Ground Control Issues

Poorly Marked GCPs

Signs of GCP issue:

• Center point of GCP is unclear and ambiguous

  • Remember, if you cannot clearly see the center point of the ground control point, then that point cannot be accurately marked during photogrammetric processing. An accurate reading from your GPS rover only gets you so far. If you cannot accurately mark the center point, then you are introducing error into the model.

• GCP visibility obstructed by an object

Troubleshoot:

• Trial and error GCP marking

• Clarify GCP marker/center point with the field crew

Possible recommendations:

• Aerotas Store: 16” x 16” Ground Control Points (GCPs)

• Use a checkerboard pattern GCP with a clearly defined center

• Mark GCPs larger

• Mark GCPs in open space

• Mark GCPs with color contrast to the ground

• Better secure GCPs when a strong wind is present

Low Quantity of GCPs

Signs of GCP issue:

• Less than 5 GCPs set or visible on-site

Troubleshoot:

• Process with caution and caveat of lower accuracy

Possible recommendations:

• Without 5 GCPs, we cannot verify the accuracy of the overall project. Use data with caution.

• Set a minimum of 5 GCPs

• Anything less than 5 GCPs is not sufficient for survey-grade quality

• Submit additional visual ID points or checkpoint

• Re-fly with additional GCPs

GCP Distribution SUBOPTIMAL

Signs of GCP issue:

• GCP not included in the flight path

• GCPs not evenly spread throughout the entire site

• GCP crowding in one area

Possible recommendations:

• Aerotas Resource: GCP Best Practices

• Set GCPs closer inside the project area or fly a slightly larger area

• Fly an additional flight line beyond the GCPs to ensure good accuracy

• Place GCP in the center and corners of the project

• If needed, submit visual tie points

GCP Showing Error

Signs of GCP error:

• GCPs showing error above our tolerance >0.1’

• Inconsistent errors between GCPs

Possible reasons for error:

• The base station was moved between flights

• GCPs appear outside of flight path

• GCP marker was moved

• Inability to properly identify the point of measurement on GCP

• Extensive vegetation coverage

• AeroPoints not left outside long enough (need at least 45 mins - 1 hr)

• Measurement error in the field

• Incorrect GCP file

Troubleshoot:

• Remove GCP(s) from processing

• Use visual tie points

Possible recommendations:

• Consider GCP error acceptable and deliver as is

• Submit any checkpoints or visual ID points

• Double-check GCP coordinates

• Mark GCPs more clearly