Transform Job to Control
This option will use a set of Control points that you enter the (AMG) co-ordinates.
These Control co-ords can be stored an and CSV file (name, E, N, H).
Another option is to create Control points in the job usinge Points/Control menu option.
If a control file name is entered, it will be used.
If no filename is entered, the program will use the CONTROL points in the job.
The program will use the new (AMG) co-ords and the existing co-ords to create a helmert transform matrix.
This transformatiuon will be used to shift/rotate/scale the job onto the final co-ords.
Using a Control Point File
A control point file can be established for all known points in an area and this is referred
to whenever a job has to be moved from a local co-ordinate system to the control datum.
Control points are matched by the point name and where a control point has been incorporated
in the job, that point must be given its control point name.
A point can be made into a control point by selecting the point, opening the Properties
(on Points toolbar), change the point Type to a
Then enter the point mark name. e.g. SSM12345
- Reference Mark
- Permanent Mark
- Trig Station
When the Transform option is started, these points will now appear in the list of control points,
usually in the ‘Selected’ list.
Making a Control File
A Control file is just a standard points file in .ACS format. Start a new job and read in any
control information you have. The name of each point must be stored in the ‘name’ attribute.
These can be set in the program by changing the point properties. e.g. SSM12345.
Note that these control point names must be unique. Save this file as an obvious job name like CTL.ACS
Using a Control File
You can transform a job onto the control co-ordinates system by identifying the control points
in the job and specifying their name as the control point name. Say the job name is J12345.ACS,
in that job point 10 is SSM6243. Change the point properties for point 10 to set the point type
to SSM and set the name to ‘SSM6243’. Repeat this process for each control point in the job.
Use the Points/Transform Job onto Control option to perform the transformation.
This part should be performed in 4 steps.
Transform Job to Control Option. This option will find all the points in the current job
which are flagged as ‘Control’ and will display the list of points for the user to select which
control points to use for the transformation. You can enable or disable control points in the
list which will be used to compute the transformation. You need a minimum of two points, and can
have a maximum of 100 points.
1. Control File. Next enter the name of the control file (say CTL.ACS) to use to compute
the transformation. This file must contain the control co-ordinates for each of the nominated
2. Use the Check Control Fits the Job button to search the control file for
the control co-ordinates for those points. It will use a “Least Squares” procedure to compute
parameters to rotate, scale and shift the co-ordinates and will apply these to the job control points.
The difference between the transformed values and the values held in the control file will be
displayed in the window. This enables the user to check how the control fits and isolates any
erroneous control values.
Points can be made active and inactive as appropriate and the transformation recalculated until
you are satisfied that the control is satisfactory.
The program will compute the average delta Z (average height difference) using the selected control points.
The value will be displayed as the proposed Height Shift to be used on all points.
You can type in your own value if required.
Leave this field Blank - No changes will be made to the height values
3. Transform Job to Control If the control differences are small enough you can use the
Transform Job to Control button to use the computed transformation parameters to
transform every point in the job onto the new co-ordinates system.
Enter a listing file if required and the transformation scale, rotation and control point
residuals will be written to the listing file when you do a ‘Check Control Fits the Job’.
Use Close to exit from the transformation screen and if the job is then on a geodetic datum,
use the Change Zone option to set the new datum and zone.
Automatic Rejection of Bad Control Points
The transformation may not use all the control points which you select.
If more than four points are enabled, the program will check the control point with the largest
residuals and if that control point is more than 2.5 standard deviations from the rest, it will automatically be excluded. Points more than 2.5 SD from the others have about a 1.5% probability of being part of the set.
How It Works
The transform uses the control values to compute the parameters for a helmert transformation between the two systems.
This provides for a shift in X , a shift in Y a rotation and a scale change.
If two control values are used you get a unique solution.
If more than two control values are used there is redundent data and a least squares procedure is used to calculate the parameters.
It is recommended that more than two control values are used so that you can get a measure of the way that the control
values fit with the job data.
After the transformation matrix is computed, the job coordinates for the control points are transformed and compared to the actual
control values. The differences are shown as residuals in the report.
Point Point Point Final Final ---------Residuals--------
Active No Name Easting Northing dX dY dZ
Yes 1202 3 458489.888 8145233.016 0.423 0.362 -19.991
Yes 1203 148 455342.389 8145170.582 -0.152 0.087
Yes 1492 2 458496.164 8146285.149 -0.271 -0.448 -19.660
As you can see, point 3 has residuals of 0.423 in X and 0.362 in north and point 2 has residuals of -0.271 in X and -0.448 in Y
So there is a missmatch of a couple of metres between the shape shown by the control and the shape for the same points in the survey.
If you look at the actual distances between the points in both systems you will see that the distance 1203 - 1492 only about 100mm different
from the control distance between these points, but 1202-1203 and 1492- 1202 each have differences of over a metre ane each is in adifferent direction.
So, it appears that point 1202 may be a bit suspect.
However, if you have no other information and have to adopt all values, each point in the job will be transformed by the transformation matrix.
The control point however will simply adopt the control coordinates and not the transformed values.
Because of this a point which started say 1mm from point 3 would be about 0.27 in X and 0.45 in Y from that point after transformation.