Everything You Need to Know about Ground Control Points
What are Ground Control Points and What Purpose Do They Serve?
Ground Control Points (GCPs) are large photo identifiable targets that are placed on the ground within the boundary of your drone survey. GCPs are the not so secret weapon that turn maps from low cost consumer drones into survey grade products. The resulting data can be used for engineering design, construction progress monitoring and precise mapping applications.
You may map using low cost drones without ground control points. The relative height of a stockpile will be precise, but the absolute position of that same stockpile or object will deviate about 5m (15′) horizontally and 20m (60′) vertically from it’s true location. The high relative precision allows you to calculate good stockpile volumes or measure the length of a field within a few percent. However, the low absolute accuracy makes it impossible to use for engineering design, or to track change through cut-fill analysis.
Size and Shape of Ground Control Points
The standard size is a square about 60cm x 60cm (2’ x 2’). This shape and size is easily detectable when creating GCP marks on images before processing the flight. For ultra high resolution cameras or low altitude flights smaller GCPs may be acceptable. Larger GCPs are typically only required for manned flights. There are numerous variations on shape, size and color.
Temporary Ground Control Points
Reusable black and white checker vinyl targets can be purchased and used repeatedly. A freshly painted ‘X’ on the ground typically shows up very well when picking GCPs from drone images.
The ideal color will depend entirely on the background terrain colors. This is affected by where you live and what time of year it is. The contrast within the GCP itself is also important.
- Black and white works reliably and has been a standard for a long time
- Orange spray paint is very visible
- White V shapes work well
Exceptions include snowy winter operations which require clearing snow and using colors that stand out. Fortunately the black on a black and white checker pattern works well even in winter, provided it isn’t snow covered!
Permanent Ground Control Points
Installing permanent ground control points is a big time and cost savings for projects that are flown repeatedly or where you may not have access to survey grade GPS whenever you do a drone flight. Our favorite permanent ground control point is a 30″ x 24″ patio block. White and black vinyl tiles are glued to the patio block using high strength construction glue. In a vegetated environment it can pay off to put down some landscape fabric around the Ground Control Point to prevent it from being grown over and becoming invisible over time. Read more about installing permanent GCPs.
Leverage existing features as ground control points
Use of natural photo identifiable targets such as traffic arrows, catch basins etc. Natural targets are useful whether for a single site survey or when revisiting a site regularly. Best practice is to take a picture of the object being used as a permanent ground control points so that it’s easy to distinguish which manhole, paint line etc. is the GCPs and which ones are not. You don’t want to accidentally tag the wrong manhole!
How Many and Where to Place GCPs
While technically many drone mapping software’s can work with as little as 3 GCPs best practice is to use a minimum of 5 GCPs for a flight area.
Placing GCPs is similar to holding down a tarp in the wind. You want to start by placing a GCP at a corner or sharp changes of direction in your project area. However, if a gust of wind catches your tarp the middle can easily blow up in the wind. The exact same thing can happen with a drone map. It’s called doming. To prevent doming you want to place one or more GCP points inside the mapping area.
The effectiveness of a GCP is inversely proportional to the distance from a GCP. The area close to a GCP will be very accurate, the further from the GCP the less accurate the results. It can be beneficial to draw an imaginary line circle around each GCP with a 200m radius. The area that falls outside the circles will be of lower quality. The 200m radius is for a low end camera / drone such as the mavi mini. For better cameras such as the Phantom 4 pro the imaginary circle can be larger.
Example 1: GCPs for a 20 acre Landfill Cell
Mapping a medium size site such as a 20 acre landfill cell should have a minimum of 5 GCPs, but 7 is a safer number to use. The example below shows a well controlled drone survey of a 20 acre site.
Key attributes of this GCP layout.
- GCPs are set at each corner, but inset at least 15m (50’) from the mapping boundary.
- The central portion of the survey has 3 GCPs to prevent any “doming” in the drone survey.
- GCPs are set at the highest and lowest points of the site.
Example 2: Surveying a 160 acre quarter section
Mapping a large area such as a full half mile by half mile quarter section for survey grade accuracy using GCPs is a significant undertaking. The example GCP layout below has 13 GCPs spread throughout the site.
The edge and corners are well marked out with GCPs, but note that the GCPs are all set slightly inwards on the project area to ensure that they show up in enough images when flying.
Example 3: Surveying a Road or Corridor with GCPs
Mapping corridors is slightly different from mapping areas. The three best practices for placing GCPs on a corridor are as follows.
- Place them as far off the centerline as possible.
- Alternate sides of the corridor
- Place a GCP every 200 – 300m along the corridor.
The example below shows a straight one mile long road segment with 9 GCP locations distributed along the length of the road. Key details of the GCP placement here.
- Not placed near the centre line of the road or on the road itself, but instead 50m / 150ft each side. This controls the ‘roll’ of the drone survey.
- Spaced every 200m (600’) along the road.
- Staggered side to side for good geometric control.
How to Survey GCPs
Obtaining high accuracy survey grade coordinates for your GCPs is critical for the GCPs to have any value. Most of the time this is done with survey grade RTK GPS. However alternatives exist such as smart targets and GCP-as-a-service.
Affordable drones combined with the need for better insights is leading many gravel pits, landfills and construction sites to set up their own low cost drone programs. However, the GPS systems to survey the GCPs can cost anywhere from $5,000 to $50,000 USD.
For regular site specific drone operations we recommend skipping the purchase of survey GPS. Instead install permanent Ground Control Points and use the Ground Control Point as a Service solution.
RTK GPS
Options include:
- Purchasing a surveying system from one of the big three (Trimble, Topcon, and Leica) for $40 – $50K.
- Purchasing a discount GPS set from a company such as EMLID for about $5K
- Purchase a subscription system such as Trimble’s Catalyst
GCP as a Service
Civil Tracker provides GCP-as-a-Service. Which includes the equipment and processing of the GCPs, and the customer installs the GCPs and uses our one button system to collect data.
Figure out what works for you
The best way to become comfortable is to experiment. Try different numbers of GCPs. It can be very valuable to survey more GCPs than you plan on processing with, and using the remaining GCPs for independent checks to insure data quality.