Using Drones for Land Surveying

What is Land Surveying?

First of all we must deinfe what land surveying actually is. The book defintion is:

“The technique of determining the terrestrial or three-dimensional position of points and distances and angles between them”

Using a drone we take hundreds if not thousands of pictures of the pjoect site which overlap. Each picture is geotagged to so it knows the GNSS (GPS) position of the subject matter.

We take 2 types of image depending on what the requirements of the survey are.

Nadir Images

The drone camera is kept perpendicular to the ground. The image looks straight down on the subject.

Nadir images are mostly used for surveying, land maps etc.

Oblique Image

The drone camera is not perpendicular to the ground. The image looks across at the subject at about 60°.

Oblique images are used for Point Maps and 3D models of structures.

Post Processing Drone Images

Once the images have been taken they are then "stiched" together using specialised software to create a large, very accurate, map of the site. These maps can then be output in various formats depending on client requirements.

What Land Surveying Can Drones Be Used For?

• To establish land boundaries
• Create land maps for planning
• Construction site progress reporting
• Stockpile monitoring
• Agriculture field planning
• Monitoring crop health
• Marketing
• Proof of concept
• Overlaying of CAD drawings
• Assessing "As Built" accuracy

Who Can Make Best Use Of Drones For Land Surveying?

The list of potential users for drones to carry out land surveying include, but are not limited to:

• Architects
• Surveyors
• Construction Companies
• Infrastructure Companies building roads, bridges and railways
• Estate Agents determining areas of land for deeds & ownership
• Farmers
• Land managers

So Why Should We Be Using Drones to Survey Land?

• Much faster to collect accurate data compared to traditional methods
• We can catch millions of data points within minutes using a drone
• Easily & quickly generate orthomosaics maps (for more on orthomosaic maps read here)
• Create Digital Elevation Models (DEMs). We can create 2 types of DEMs with drones:
• Digital Terrain Models (DTMs) – represents the earth’s surface
• Digital Surface Models (DSMs) – represents the earth’s surface and objects such as trees, plants & structures
• Generate maps with contour lines
• Create 3D models & point clouds

Most of these data types rely entirely on the use of drones.

By using drones you are able to accurately visualise features of the site that you, previously, would not have been able to.

With drones we can survey land or areas that are too dangerous or inaccessible for traditional methods.

How Accurate Can A Drone Survey Be?

There are 2 types of drone land survey accuracy types. Which one is appropriate depends, obvioulsy, on the requirements of the project.

Relative (or local) Accuracy

Management, Measurement and Marketing

Relative accuracy is the degree to which a given point on a map is accurate relative to other points within that same map. In other words, if a distance between two points measures 10 cms in the real world, it also measures 10 cms on your map.

Relative accuracy is usually sufficient for projects like taking small-scale measurements, visually inspecting progress on construction sites and monitoring fields for crop health.

Essentially, any time you need to gather information from within a map itself, but do not necessarily need to place that map accurately in space.

Because drone maps contain thousands of points, compared to the few hundred points obtained through traditional survey methods, maps with relative accuracy give a highly detailed overview of terrain that works well for these types of projects.

Projects for which relative accuracy is usually sufficient:

• Small-scale measurements — eg. area of a field, length of a fence, width of a stockpile
• Management and oversight — eg. keeping tabs on general progress of a construction site
• Crop scouting — eg. assessing damage after a storm, monitoring crop health
• Marketing — eg. creating a time-lapse or 3D model project to share with prospective clients

Absolute (or global) Accuracy

Surveys and Overlays

Absolute accuracy is the degree to which a point on a map corresponds to a fixed coordinate system in the real world. If a map has a high level of global accuracy, the latitude and longitude of a point on that map will correspond accurately with actual GNSS coordinates.

Generating Absolute Accuracy

Standard GNSS signals that most drones receive, whilst highly accurate, are not accurate enough to create maps with absolute accuracy due to small errors in the GNSS position signal. A ground base station with a known position can send signals to GNSS receivers that remove these small errors allowing for highly accurate positions to be determined.

Real-time Kinematic Drones or Ground Control Points with Post Processing Kinematic are required to generate maps with absolute accuracy.

Real-time Kinematic Drones (RTK)

A specially equipped RTK drone records GNSS (satellite position) for each image taken. The satellite accuracy is enhanced using a ground base station that removes position errors from the satellite feed.

Ground Control Points (GCP)

Used when not using an RTK drone to achieve absolute accuracy. These are points on the ground with known coordinates. The coordinates being assessed with a GNSS receiver using enhanced accuracy from a ground base station.

Five to ten markers are placed on the ground at the GCPs and used in post-processing to ensure that the image is correctly georeferenced. This is called Post Processed Kinematic (PPK).

Measuring Accuracy

Ground Sampling Distance (GSD)

“The distance between two consecutive pixel centre measured on the ground”

The accuracy of a survey is accepted to be 2 x GSD.

Example

A GSD of 1.25cm per pixel equals 1.25 cm in the real world per pixel of the image.

1.25 x 2 = 2.5

Accuracy of +/- 2.5 cm from survey