Transform - Transforming in 2D

The Point 2D option allows you to transform the co-ordinate system of a model in plan.   This is normally because a survey has been carried out assuming an arbitrary co-ordinate system and, later, you wish to transform this into another co-ordinate system such as OS National Grid.  To achieve this, you will need the co-ordinates of at least two points in both co-ordinate systems.  If the current pick mode is not set to list, you will be asked to indicate the points using the mouse cursor.  If the current pick mode is set to list, it is assumed that the points that are currently selected will be used in the transformation.

If you only indicate two points, the dialog box shown below will be displayed.  The First group shows you the co-ordinates of the first point.  In the field titles, the current co-ordinates are displayed and the fields themselves allow you to enter the new co-ordinates.  The Second group performs the same function for the second point. 

You may have already defined the transformation co-ordinates and stored them in a co-ordinate block.  If this is the case, you can load these co-ordinates from the co-ordinate block using the Load button.  The co-ordinate block will be searched for the points and will be matched by both code and number.  When transforming stations, co-ordinates will be loaded from a block but only the code will be checked.

As you enter new values, the values for Scale Factor and Angular Rotation will update automatically.  The scale factor is an indication as to the quality of the transformation and, ideally, its value should be as close to 1 as possible.  The Apply Scale Factor check box allows you to apply the calculated scale factor to the point co-ordinates, if it is checked.

The transformation normally takes place by fixing the first indicated point and then applying any translation, scaling and rotating about it.  You can change this by using the Swap button which will reverse the points and change the values displayed in the First and Second groups.  If you swap the points, the values for the scale factor and angular rotation will not change.

Unless you apply the calculated scale to the model, transforming about the first point gives this point a higher weighting in the calculations.  The Centre Origin allows you to move the transformation origin to the point that is midway between the two indicated points.  This has the effect of weighting the two points equally.  Again, the scale factor and the angular rotation will not change if you transform about the centre point.

The Save Transformation check box allows you to save the transformation for future use.  If the check box is ticked, when you select the OK button, you will be asked to give the transformation a name and it will be stored in the current project.

If you indicate three or more points for a 2D transformation, the dialog box below will be displayed. 

The list control shows the indicated points with both the original and the entered co-ordinates.  To enter new co-ordinates for a point, highlight it in the list and enter the values in the Co-ordinates group to the bottom left.  You can load new co-ordinates for the points from a co-ordinate block by using the Load button and points will be matched by both code and number.  The green tick to the left means that the point will be used in calculation of the transformation matrix.  If you have selected more than three points, you can remove points until there are three remaining by double-clicking it in the list control.  The green tick will be replaced by a red cross.  A further double-click will reinstate the green tick.

The Transformation Parameters group shows the accuracy of your transformation.  The values displayed are as follows.

Std Error - This is the standard error for the transformation and is in units of metres.  It is the sum of the differences between the co-ordinates that have been entered and the co-ordinates calculation using the transformation.  The value should be close to 0 and the larger it is, the less accurate the transformation.

Delta X - This is the offset through which the model will move in the easting direction.

Delta Y - This is the offset through which the model will move in the northing direction.

Angle - This is the angle by which the model will be rotated

Scale - This is the scale that could be applied to the model and should be as close to 1 as possible.

The Force Scale check box allows you to ignore the calculated scale and apply one of your own.  If you wish to preserve the scale of the model, make sure that this check button is ticked and that a value of 1 has been entered in the Value field.  Then press the Apply button.  The Report button gives you the option to create a report of the transformation.  This gives all the details in the dialog box and can be copied and pasted into another application, saved to file or printed for hard copy.