Using Control Points

Prerequisites

Control point files contain the surveyed coordinates of control points from total stations.

If you want to use control points, you must prepare these files before post-processing.

The control point file must meet the following conditions:

• Control points must be surveyed with high accuracy using total stations.

• Surveyed coordinates must be in a right-handed coordinate system.

• For each dataset, at least 3 control points need to be recorded and need to be contained in the surveyed control point file

• All coordinate values must be in the metric system.

• Geo-referenced coordinates can be used.

• If you are using a local coordinate system (the origin is arbitrarily self-defined), then the origin should not be too far from the scanned site, ideally no more than a few kilometers. If the data is more than 20,000 meters away from your site origin, you will not be able to add your data to your NavVis IVION site.

Control point file format

The surveyed control points can be in .txt or .csv format.

CSV format

.csv files can be exported from e.g. Microsoft Excel. Make sure to use the following format:

CP00001, 1000.458, 1964.458, 0.1
CP00002, 1005.000, 2000.000, 0.2
CP00003, 1002.862, 2025.802, 0.3
# CP-wrong, 9002.212, 3025.432, 5.3. This entry will be ignored with "#" symbol

• Each line must contain the following values for each control point: ID (name), X, Y, Z (coordinates).

• Separate all values in each line with commas. Other delimiters cannot be interpreted by the software.

• Empty lines and data beyond the fourth column will be ignored.

• Exclude lines by adding # to the beginning of the line.

TXT format

A .txt control point file must be set up in the following format:

CP00001 1000.458 1964.458 0.1
CP00002 1005.000 2000.000 0.2
CP00003 1002.862 2025.802 0.3
# CP-wrong 9002.212 3025.432 5.3 This entry will be ignored with "#" symbol

• Each line must contain the following values for each control point: ID (name), X, Y, Z (coordinates).

• Separate the fields with a space.

• Empty rows and data beyond the fourth column will be ignored.

• Exclude lines by adding # to the beginning of the line.

Checking Control Point Errors after Processing

Review Control Point Error logs

Post-processing generates Error Log files that indicate drift errors in the device's trajectory before and after processing with surveyed control points.

An introduction of drift errors and how control points are effective on reducing the errors.

Error Logs can be found in each dataset folder under datasets_proc, for example:

/datasets_proc/2024-08-24_14.16.03/anchors/

There exist two important files indicating the control point errors:

anchors_alignment.log

This file reflects the control point error before the processing. It is created by matching the recorded control points from NavVis devices with the surveyed control point values.

A sample anchors_alignment.log:

1714466412.981071803	"6"   -0.10635   -0.01164   +0.00129   +0.00000 false true true true
1714466628.554513557	"7"   +0.00741   -0.00850   -0.00619   +0.00000 false true true true
1714466808.296838821	"8"   +0.00819   -0.02187   +0.00977   +0.00000 false true true true
1714467051.500836058	"9"   +0.01973   -0.05764   -0.00918   +0.00000 false true true true
1714467486.676680874	"1"   +0.07102   +0.09964   +0.00431   +0.00000 false true true true

Each line logs the following information:

• The time when the device was placed on the control point (Unix time).

• The control point's unique identifier (ID).

• The remaining position error (x, y, z).

• The orientation error (yaw; can be ignored).

• The flags (true/false) for the control point optimization stages are also displayed but can be ignored by normal users.

anchors.log

This file reflects the remaining control point errors after processing. It is created by the recorded control points from NavVis devices with the surveyed control point values.

A sample anchors.log:

1714466412.981071803	"6"   +0.00015   +0.00062   -0.00001   +0.00000 false true true true
1714466628.554513557	"7"   +0.00015   -0.00014   -0.00002   +0.00000 false true true true
1714466808.296838821	"8"   +0.00001   +0.00030   -0.00026   +0.00000 false true true true
1714467051.500836058	"9"   -0.00004   -0.00040   -0.00021   +0.00000 false true true true
1714467486.676680874	"1"   -0.00043   +0.00000   +0.00019   +0.00000 false true true true

The file format is the same as the anchors_alignment.log’s above.

We can see that control point errors are better optimized after being processed by our software.

Review Control Point Error Graphs

Another way to analyze control point errors visually is to check the residuals graph. These are visual representations of the values from the above control point errors.

The control point error graphs are generated under datasets_proc, for example under this path:

/datasets_proc/2024-08-24_14.16.03/internal/anchors/alignment
or
/datasets_proc/2024-08-24_14.16.03/internal/anchors/optimization

alignment/residuals.png

The residuals.png in the alignment folder shows the errors for each control point before post-processing.

optimization/residuals.png

The residuals.png in the alignment folder shows the errors for each control point after post-processing.

Note: the scale of the graph is auto-adjusted. By comparing both residual graphs, we can see that the control points have been optimized after post-processing.