Collecting ground control points manually

If you are working in a project with a large volume of images, it is a good practice to first enter the ground control points (GCP) for a limited number of images (say, up to five), complete the calculation of the math model, and then check for errors before continuing. It is easier to locate bad points in a few images than the entire project.

When you have collected stereo GCPs, the Reference Image GCPs and Candidates table displays other images that contain the same GCP as the one selected in the Accepted Points table.

To collect GCPs manually

In the Open Image window, double-click the uncorrected image or images on which you want to collect points.
In the GCP Collection window, click the Ground control source list, and then click the method you want.
You can select from the following methods:
- Manual entry: enter the coordinate values manually.
- Geocodedimage: enter the coordinate values by using a geocoded image as the source or the "master" image when you are creating a project to mosaic raw images.
  In the Filename box, type the path and file name of the image or, to select a file, click Browse.
  
  The image opens in a viewer. To open additional reference images, click Browse, and then select the image you want.
- Geocoded vectors: enter the coordinate values using geocoded vectors as the source.
  In the Filename box, type the path and file name containing the geocoded vectors or, to select a file, click Browse.
  
  To select multiple layers, select a segment, and then for each layer, click Load. The vectors open in a viewer.
- Chip database: enter the coordinate values by using a chip database as the source.
  In the Filename box, type the path and file name of the chip database or, to select a file, click click Browse.
  
  To display the geocoded locations of the chips in your raw image after you have enough information to calculate the math model, select the Display Chips check box.
- PIX/Text file: enter the coordinate values from a PCIDSK (.pix) or text file.
If you want to use a DEM to supply the elevation information for the points, type the path and file name of the DEM in the DEM box or click Browse to select the file.
Depending on the math model you selected when you created your project, various additional options may be available from which you can select, as follows:
- If you selected a Rational Function math model, where you compute the coefficients from the GCPs, in the No. of coefficients list you can select the number of coefficients you want to use to calculate the math model.
- If you selected a Rational Function math model, where you import the coefficients from the Rapid Positioning Capability (RPC) data, in the RPC adjustment order list you can select the polynomial order you want to use to calculate the math model.
  In the GCP Collection window, you can change the order of the working photo or change the order of all photos by clicking Apply to all images.
  
  For more information about RPC adjustment, including each available order, see Understanding RPC adjustment order.
- If you selected a Polynomial math model, in the Polynomial order list you can select which polynomial you want to use to calculate the math model.
- To help with collecting the points, you can select the Auto locate check box, the Compute model check box, or both.
If necessary, in the Point ID box, you can enter a new value.
That is, the Point ID is generated automatically. While you can enter a new value, all points (GCPs, independent check points, tie points (TP), and elevation match points) in the image must have unique values. When collecting stereo GCPs (for the same GCP in the overlap areas of different images), use the same Point ID in each image.
In the list beside the Point ID box, click:
- GCP: to use the point as an active GCP in the math model.
- Check: to use the point as an independent check point to check the accuracy of the math model.
- Inactive: to use the point as an inactive GCP.
Note: Changing a stereo GCP to inactive applies only to the selected image.
Enter the coordinates of the GCP from the source according to the method you selected in the Ground control source list:
- Manual entry: In the Easting (X) box and the Northing (Y) box, type the ground coordinate of the GCP.
- Geocoded image: Zoom to a level where you can see the detail in the source or "master" image, position the cursor precisely on the feature you will use as a GCP, and then click Use Point.
  The coordinates transfer to the GCP Collection window. For more information, see Using geocoded or raw images for collecting ground control points.
- Geocoded vectors: Zoom to a level where you can see the detail in the vector viewer, position the cursor precisely on the feature you will use as a GCP, and then click Use Point.
  The geocoded Easting and Northing coordinates transfer to the GCP Collection window. For more information, see Using vectors for collecting ground control points.
- Chip database: In the OrthoEngine ChipDatabase window, select a chip, and then click Use Image Chip.
  The geocoded Easting and Northing coordinates transfer to the GCP Collection window. For more information about using this window, see Understanding chip databases.
- PIX/Text file: In the Read GCP from PIX/Text File window, select the file containing the GCPs.
  You can also open this window by clicking Select PIX/Text File. For more information about using this window, see Importing ground control points from a file.
In the Elevation (Z) box, type the elevation of the GCP (if it has not appeared automatically) or click Extract Elevation to use the DEM that you set to determine the elevation of your GCP.
Zoom to a level where you can see the detail in the raw image, position the cursor precisely on the feature you will use as a GCP, and then click Use Point.
If you are collecting a stereo GCP, collect the point on each image by repeating this step in each image before proceeding to the next step.
In the +/- boxes beside the Image pixel box, the Image line box, the Easting (X) box, the Northing (Y) box, and the Elevation (Z) box, type the estimated error for each.
Note: You can edit the error estimate in the +/- boxes to correspond to your ability to precisely identify a feature in the image. For example, if you use coarse imagery, you can probably only measure to the closest pixel. If you use imagery that was compressed or poorly scanned, you may only be able to measure to the closest two pixels. Even if you identify a GCP to the closest pixel, the coordinate may only be accurate to a certain number of meters.
Click Accept.

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