Ground Control Placement and Distribution


What are Ground Control Points?

Ground Control Points (GCPs) are visually identifiable points on the ground with a known northing, easting, and elevation (or latitude, longitude, and elevation) that can turn drone image data into highly accurate, survey-grade models.

Propeller supports both AeroPoints and custom GCPs as ground control. AeroPoints are Propeller's smart ground control points: durable visual targets with integrated GPS and precision post-processing. 

Insufficient ground control can introduce noise, warping, and increased error levels in your model.

An easy way to think about ground control distribution would be to compare it to weighing down a tablecloth. If you don’t have weights (or GCPs) evenly distributed around the edges of the tablecloth, it can move in the wind. Likewise, if you don’t have weights in the center of the tablecloth, it can still flap slightly in a gust. The model can move if ground control isn't sufficiently placed around the edges and in the center of your survey site.  This can create distortion or warping in the model.

If your drone has a high-accuracy GPS that can be corrected using PPK or RTK, it is similar to having a much thicker, heavier tablecloth. It can still move slightly in the wind if there is no ground control at all, so weights are still needed, but the required numbers are greatly reduced. Even distribution is still required for larger tables and tablecloths to hold their shape.

For more information about GCP numbers, see How Many AeroPoints Do I Need for My Survey?  Additionally, if you have further questions about the recommended number of GCPs to use for a survey area, please get in touch with your Customer Success Engineer to discuss this in greater detail.

The following examples cover scenarios where there is insufficient ground control if you use a non-PPK/RTK capable drone.

Examples of inadequate GCP distribution

Insufficient placement around the edges and center of the survey

In this example, the west side of the survey area does not have adequate ground control. As you can see, it creates a warped and distorted model.

GCP_Placement_Warping.png

GCPs clustered in one area of the survey

Clustered GCPs will drag the model toward a single area.  Evenly distributed GCPs will ensure the model isn’t skewed in one direction.

In this example, the clustered GCPs lead to warping in the Southeast section of the work site.

GCP_Placement_Clustered.png

Additionally, Ground Control Points should always be at least 6 feet (or 2 meters) apart to decrease the risk of marking the wrong GCP during data processing.  

Insufficient ground control placement will not produce a reliably accurate model

Correct placement of GCPs is also crucial for an accurate survey.  

Ground control points should be evenly distributed along the perimeter, in the middle, and at the high and low elevations of the site.

Example 1:

GCP_Placement_Examples.png

Example 2:

GCP_Placement_HighLow_Example.png

See how the AeroPoints are evenly distributed throughout the survey area, reach out to the edges of the survey area, and capture the topographic high and topographic low areas.

Other considerations

As obvious as it might sound, make sure you fly over your markers after you’ve marked them. An avoidable common mistake is for pilots and surveyors to plan out ground control only to have the flight path fall short of capturing the marker.

It may be best to plan a flight path that covers an area slightly larger than your intended survey area to ensure you have captured all the markers in multiple images.


I still can't do it!

We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.

 

If you're stuck, the Propeller data success team may be able to help. You can contact our support team by clicking the support tab on the top-right pane of your user portal

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