Best Practices Ground Control Points (GCPs)

1. Use a minimum of 4 large quality markers:

• DroneDeploy requires a minimum of 4 GCPs and we suggest using 8 GCPs for maps up to 50 acres. For maps over 50 acres, we recommend adding at least 1 Checkpoint for every additional 10 acres.
• An ideal GCP marker can be as simple as two intersecting lines. The goal is to create a visible feature on the area that is being flown.
• The GCP should be visible on the map at the given altitude that the mission was performed. If you cannot see the center point of the GCP in your map photos, it cannot be accurately marked during processing. If your GCP is obscured, poorly marked, or if you cannot confidently place your cursor on your photos within an inch of the point where the GCP was measured with GPS, it does not matter that your GPS took a centimeter or millimeter accurate reading, your GCP will only be as accurate as the center can be marked.
• Adding a center mark to the GCP, as seen in the images below, can also eliminate confusion about where the center point is located.
  This is an example of a well-constructed GCP. The marker is large enough that it is visible from far away, but the location will be measured from the center to increase accuracy.
• If you cannot mark your GCPs with paint, there are various low-cost ways to make excellent markers with items from the internet or home depot. These sheets of roofing rubber with these vinyl stickers ($1 per 6" x 12" sheet) can be used to make a simple checker pattern but they need to be staked into the ground if it's windy.
  These rubber and vinyl markers cost $5 each and are extremely weather-resistant. 

2. Distribute your GCPs evenly: 

• The placement of GCP points should be scattered throughout the area of interest being mapped.
• If the area being mapped has noticeable elevation changes, make sure your GCP placement (hills, mines, valleys, etc.) accounts for this by placing them throughout the different elevations on a relatively flat surface.
• Ensure the region of interest (where you want the map accuracy to be highest) has GCP points surrounding it and scattered throughout the region.
  
  Measuring GPS from the center of the point helps increase accuracy. 

3. Create a Buffer zone:

• When placing GCPs around the perimeter, DroneDeploy recommends a 50 ft (15m) buffer zone between the edge of the map and the GCPs location. This will ensure there is enough image coverage to carry out the processing.

Further Tips:

• Ensure the Calibration point is within polygon of your map

• Take the coordinate from the centre of the intersecting lines.
• Use an RTK or PPK GPS for the most accurate reading.

• Include consistent techniques

• Include checkpoints

• Fly with a high-resolution (20MP or greater), mechanical shutter camera like the DJI Phantom 4 Pro.
• *Not using RTK or PPK GPS systems may cause issues with mapping accuracy.