Collecting tie points manually

Used in rigorous models, such as Aerial Photography, Optical Satellite Modelling, and Radar Satellite Modelling math models, tie points (TP) identify how the images in your project relate to each other. You can collect TPs and ground control points (GCP) to compute a transformation to improve the fit between the images.

Tip: To make navigation easier while you complete the procedure, position the windows on your desktop so you can view them all at the same time.

In OrthoEngine, you can identify easily images that overlap the current working image, so that TPs can be collected quickly and efficiently from overlapping images. After collecting a TP, you can identify other images in which the same point can be collected potentially, which is particularly useful with large projects containing many images.

Tip: You can use the edges of lakes, parking lots, or flat fields as TPs with elevation, because it is easy to estimate the elevation of these points from topographic maps or DEMs.

To collect tie points manually

In the Open Image window, do the following:
1. Open an image for which you want to collect TPs.
  Note: When a new image is opened, it is automatically set to Working. The list of overlapping images displayed changes according to the current working image.
2. If you only want to show the images that overlap your current working image, select the Display working image overlaps only check box.
3. Select another image from the list, and then click Open.
  Repeat this step until you open all of the images of interest.
In the Tie Point Collection window, if necessary, type a new label in the Tie point ID box.
That is, the value in the Tie point ID box is generated automatically. You can change the value; however, all points (GCPs, check points, TPs, and elevation match points) in the image must have unique values.
You can also, as an option, enter the elevation of the TP as follows:
- Select the Tie point elevation check box, type the elevation in the Tie point elevation box, and then type the estimated error in the +/- box.
  
  By doing so, the elevation of the TP will be incorporated into the math model.
To aid in collection, you can also select the Auto locate check box.
When you have collected a sufficient number of TPs, the image cursor automatically relocates to the appropriate image location in any open reference images.
If available, in the RPC adjustment order list, select the polynomial order you want to use to calculate the math model, and then to update the order of the working image or all of the images, as applicable, select the Apply to all images check box; otherwise, proceed to the next step.
That is, if when you created your project you selected a Rational Function math model, where you import the coefficients from the Rapid Positioning Capability (RPC) data, you can select the polynomial order you want to use to calculate the math model, and then update the image or images, accordingly. For more information about RPC adjustment, including each available order, see Understanding RPC adjustment order.
In the Viewer window, zoom to a level where you can see the detail in the raw image, position the cursor precisely on the feature that you want to use as a TP, and then click Use Point.
Under Working Image, beside Image pixel and Image line, type the estimated error in the +/- boxes.
By default, all TPs added are active. To make a selected point inactive, clear the Active TPcheck box.

Note: Changing a TP to inactive applies only to the selected image.
Collect a TP for one or more candidate images.
That is, the Reference Image Tie Points and Candidates table also displays the image IDs of other image "candidates" from which you can collect the same point, potentially.
1. To collect a TP for the image candidate, in the table, select a record, and then click Open Image.
  The image opens in the Viewer window.
2. In the Viewer, position the cursor at the corresponding location, and then click Use Point.
In the Tie Point Collection window, under Bundle Adjustment Options, do the following, as applicable:
1. To update the model, select the Compute model check box.
  Selecting this check box provides the RMS error for each TP, and the computed ground coordinates (x, y, and z). The computed coordinates will appear only for active images. The coordinates for inactive images will be blank.
2. To incorporate the elevation from the digital elevation model (DEM) into the model and use the DEM to constrain the heights of the TPs, click Select.
  If a DEM is not specified, the z-value for each TP will be calculated based on the model and will not be tied to a DEM.
  
  In the DEM File window, follow the steps in Setting DEM file options.
  Note: The File Selector window appears, in which you can select a DEM file.
  
  To remove the selected DEM, click Clear.
In the Tie Point Collection window, click Accept.

Tip: To close all open images, beside the table of reference images, click Close Images.