Adjust absolute image positions

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Adjust absolute image positions

Adjust absolute image positions

Add control points to the tie point file

For airborne projects, the GPCs’ coordinates are usually imported from a text file into TerraPhoto. The text file must store the coordinates of the GCPs in one of the formats:

Easting Northing Elevation

Identifier Easting Northing Elevation

1. Use Import known points command in the Tie points window in order to import the text file into the Tie points window.

The software adds the control points as Known xyz tie points. The images that see the location of a GCP are marked with the asterisk character (*) next to the image number in the upper list of the Tie points window. For each image, the tie points are listed in the lower list.

tie_points_known

2. Select an image marked with an asterisk. Select the first line of a Known xyz tie point in the tie points list.

The selected image is displayed in the Active detail view and a secondary image is displayed in the Secondary detail view.

3. Select Enter position command from the Pixel pulldown menu in the Tie points window.

The location of the GCP coordinates is marked with a cross if the mouse pointer is moved into the Active detail view or the Secondary detail view. This can be used as indication where the point is located in the image.

4. Enter the tie point with a data click in the Active detail view.

This places the tie point pixel in the active image. The distance between the tie point location in the image and GCP coordinates is shown in the tie point list.

5. Enter the tie point with a data click in the Secondary detail view.

6. Select the next undefined pixel of the tie point in the tie point list with a data click or the <Arrow down> key.

Continue with step 5 until all pixels of the tie point are defined.

7. Continue with step 2 until all GPCs are defined as tie points.

8. Use Save As command from the File pulldown menu in order to save the tie points into a file.

A good practice is to document an accurate description of the location of a GCPs before starting the tie points’ placement. You can also use TerraScan, TerraSurvey, or MicroStation tools to read in the GCP text file and draw the points into the design file.

If the elevation coordinate of a GCP is not usable, you can change the type of the tie point to Known xy using the Edit information command in the Tie points window.

Calculate xy shift values

The software uses the Known xyz or Known xy tie points for determining whether there is a systematic xy shift between the images and the GCPs. It suggests correction values for adjusting the positions of the images.

1. Create an Output report from the File pulldown menu in the Tie points window. The report must include the Known point comparison option.

The Known point comparison part of the report contains a table that lists all GCPs, their coordinates, and the xyz shift distances at each tie point location. Then, it shows the recommended correction values that should be added to the easting and northing coordinates of the images. Finally, the average and worst shift distances are displayed. For example:

You should add    -0.070 to image eastings

You should add    +0.106 to image northings

Pixel mismatches

Average distance  14.22 cm

Worst distance    23.32 cm

Apply the xy shift to the image list

The correction values given in the tie points output report can be used to transform the image list. For this purpose, a transformation has to be defined in TerraPhoto Settings. Then, the transformation can be applied to the image list.

1. Open Coordinate transformations / Transformations in TerraPhoto Settings.

2. Define a new Linear transformation.

The recommended corrections values for easting and northing from the output report correspond to the Add constant X and Y values in the Settings dialog.

3. Select Transform positions command from the Images pulldown menu in the TPhoto Main window.

4. Apply the transformation to the image list.

5. Use Save list As command from the Images pulldown menu in order to save the image list into a new file.

If images and laser points need to be shifted with the same correction values, the transformation can be copied from TerraPhoto Settings to TerraScan Settings. Then, the transformation can be applied to the laser points using TerraScan transformation tools. Use the Create camera view command in Terra Photo to check how well laser data and images match each other.