Preparation of the Ground Control File

2.1.3 Preparation of the Ground Control File

The Ground Control File contains the coordinates of the points which are used for the absolute orientation or geo-registration of the photogrammetric unit (strip, block) being triangulated. There is no limit to the number of points that can be included in the file. It is essential that the units of the coordinates in the Ground Control file are consistent with the units used to specify the exposure station coordinates in the Frame File. The use of different units in these two instances would lead to an incorrect solution, assuming the solution would converge at all. The Ground Control File derives its name from the name of the project and has an extension of .CNT.

Structurally, the Ground Control File is the simplest of the three ASCII interface files. It is simply a sequential list of records, each of which contains the control point name, coordinates, standard deviations and type. The type of the control point will be either horizontal control, vertical control or full control. The following tables specify the format of a typical record of the Ground Control File.

Field

Contents

Data Type

Columns

1
Control Point Name
Alpha-Numeric

1-8

2
First Axis Coordinate
(Longitude, X, ... )

Floating Point

9-20

3
Second Axis Coordinate
(Latitude, Y, ... )

Floating Point

21-32

4
Third Axis Coordinate
(Height, Z, ... )

Floating Point

33-44

5
Standard Deviation of
First Axis Coordinate

Floating Point

45-54

6
Standard Deviation of
Second Axis Coordinate

Floating Point

55-64

7
Standard Deviation of
Third Axis Coordinate

Floating Point

65-74

8
Control Point Type Code
Integer

80

The Control Point type is encoded according to the definition in the following table. Literally, the code identifies which coordinate is not to be treated as control: a code of 1 means the first coordinate is not known; 2 means the second coordinate is not known; 4 means the third coordinate is not known. Their additive combinations generate the following 8 control point types.

Code

First Coordinate

Second Coordinate

Third Coordinate

Remarks

0
Known Known Known Conventional Full Control

1
Unknown Known Known

2
Known Unknown Known

3
Unknown Unknown Known Conventional Vertical Control

4
Known Known Unknown Conventional Horizontal Control

5
Unknown Known Unknown

6
Known Unknown Unknown

7
Unknown Unknown Unknown Completely Unknown Point

Note that the definitions for the control point definition codes are opposite to the definitions of the frame position and attitude coordinate angle constraint flags.

2.1.3.1 Verification of the Ground Control File

GIANT offers a facility for automatically checking the structural integrity of a project's Ground Control File. It is recommended that this feature be exercised as a first-order means of data validation before executing the triangulation adjustment. The verification feature is accessed from the main GIANT Process Options screen. If the project of interest is not displayed in the Project field, use the Browser button and Path field to select the appropriate project. (Section 3.1 gives detailed instructions for using the Browser, Path and Project features of the Process Options screen). Use the mouse or Alt-V key combination to select the Verify menu option. A pull-down menu then appears. Selection of the Ground Control File option will invoke the file verification procedure.

The verification procedure conducts two integrity checks on the structure of the Ground Control File.

Blank records.
Duplicate control point names.

Failure to meet the checks is identified by a pop-up message that alerts the user to the failed condition(s). Upon notification of such an integrity check failure, exit GIANT and use a text editor to correct the identified condition(s) in the dataset. When the corrections have been made, re-start GIANT and repeat the verification test. Continue this iterative correction phase until the verification executes without failure. Do not attempt to execute the triangulation adjustment with known errors in the input data stream. Doing so will only result in an abnormal termination of the program.

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