Terrestrial Recording Strategies - 1. Basic Rules

        Basic Rules

• 3 pictures per point to connect data
• go from coarse to fine
• only use the inner 2/3 of the image area
• calculate the optimal distance to the object
• horizontal overlap must be about 80%
• the offset must be calculated by: distance between two images = distance to the object / 3 
• for vertical overlap the camera can only be tilted by 20° or less
• mind flat angles, a recording angle cannot be smaller than 10°
• mind blind angles, no data collection there

3 Pictures per Point

The software searches the same points on different recordings to connect the pictures and create the model. However, this is only possible if the software can find the same point on at least three pictures. The first two points are necessary to create 3 dimensions in the digital space; the third point is there for corrections, otherwise the error rate will be too high.

From coarse to fine

In general, it is useful to work from coarse to fine. Also, the smaller the distance the better the target resolution, but the more pictures will be necessary.  

Usable image area

For high quality recordings the entire image area cannot be used due to geometric properties of the camera lenses (focal length, distortion). Only the inner 2/3 from the centre to the edge are qualitatively usable; the outmost 1/3 is only to be used in case of need for recordings and evaluations. 

Furthermore, the images can be taken in a wide format as well as a high format. Here, only the handling of the hardware (e.g., the antenna might be sideways) must be considered.

Distance to the object

The distance between the recording position and the object must be calculated by either defining the target resolution or the following rule: Distance = (Focal length * Resolution) / Pixel Size

In general, the distance from the spots recording to the centre or a fictional line of the object should always be the same, so the target resolution stays the same. If there is a need for changing the target resolution the scale strategy must be applied.

Overlap and Offset 

Each reconstructed point must be on at least three pictures. Two of the pictures are needed for the spatial view and the third point is used for the quality control. Depending on the distance to the object, the overlap distance between two images must be calculated. It works by the following rule: dist. between two images = dist. to the object / 3

Each picture must be taken from another position as the others and must overlap horizontally by about 80%. 

This also applies when taking pictures in a vertical motion, which might be necessary for tall objects. Here the camera is to be tilted with a maximum of 20° from one image to the next.

Also, the surfaces shouldn’t be recorded in a too flat angle. The angle alpha cannot be smaller than 10°. Beaware of dead blind as well.