What are the outputs obtained in aerial surveying using drones?
Drones (also called as UAVs)are getting popular in engineering applications, but what do we get out of it?
In aerial surveying, we find that the practitioners use a lot of technical words which seem to be confusing most of the times. Let us look at some of the them which are the most popular ones:
1. Orthomosaic
Orthomosaic is made from two words Ortho (meaning perpendicular) and Mosaic (meaning small pieces joined together to make a big piece). An orthophoto, or orthomosaic is a large image of an area which can be used to measure actual distances between any two given points on the image. A normal photograph, as taken from above is not uniform in terms of scale (It means that the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground is not equal). However, an orthomosaic is such that the scale of the image to the actual ground is uniform, thus representing the Earth’s actual surface.
In simple words, an orthomosaic is an accurate representation of the Earth’s surface from top view in 2D.
2. Digital Elevation Model
Elevation simply means height. In simplest terms, it can be referred to as Digital Height Model. Thus, a digital elevation model (DEM) is a representation of a terrain’s surface only in terms of its height. DEMs are the superset of digital surface models (DSM) and digital terrain models (DTM).
Digital Surface Model (DSM): As the name suggests, a digital surface model is a digital representation of the Earth’s surface (in terms of height). In other words, DSM provides the height of all the points on Earth as seen from above such as buildings, trees etc. It can be obtained directly from any standard photogrammetric software.
Digital Terrain Model (DTM): It is similar to a digital surface model except it represents only the bare surface of the earth without including any height information about the structures above ground such as buildings, trees. It is a bare earth model. A DTM can be obtained from further processing of a DSM, and is used to generate contour of the given area.
3. Point Cloud
Imagine the real-world in form of points. That’s what a point cloud is — a 3D representation of the real-world in form of points. In other words, it is a collection of large number of points/pixels (>1 crore) where each point has X, Y and Z location in a given 3D coordinate system, and also a corresponding R, G, B colour value, such that all the points when combined according to their position in the coordinate system, it looks a digital copy of the real-world. Unlike Orthomosaic, where we can do only 2D measurement (distance/length), Point Cloud gives us the freedom of measuring in 3-dimension (3D), and hence enabling us to find out height difference between two points, volume estimation of a specified region, elevation profile of a section etc.
4. 3D Mesh
A 3D Mesh is the most accurate representation of the real-world in every form. Unlike point cloud, which is a discrete set of points, a 3D Mesh is continuous similar to the real-world. It also gives us the freedom of measuring in 3-dimension (3D), and hence enabling us to find out height difference between two points, volume estimation of a specified region, elevation profile of a section etc.
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