Ground Control Point (GCP)
What are Ground Control Points (GCPs)?
GCPs are coordinate data used to increase accuracy of geometrical correction on drone-captured imagery during ortho-processing in which perspective projection and terrain displacement are rectified. In geospatial application, GCPs are commonly used to georeference drone-captured images to the correct position on Earth.
GCPs allow you to accurately locate the site being surveyed and correctly scale the project in 3 dimensions. Coordinates of GCPs are usually collected with survey-grade GPS unit. Coordinates are either recorded in northing, easting, and height or latitude, longitude, and elevation format.
The accuracy of GCPs depends mainly on hardware, software, processing method and the experience of the data capture team.
Why is there a need for GCPs?
GCPs are required to improve geometrical accuracy of images during post-processing because the accuracy of unprocessed images is often insufficient to meet end user requirements.
GCPs are important for precise geometrical correction of:
- LiDAR and IFSAR surface models
- Photogrammetric Mapping (Orthomosaic)
- Remotely Sensed Imagery
- Image and Mapping Data Quality Assessment
What makes a good GCP target?
A good GCP target is a noticeable feature on the area that is being flown by drone at 80 to 180 meters. In other words, GCP should be visible on the captured images for image processing.
There are 3 criteria to take note of:
- Contrast – The color of GCPs should have strong visual contrast compared to the surround area. Black or white is usually chosen to provide high visibility on images.
- Visibility – GCPs target should be placed on flat ground with no overhead obstructions blocking the view from above.
- Precise Pick Point – It can be an ‘X’ painted on ground or a tip of an arrow. Targets without definite shape will introduce positional inaccuracy as it is very hard to identify the actual center point of GCP.
Example of GCP targets:
- Spray paint / paint with permanent center
- Non-glare ceramic tiles
In a nut shell, GCP can be as simple as two intersecting lines drawn by chalk or a cardboard with a cross painted in a contrasting color. We recommend that you to add a center mark to the GCP and to take the coordinate from the center of GCP so as to eliminate any confusion during post-processing.
What makes a bad GCP target?
Bad GCP targets is definitely not better than no target at all. It is important to avoid all the mistakes mentioned below in order to establish good GCP targets.
- Size – Make your GCP targets at least 1 meter across in order to ensure the visibility of GCP targets from 60-180 meters in altitude.
- Trees/Vegetation – Do not put GCP targets under a tree or any vegetation because they need to be visible without obstruction from above.
- Non-flat ground – Make sure to place all GCP targets on flat ground to reduce the accuracy error in Z axis as much as possible.
- Movement – GCP targets should stay stationary on the designated position during the flight otherwise it defeats the purpose of having GCP targets.
- Boundary – Do not put GCP targets too close to the boundaries of area of interest because they need to be visible in as much captured images as possible.
How should I position GCP targets?
We have 3 recommendations for you to position your GCP targets in order to achieve higher resolution of topographic mapping based on drone-captured images.
- Place all GCPs homogeneously within an area of interest (AOI) before flight. Bear in mind that you should not place GCPs too near to each other because it will reduce accuracy of geolocation of captured images which are further away from the GCPs. Image below shows the placement of GCPs within an AOI during flight planning phase in Garuda Plex.
- Place GCPs in flat open area with as few obstruction as possible (e.g. trees and building structures).
- Place GCPs on surrounding ground which has high contrast to the GCPs and will allow you to locate them easily during image processing.
What is GPS?
Global Positioning System (GPS) refers to a satellite-based navigation network consisting of at least 24 satellites. GPS works anytime and anywhere in the world regardless of weather conditions without any subscription fees.
GPS devices are able to receive signal transmitted by the satellites and then to triangulates your exact position on the Earth to the nearest few meters.
Advice on using GPS device.
- To ensure the accuracy of data, GPS devices should be used in the open field to avoid obstruction from building and trees which can block the signals from satellite and reduced the number of available satellites.
- It is not recommended to record coordinate data on a cloudy day because the density of clouds in the sky can affect the signal strength of satellites.
- A GPS device should receive signals from at least 3-5 satellites to be able to calculate the position (latitude, longitude and altitude). In other words, accuracy of data increases as the number of available satellite increases.