Digital Elevation Models (DEMs) specifications

The Globhe standards for Digital Elevation Models (DEMs)‍

The Globhe standards will be applied if others are not specified in your order.

Digital Elevation Models (DEMs) are digital representations of the Earth's surface, providing three-dimensional reconstructions of terrain relief/elevation. DEMs are widely employed in a range of applications, from environmental monitoring and urban planning to disaster management and engineering projects.

They offer an accurate representation of the terrain, structures, and objects within the mapped area, allowing for detailed analysis, measurement, and visualization.

Digital Surface Models (DSMs) & Digital Terrain Models (DTMs)

DEMs can be either Digital Surface Models (DSMs) or Digital Terrain Models (DTMs), which differ in what elements of the terrain they include, making them suitable for different applications.

Digital Surface Models (DSMs) are DEMs that represent the Earth's surface, including all features and objects present on the terrain, both natural and man-made. This means they include not only the bare Earth (terrain) but also any above-ground features such as buildings, vegetation, trees, bridges, and other infrastructure elements. DSMs are derived from various data sources, including RGB drone data, LiDAR, and aerial photogrammetry. The elevation values in a DSM represent the heights of all objects above the reference plane (usually sea level), including buildings, trees, and other structures. DSMs can be used in urban planning, forestry, telecommunication, and 3D visualization projects, where the presence of above-ground features is crucial for decision-making and visualizations.

Digital Terrain Models (DTMs) are DEMs that represent only the bare Earth's surface, excluding all above-ground objects and features (such as trees or buildings). They provide a clean representation of the terrain's topography, free from any obstructions caused by buildings or vegetation. DTMs are derived from elevation data captured using RGB drone data, LiDAR, and aerial surveying. The elevation values in a DTM represent the heights of the bare Earth surface above the reference plane (usually sea level), providing vital information for analyzing slopes, drainage patterns, and watershed delineation. These are particularly useful in applications such as hydrological modeling, flood risk assessment, geological analysis, and any scenario where a clear understanding of the terrain's natural shape is essential.

Deliverable specifications

Flight height

Definition: the altitude or elevation at which a drone operates during an aerial mission or flight.

Globhe standard: 110 meters AGL*, in line with national and local regulations in place.

*Above Ground Level = the altitude or vertical distance between the drone's current position and the Earth's surface.

Preferably using the terrain follow mode*, where possible.

*Terrain follow mode = flight mode available on certain drones that allows them to automatically adjust their altitude and maintain a consistent distance above the ground or terrain below. When the terrain follow mode is engaged, the drone utilizes various sensors and algorithms to detect and track the ground's elevation. It uses available terrain models as a reference to continuously adjusts its flight altitude and compensate for changes in the terrain, such as hills, valleys, or uneven surfaces. By following the terrain contours, the drone can capture consistent and precise data, while enhancing the safety and efficiency of drone operations.

Speed

Definition: the rate at which the drone can travel through the air or move from one location to another. It is a measure of how quickly the drone can cover a certain distance within a given period of time. The speed of a drone is typically expressed in terms of a linear velocity, often measured in meters per second (m/s). The speed of a drone can vary depending on various factors, including its design, size, weight, propulsion system, flight mode, and external factors (such as wind speed and direction).

Globhe standard: 3 - 5 m/s and adjusted to meet the technical requirements.

Image overlap

Definition: the degree of redundancy or overlap between consecutive images captured during a drone flight mission. It is typically expressed as the percentage (%) of overlap between adjacent images along (front overlap) and perpendicular (side overlap) to the flight direction. Image overlap is needed to ensure the accuracy and quality of the resulting drone data. The overlap allows for better stitching and alignment of the images during post-processing (photogrammetry), enabling the creation of seamless DEMs.

Globhe standard: 75% front overlap, 70% side overlap.

Spatial resolution / Ground Sampling Distance (GSD)

Definition: the level of detail and clarity with which the physical features on the Earth's surface are captured and represented in 2D orthomosaic maps. The spatial resolution defines the size of the smallest object that can be distinguished in drone images and it is typically measured in terms of the ground sampling distance (GSD), which represents the physical distance on the ground covered by each pixel. The GSD achieved is a function of the drone sensor/camera specifications and the flight height.

Globhe standard: normally 3 or 4 times GSD of the drone imagery/2D orthomosaic. I.e. imagery GSD: 2.5 cm/px --> DEMs GSD: 8 - 10 cm/px.

Ground Control Points (GCPs)

Definition: precisely located reference points on the Earth's surface that are used in conjunction with drone imagery or data to enhance the accuracy and georeferencing of the captured information. GCPs serve as known geographic control markers that provide a reliable connection between the drone's coordinate system and real-world geographical coordinates. GCPs are measured using markers placed on the ground, such as aerial targets with known coordinates. These markers are visible and easily identifiable in the drone imagery. The GCPs are surveyed or measured using high-precision surveying equipment, such as GNSS stations/receivers to determine their precise three-dimensional coordinates (latitude, longitude, altitude).

Globhe standard: not needed for simple mapping purposes. Around 10 GCPs every 100 hectares are required for more accurate work such as flood mapping/modelling or infrastructure surveys.

Absolute accuracy

Definition: the degree of conformity between the recorded or measured geographical positioning and the true or actual values of the features or properties being captured by drones. It represents the level of correctness, precision, and reliability of the data or measurements obtained from drone operations. I.e. if the position of a road in the reconstructed model is close to its actual position on the Earth, then the absolute accuracy is high.

Adding precisely measured GCPs or using RTK/PPK drones can greatly improve absolute accuracy. So the accuracy of the deliverables highly depends on the accuracy of the GNSS receiver of the drone or the GNSS station, and if GCPs are used or not.

Globhe standard: highly dependant on the deliverables GSD and application. Few meters if no GCPs are used and up to 10 cm of horizontal and vertical accuracy if GCPs are used.

Coordinate Reference System (CRS)

Definition: standardized system for defining and interpreting geographic coordinates, allowing for accurate and consistent positioning of features on the Earth's surface. In the context of drone data and mapping, the CRS plays a vital role in establishing a common reference point for geospatial data captured by drones. It consists of three key elements: coordinate system, map projection, and datum. The coordinate system specifies how locations are defined on a two- or three-dimensional plane using coordinate values. The map projection converts the curved surface of the Earth into a flat surface, facilitating the representation of spatial data on maps or digital screens. The datum provides a reference frame for measuring and aligning coordinates to the Earth's surface. There are various types of CRSs used in geospatial applications, including geographic coordinate systems (e.g., latitude and longitude), projected coordinate systems (e.g., Universal Transverse Mercator), and local coordinate systems (e.g., State Plane Coordinate System). Each CRS has its own set of properties, units of measurement, and accuracy characteristics.

Globhe standard: WGS 84 / (EPSG: 4326) / Meters.

Data format

Globhe standard: geotagged DEMs - GeoTIFF (.tiff).

Definition: geotagging consists in adding geographic information to the image file, such as the latitude and longitude coordinates of where the image was taken. Overall a geotagged 2D orthomosaic map combines the visual information captured by a drone's camera with embedded geographical metadata, providing valuable location-based context to the processed orthomosaic map.

Drone models and sensors

Globhe relies on a wide range of commercial drone models available through our Crowddroners, including but not limited to multirotors (i.e. quadcopters, hexacopter, octocopters) and fixed-wing, mounted with every sort of sensor as payload*.

*Payload = the additional equipment or devices that are carried or attached to a drone in order to perform specific functions or tasks. These payloads can vary depending on the purpose and capabilities of the drone. The most common drone payloads are RGB cameras for aerial photography or videography, sensors for data collection (such as thermal, multispectral imaging, or LiDAR sensors), and specialized equipment for tasks like seedlings, crop spraying, or search and rescue operations.

If the drone model and/or sensor are not selected, the GLOBHE team will choose the suitable drone needed to meet technical requirements, also based on availability.