control points

Development Asset Creation
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1

Hi,
What are the differences between control points type? (tie points, ground control, ground test) and how they influence the scale of the model?
Luciano

2

Hi Luciano

TIE points = used to set scale
GROUND CONTROL = used in GIS with GPS coordinates
GROUND TEST = used in GIS for GPS coordinates checking

3

So the only difference is that Ground TEST points have no influence on alignment whereas Ground Control points do?

4

I’m trying to understand this as well.

There are at least two roles for these points: manually overriding the alignment process, and imposing scale/a coordinate system.

I think tie points and GCPs can both link components that the alignment algorithms haven’t connected. (My experience is that this is a powerful tool, but there will usually be a step in the mesh between the two components.) So I suppose both behave the same in this respect?

For completeness - will ground test points do the same?

Tie points can be used as end points of scale constraints. GCPs are given coordinates. A question here - if I do both, are both taken into account? Or do GCPs override scale constraints?

One thing I’ve been wondering about though is whether GCPs can influence alignment other than by linking compoents. If alignment is (re)run after GCPs are defined, might model geometry be changed by the GCPs? Or will they simply be used to set the coordinate system from a “best fit” between the points defined on the model and the coordinates specified? I’m thinking about a long, narrow model where accumulated errors might result in a false curvature, would GCPs suppress this? Or would they simply highlight the error, by showing the varying fit along the length?

Thanks.

5

I also recommend you to read the Control Points tutorial in the Help section of the software: Help (SW documentation/manual)

6

Hi Hamish (or Harvey?),

as you know from above, the exact use of TEST points is also unclear to me.

CPs simply tell RC which points on different images should be in the same spot in 3D.
The weight setting defines how many images in the calculation process it simulates.
E.g. RC has 7 images of one particular spot but there is a problem.
So you add a CP with weight setting 10, then the CP counts like 10 images (as opposed to the 7 from RCs automatic detection) and corrects the geometry accordingly.

GCPs are the same as CPs only with, as you said, given coordinates - be they from GPS or theodolite survey or other means.
And yes, they will pull the spot to the “real” coordinates within a certain limit.
However, you need to check this - RC gives you the error measurements.
Sometimes it can happen that even though there are GCPs with coordinates, the model will stubbornly remain in a faulty geometry.
Then you need to delete ALL components and re-align…

The steps you mention result froman area that is covered by two groups of images (in the same component) that are not or only insufficiently linked by tie points (the automatic ones detected by RC). This can be remedied by adding sufficient CPs on images of both groups.