Ground Control Point (GCP) Guide

Ground Control Point (GCP) Guide


  1. 1. Overview 

Mosaics from different dates must be precisely aligned, which can be achieved efficiently by installing and surveying GCPs.  

  1. 2 Equipment

Traditional GCP surveying equipment can be expensive and extremely complicated to use. Sentera has an alternative: an intuitive and inexpensive method to promote to customers. 
Sentera recommends using white, 5-gallon bucket lids secured to the ground using wooden stakes as cost-effective GCP’s (Table 1). 


Ground Control Point 

 

White bucket lids can be purchased at most home improvement stores for between $1.00 and $3.00 dollars per lid.  


Stakes 

 

Wooden or plastic stakes can be purchased from various online retailers as well as home improvement stores. Oftentimes stakes are sold in bulk for an average price between $0.20 and $0.50 per stake. 

 

Note: Sentera advises against using metal stakes for GCP’s. Metal stakes may puncture tires and damage equipment. 


Table 1: Ground Control Point Equipment List 

Some locations require GCP’s to be surveyed using RTK (Real Time Kinematics) or PPK (Post Processing Kinematics) equipment. Please ask your project manager or a Sentera representative if your site requires this equipment. 


2.2.1 Propeller AeroPoints

Sentera recommends using Propeller AeroPoints, a simple to use PPK system. AeroPoints do not require a radio link and are powered by a built-in solar panel for long battery life. 

Propeller AeroPoint:  2022 Retail Price (set of 10): $8,000.00 

 

How to use Propeller AeroPoints:
1. Leave AeroPoints in a well sunlit area for a few hours to charge prior to mission 
2. Place one AeroPoint precisely centered on each GCP and press the button on each. The light will become solid red.  
3. Leave AeroPoints in place and wait 1 hour
4. Power on Wi-Fi.  Ensure that the Wi-Fi hotspot is active with the SSID* “propeller”. SSID can reset if iPad is locked or goes through a power cycle. Ensure the SSID is “propeller” at all times during upload. 
5. Go to each AeroPoint, press the button. The light will turn from solid to blinking slowly. Once blinking slowly, pick it up
6. When in range the AeroPoint will automatically connect to the Wi-Fi hotspot and upload GPS data. While uploading data a fast blink will occur on each AeroPoint. Leave the Wi-Fi hotspot with all AeroPoints for 30 mins. It may take a few cycles to complete the upload process. Go through the “blinking fast” or “upload” stage a few times to ensure data was uploaded. 
7. Login to your propeller account and confirm GPS data was captured.  

  1. 2.2.2 Trimble DA2 

A viable alternative to AeroPoints is the Trimble DA2 GPS RTK system. 

 

Trimble DA2 Kit: 2022 Retail Price (per unit): < $1,000.00 


How to use the Trimble DA2 kit:

1. Prior to field visit: Download the Trimble Manager app to your device and create an account. Assemble Trimble unit. 

2. Power on Trimble unit and connect to your device via Bluetooth. 
3. On the app home screen, select DA2 as position source and ensure toggle button is switched on  
4. Check power button LED status: 

Flashing Blue
Ready to be paired
Solid Blue
Paired and operable
Red
Fault 
Yellow
Software update 

5. In app, activate 1-hour token to begin surveying 
6. Place the pointed end of the rod in the center of the desired location. Keep unit level using the bubble. 
7. Screenshot location data and upload to the field in “Desktop View” in FieldAgent Desktop.   

3. Field Preparation  

It is important that the GCP’s can be spotted in the aerial imagery over the course of the entire season. Sentera highly recommends leaving a 5-to-10-foot bare (or dirt) border around the perimeter of a site where the GCP’s will be placed (Figure 1). This ensures the GCP’s are never covered by growing crops and are easily visible by everyone in the area. 


For canopy height/lodging missions it is equally important to have a clearing and GCP in/around the center of the site as seen in Figure 1. A good rule of thumb is to ensure that the GCP is as far away from the crop as the crop is tall (at its peak). Sentera pilots will not destroy crops to create clearings. 

  1. 4. GCP Placement 

It is imperative that before placing the ground control points the landowner/grower and project representative are aware of the GCP’s presence. If GCP’s are picked up or destroyed by mowers/tillers it is near impossible to replace it in the same location.  

 

A minimum of 8 GCP’s per site/mosaic are recommended for best results.  



Figure 1: GCP Placement Location Guide  


GCP’s need to be recovered at the end of the season to reduce the risk of damage to equipment and the surrounding environment.  

 

  1. 5. GCP Maintenance 

Dirt, weeds, and other debris can accumulate on GCP’s over the course of a season. Pilots need to visually inspect and clean the GCP’s on each subsequent field visit.  

 

If GCP’s have been removed or damaged, pilots should replace the missing/damaged GCP with a new one in a noticeably different location to avoid confusion. Pilots should notify the project manager of the change.  

  1. 6. GCP Removal 

It is important to remove every GCP and its corresponding stake(s) after the last flight has been completed. Do not leave the field until all equipment is accounted for. Ground control points and stakes left in the field can cause damage to equipment and the surrounding environment. 





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