Baseheight Ratio and Stereoscopic Coverage

From Figure 3-6 it is also evident that the stereoscopic parallax p and thus depth impression can be increased when the eye base B is increased, an option we do not have in reallife viewing, but is possible and desirable in artificial stereoscopic viewing. If the base between the exposure stations in relation to their height (the photographic base-height ratio) is larger than the base of our eyes in relation to the viewing distance (the stereoviewing base-height ratio), the stereomodel appears exaggerated in height by the same factor (Wolf and Dewitt, 2000). The exaggeration factor varies with image overlap and focal length and is typically between two and six times for conventional aerial photographs.

Stereoscopic photographs from professional aerial surveys are acquired in blocks of multiple flightlines in such a way that full stereoscopic coverage of the area is ensured with multiple stereopairs (Fig. 3-8). Each photograph overlaps the next photograph in a line by approximately 60% (forward overlap or endlap), while adjacent lines overlap by 20-30% (sidelap). In mountainous terrain, the overlap can be

Flight Path Side Lap Overlap
FIGURE 3-8 Block of aerial photographs with 60% endlap in three flightlines with 25% sidelap. This survey design is ideal for ensuring gapless stereoscopic coverage while minimizing image number and redundancy.
Sidelap Overlap

increased in order to avoid gaps of stereoscopic coverage by relief displacement and sight shadowing.

The required air base or distance between exposure stations B is dependent on the dimensions of the image footprint and the desired endlap. If D is the image coverage in direction of the flightline and PE the percent endlap, B calculates as:

(Equation 3-5)

The same equation can be used for calculating the required distance between adjacent flightlines for a desired sidelap PS; keep in mind, however, that SFAP cameras most probably feature a rectangular image format (¿l x dw) and a decision has to be made as to its longitudinal or transversal orientation along the flightline (substituting D in Eq. 3-5 by Dl or Dw, respectively).

For platforms with approximately constant ground velocity Vg, the time interval AT between exposures is:

(Equation 3-6)

The more precisely a platform is navigable, the easier it is to follow a pre-arranged flightplan with systematic design. Figure 3-9 shows the actual flightpath of a survey taken by the autopiloted model airplane presented in Chapter 8.5.2, which comes quite close to the conventional blocks of aerial photography. However, with most SFAP platforms, it is usually difficult to achieve such a regular flightline pattern, and it is indeed not a prerequisite for stereoscopic analysis. Stereomodels also can be created from images which overlap in a much more irregular pattern as long as all parts of the area are covered by at least two photos each with sufficient base-height ratio (Fig. 3-10). It should be pointed out, however, that some digital photo-grammetry systems designed for analyzing standard aerial

(Equation 3-5)

(Equation 3-6)

Focal Length And Aerial Photographs
FIGURE 3-10 Block of aerial photographs taken by a tethered hotair blimp from ~120m flying height. Although the image alignment and overlaps are irregular and the photographs even have slightly different scales, full stereoscopic coverage of the desired ground area was given.

imagery may have difficulties in dealing with irregular image blocks.

The vertical exaggeration of relief in the stereomodel plays an important role in 3D analysis. However, which base-height ratio is "sufficient" for stereoscopic analysis depends on the desired degree of depth impression and (if applicable) measurement accuracy for the particular relief type. Small height variations of gently undulating terrain are better viewed and measured with an extra-large base-height ratio, while the analysis of high-relief terrain, which already exhibits larger stereo-parallaxes due to radial distortion, tolerates lower base-height ratios. In general, errors in planimetry and height decrease with increasing base-height ratio and increasing stereo-parallaxes (Wolf and Dewitt, 2000; Kraus, 2004; Kraus et al., 2007).

For a stereo survey with a given image scale S, wide-angle lenses may be used at lower flying heights than smaller-angle lenses, thus increasing the base-height ratio (Fig. 3-11). In addition, wide-angle lenses cause larger relief displacement and thus larger stereoscopic parallaxes than smaller-angle lenses, even if both are taken with the same base-height ratio. For stereoscopic purposes, images taken with wide-angle lenses from lower flying heights are therefore preferable to images taken with smaller-angle lenses from larger flying heights—quite contrary to the case of monoscopic mapping or mosaicking (see below) where distortion-minimized images are desired.

A typical SFAP problem regarding the stereo-capability of photographs taken from kites, balloons, and drones is the variability of scales and tilting angles that is often unavoidable. From the authors' experience, difficulties with stereoscopic viewing and image matching (see below) may arise if the image scales differ more than 10% or so. Stereoviewing is also hampered by image obliqueness. For unfavorable constellations of nadir angles, tilting may even have the additional effect of extinguishing the stereo-parallaxes regardless of good base-height ratios.

The only solution is to take as many photographs as possible during a survey and carefully choose the best of them for analysis (see Chapter 11.5.). Taking more photographs is always a good way to increase chances for success, regardless of the mission's nature or purpose.

Digital Cameras For Beginners

Digital Cameras For Beginners

Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.

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Responses

  • Niklas
    What is stereoscopic coverage?
    8 years ago
  • Juan
    Why we keep side lap in aerial photograph?
    7 years ago
  • nebay
    Can achieve same stereoscopic coverage using constant flying height?
    4 years ago
  • timothy
    What is stereoscopic coverage in aerial photography?
    2 years ago
  • juho
    What is flight direction of aerial photographs?
    2 years ago
  • MONIKA
    How to get contact prints for aerial phorographs stereoscopic viewing?
    6 months ago

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