What are ground control points and why are they so important?
Updated: Oct 28, 2019
Nowadays, satellite navigation systems have become a fundamental part of all possible applications. There is an increasing interest in positioning techniques based on Global Navigation Satellite Systems (GNSS) such as navigation, tracking systems, precise positioning, timing and, monitoring applications, photogrammetry, cadaster, intelligent transportation systems, automatic vehicle location, etc. However, to ensure that most of these uses provide us with accurate information, thousands of control points are collected daily in the world.
In photogrammetry, as more accuracy is required and user expectations increase, the need for ground control points or GCPs is becoming increasingly important. Thanks to the use of GCP, our products are more accurate and cost-effective than it was in the past.
In our experience, this technology helps us ensure that the images are correctly positioned according to the coordinates of the ground.
In LAS, ground control points are important and crucial for orthorectification. If you do geometrical corrections, you can have centimeter-level precision. Where in contrast you could have two meters or more of error if you were just using the GPS inside the drone.
So, if you want to create an orthophoto with a high precision you need at least 3 ground control points.
The method´s precision varies depending on the type of receiver used, the distance between the survey points and the reference point, and the acquisition mode. These methods can be:
Static Mode: It consists of collecting the information in place, for several hours. Therefore, the quality of the measurement is of millimeters, with high precision receivers.
Fast static mode: It consists of collecting information staying at a point a few minutes, so you can take many control points a day.
Kinematic model: Allows surveys from a mobile device, with this, you can indicate the trace of a path, terrain profiles with a precision of a few centimeters. Most Kinematic methods need a static initialization, that is, we need to stay a few minutes at the starting point before carrying out the survey
In the Kinematic model, there are different ways to collect coordinates with high precision. One of them is real-time kinematic (RTK). RTK is a technique used to increase the accuracy of GNSS signals by using a fixed base station that wirelessly sends out corrections to a moving receiver. This method uses some sophisticated statistical methods to align the phase of these signals to eliminate the majority of normal GNSS type errors.
The necessity to do topographic surveys in less time and high precision has allowed us to incorporate the post-processing and real-time kinematic techniques in drones.
Our drones are also capable of using RTK-PPK systems. These systems are dual-frequency GPS/GLONASS/BeiDou receivers. They allow cm-level position referencing of orthomosaic photos and 3D models without the ground control points.
By: Yolanda Villaverde
LAS Technical Lead.
For more information visit: www.latitudeas.com
Phone: +1 347 960 6444