be specified rather than forward and side overlap.
A thorough understanding of the geometric principles of flight planning allows users of orthophoto services to accurately analyze the performance of available cameras, compare their performance, and ultimately, make the right choice for each individual project. This careful assessment will allow users to conserve material resources and save time.
© Yu. Raizman, 2012
