The implementation of drones as a survey instrument has been a welcome addition in the repertoire of many land survey companies across the globe. Not only have they proved to save land surveyors valuable time and increase profit, but they are also able to deliver highly accurate maps and 3D models. This goes without saying the correct practices and technology need to be implemented by skilled individuals to achieve these results.
What does accuracy mean in the land surveying industry?
As we are dealing with drones which create maps by taking successive, high-quality photos, the man on the street would assume that high accuracy in drone surveys would refer to high resolution or pixels. Although high-quality photos are a contributing factor to increased accuracy, it is but a small drop in the ocean.
In the land surveying industry accuracy can be measured as relative or absolute accuracy. The level of accuracy which is required for a survey (relative or absolute) depends on the desired outcome of the survey according to the specifications of the client.
Relative and absolute accuracy can be explained as follows:
Relative accuracy: This term refers to the accuracy of the metric measurement between two given points within a map in relation to the actual metric measurement between the same points in the “real world”.
Absolute accuracy: This term refers to the accuracy of the geospatial data of a given point in relation to that same point’s true location on the Earth’s surface which is determined by a fixed geodetic coordinate system.
To conclude, accuracy in the land surveying industry can be defined as the accuracy with which a map or 3D model can simulate the “real world”.
Hardware (drone model) and software (image processing software) highly influence the accuracy of drone survey data, but what are the other factors land surveyors can manipulate to achieve survey standard accuracy when conducting a drone survey?
The land surveyor or drone pilot has a great degree of control over the level of accuracy achieved during a drone survey. Flight parameters such as altitude, image overlap rate and flight speed are set according to the land surveyor’s calculations and requirements.
The GSD or Ground Sampling Distance refers to the distance between the centres of two pixels as measured on the surface of the earth that is being surveyed. Flights performed at lower altitudes produce data with lower GSD’s and lower GSD’s signify the elevated accuracy of data.
The amount of overlap between one photo and the next will determine how successfully your data processing software will be able to reconstruct a 3D model of the surveyed terrain. “Image stitching” only requires an overlap of 20% but due to lack of information, the map will not include elevation or three-dimensional information. Under normal circumstances, a 75% frontal overlap and a 60% side overlap will suffice. The image overlap needs to be adjusted according to the complexity of the survey terrain. The more complicated the terrain, the greater the image overlap will have to be.
Ground Control Points
Including GCP’s or Ground Control Points in your aerial survey is another effective way to greatly improve the accuracy of survey data. GCP’s are fixed and known points which are created by a land surveyor before an aerial survey is initiated. GCP’s are created by strategically placing large markers throughout the survey area. A land surveyor then determines the GPS coordinates of each marker by using a base station and a rover. The GCP’s and their coordinates are used by the image processing software to accurately position the map in relation to the “real world”.
Conducting an aerial survey during overcast and windy weather could have a detrimental effect on the accuracy of your survey data. These conditions have a direct effect on visibility and the stability of the drone during the flight which could result in blurry images and poor data capture. It is the best practice to perform a drone survey when the sun is at it’s highest and the weather is clear.
As you can see, many factors need to be taken into consideration when conducting an aerial survey. From hardware and software, calculating and setting the correct flight parameters for specific conditions, incorporating GCP’s and of course, the weather.
Although drone technology is becoming more and more accessible and affordable it is advised to take advantage of the vast knowledge of a professional in the field (if accurate data is a requirement). Without absolute and relative accuracy, drone survey data will be meaningless.
Long live the land surveyor!