The instrumental component of establishing an accurate relationship between the images in a project, the camera or sensor, and the ground is GCPs. GCPs are identifiable features located on the Earth’s surface that have known ground coordinates in X, Y, and Z. A full GCP has X,Y, and Z (elevation of the point) coordinates associated with it. Horizontal control only specifies the X,Y, while vertical control only specifies the Z. The following features on the Earth’s surface are commonly used as GCPs:
- Intersection of roads
- Utility infrastructure (for example, fire hydrants and manhole covers)
- Intersection of agricultural plots of land
- Survey benchmarks
Depending on the type of mapping project, GCPs can be collected from the following sources:
- Theodolite survey (millimeter to centimeter accuracy)
- Total station survey (millimeter to centimeter accuracy)
- Ground GPS (centimeter to meter accuracy)
- Planimetric and topographic maps (accuracy varies as a function of map scale, approximate accuracy between several meters to 40 meters or more)
- Digital orthorectified images (X and Y coordinates can be collected to an accuracy dependent on the resolution of the orthorectified image)
- DEMs (for the collection of vertical GCPs having Z coordinates associated with them, where accuracy is dependent on the resolution of the DEM and the accuracy of the input DEM)
When imagery or photography is exposed, GCPs are recorded and subsequently displayed on the photography or imagery. During GCP measurement in IMAGINE Photogrammetry Project Manager, the image positions of GCPs appearing on an image or on the overlap areas of the images are collected.
It is highly recommended that a greater number of GCPs be available than are actually used in the block triangulation. Additional GCPs can be used as check points to independently verify the overall quality and accuracy of the block triangulation solution. A check point analysis compares the photogrammetrically computed ground coordinates of the check points to the original values. The result of the analysis is an RMSE that defines the degree of correspondence between the computed values and the original values. Lower RMSE values indicate better results.
See RMS Error in Rectification for more information.
Requirements for minimum GCPs for an accurate mapping project vary with respect to the size of the project. With respect to establishing a relationship between image space and ground space, the theoretical minimum number of GCPs is two GCPs having X, Y, and Z coordinates and one GCP having a Z coordinate associated with it. This is a total of seven observations.
In establishing the mathematical relationship between image space and object space, seven parameters defining the relationship must be determined. The seven parameters include a scale factor (describing the scale difference between image space and ground space); X, Y, Z (defining the positional differences between image space and object space); and three rotation angles (omega, phi, and kappa) that define the rotational relationship between image space and ground space.
In order to compute a unique solution, at least seven known parameters must be available. In using the two X, Y, Z GCPs and one vertical (Z) GCP, the relationship can be defined. However, to increase the accuracy of a mapping project, using more GCPs is highly recommended.
The following descriptions are provided for various projects:
Processing One Image
If processing one image for the purpose of orthorectification (that is, a single frame orthorectification), the minimum number of GCPs required is three. Each GCP must have an X, Y, and Z coordinate associated with it. The GCPs should be evenly distributed to ensure that the camera or sensor is accurately modeled.
Processing a Strip of Images
If processing a strip of adjacent images, two GCPs for every third image is recommended. To increase the quality of orthorectification, measuring three GCPs at the corner edges of a strip is advantageous. Thus, during block triangulation a stronger geometry can be enforced in areas where there is less redundancy such as the corner edges of a strip or a block.
The following figure illustrates the GCP configuration for a strip of images having 60% overlap. The triangles represent the GCPs. Thus, the image positions of the GCPs are measured on the overlap areas of the imagery.
Processing Multiple Strips of Imagery
The figure below depicts the standard GCP configuration for a block of images, comprising four strips of images, each containing eight overlapping images.
GCPs in a Block of Images
In this case, the GCPs form a strong geometric network of observations. As a general rule, it is advantageous to have at least one GCP on every third image of a block. Additionally, whenever possible, locate GCPs that lie on multiple images, around the outside edges of a block, and at certain distances from one another within the block.