Transformation with Total Station Coordinates
If you have ASCII data from a Total Station theodolite in UTM or X-Y coordinate format, you can use this to rectify images from which you have recorded this data from at least four well-distributed points which are also visible in the image and perhaps marked with pegs or nails.
The asci data from the theodolite may have to be edited with Notepad before you can use it. Each line must contain precisely four items in the order specified here. These must contain:
A unique identity number for a point which you can easily see in the image to be rectified. The numbers may be in any order, and may start with any value. Gaps in numbering are permitted. However, it will later be quicker if they start with 1, are in order and have no gaps. Blank lines may be present and comment lines may start with the # character, but this will not be compatible with other software.
The East or X coordinate of the point as recorded by the theodolite.
The North or Y coordinate of the point as recorded by the theodolite.
The Height or Z coordinates of the point. This must be zero if not recorded.
Nothing else may be present in this ASCII file.
It is recommended to look at the folder where the file is located to verify that the values are correct and obey the rules above before continuing.
Load the oblique image.
Click on Options, Appearance
There, select Overwrite Target, Fine Interpolation and if you have dual displays, Use only one display.
Click on Options, File + Grid Preferences
Select the input format to correspond to that of the image which is to be rectified. Select whatever output format is needed for further work.
As a grid, select either UTM WGS84 or X-Y/E-N depending on the data furnished by the theodolite.
Select a GIS output option if required.
and select either UTM or X-Y as the choice for the coordinate system, depending on what the theodolite data contains.
Either from the control menu or by pressing Ctrl-E start Control point entry mode:
Select Number Manually only if the id numbers from the theodolite are not in sequence or don't start with 1. If you choose this option, you will be shown the next available number in a sequence when you click on a point with the magnifying cursor.
The list from the theodolite may contain identity numbers from multiple images. However, the identity numbers from the photo and in the theodolite list must always be unique corresponding pairs with no duplication. This feature may be convenient when processing multiple images perhaps made at the same place.
You can change this either by using the spinner icons next to the entry field or type in a new number automatically. If you choose the number of a point which you have already entered, this point will be deleted and a new point created. However, as stated above, it is faster not to use manual numbering provided that the ID numbers of the asci file from the theodolite are in simple ascending numerical order. Then, the default automatic control point numbering scheme in AirPhotoSE can be used.
Save the image using the Save As option after you have finished adding control points at all the places listed in the theodolite asci file.
Click on the Transform option on the main menu.
Then click on Total Station Transform:
If you have chosen UTM as your grid option and if the theodolite output has been set to this mode, you will be asked to enter the UTM zone number for the data. If the data is south of the Equator, click the south button. See the chapter on Calibration here if necessary.
Rectification of the image will now proceed automatically and the result displayed.
Behind the scenes, the programme will create a new blank square image with size equal to the diagonal of the image to be rectified. It will compute the highest, lowest and mid-point X/Y or E/N values using the chosen grid and calibrate the blank image in the chosen grid. A calibrated geo-referenced control point will then be entered automatically on the new image using the values in the theodolite list and stored in a normal AirphotoSE control point file. Then the source image will be rectified to the blank image which will be overwritten by the image data, because Overwrite has been chosen in the Options step above. This will be roughly centred in the new image which will be assigned the name of the original image plus the addition of the characters "-TS" (for Total Station) to distinguish it from the input when saved.
You can view the reprojection error of the rectification by pressing Shift-F9 if you wish.
Usually this result will be good enough, but if desired, you can optimise the output control point positions a bit by clicking on Global Optimisation on the control point menu.
This will usually reduce the reprojection error to very small values
The distant parts of the image may not be of interest or be blurred. You can then crop the image by clicking on the cropping frame button on the main toolbar, moving the frame to a desired position
and then clicking on the Crop menu item.
The result will show the area of interest and be calibrated either in UTM or in X-Y if chosen. If you want the whole image, with the focus on the transformed result, click on Undo, then click on the untransformed image to give it the focus, set the Scale slider at the bottom to approximately 100% and from the Transform menu, click on Transform and Scale.
You can overlay a grid on this or one of the earlier results in any colour, with or without a frame and more. Click on the Grid button on the main button bar:
If you wish, you can check the accuracy of the placement of control points by looking at a small part of the full result with the grid showing and the original theodolite coordinate list in the Explorer when moving the image cursor over one of the control points:
Please note that all operations after creation of the rectified image can be undone with the Undo button and repeated perhaps with different options.