Tie point values

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Tie point values

Tie point values

TerraPhoto can adjust the positioning of the images in a way that tie point objects seen in different images fall more accurately at the same location. The adjustment process is only reliable if each image has a sufficient number of tie points. If an image does not have enough tie points, the adjustment process does not necessarily make an improvement. It can even worsen the positioning in those parts of the project area where images do not have any tie points.

TerraPhoto computes a sum of tie point values for each raw image in order to indicate if an image has enough tie points (i.e. is well-defined) or not. The positional value of a tie point depends on its type and is a measure of how fixed the position of the tie point is. TerraPhoto uses the following value system:

Air point, Line

1

Ground point, Depth point, Known line

2

Known xyz point, Known xy point, Known depth point

3

Straight lines do not have a tie point value.

 

Air points and Lines are the least valuable types as they have the biggest degree of freedom. Their position is always computed as an average of the tie point rays and the solution point can move freely. Ground points, Depth points, and Known lines provide more value. Their position is computed as an average of the tie point rays but it is also fixed on the ground surface. Known point types are most valuable as their position is fixed.