What is Remotely Sensed Data?
Remote sensing, in simplest terms, means viewing something from a distance rather than by direct contact, an aerial photo is a familiar example. For the purposes of earth observation, Remotely Sensed Data is data collected by instruments/equipment that detects the electromagnetic (EM) energy that is reflected from Earth. The equipment, better known as sensors, can be on satellites, mounted on aircraft, vehicles or in a stationary position.
Remote sensors can be either passive or active. Passive sensors record EM radiation that is reflected from the object's surface, usually from the sun. Because of this, passive sensors can only be used to collect data during daylight hours. Cameras, digital or film based, are passive sensors.
In contrast, active sensors emit an EM energy beam to collect data about the object they are measuring. For example, a laser-beam remote sensing system projects a laser onto the surface of Earth and measures the time that it takes for the laser to reflect back to its sensor.
What can I do with Remotely Sensed Data?
Remotely Sensed data has a range of applications including, measuring the Earth's surface, mapping, monitoring hazard and disaster events, natural resource management, vegetation monitoring, oceanographic or geological survey and meteorological measurements to name a few.
What is meant by Resolution and Accuracy?
"Resolution" is the term used to define the ability of the remote sensor to distinguish between two adjacent objects. This can mean any of the following,
- Spatial resolution describes the ability of a sensor to identify the smallest size detail of a pattern on an image.
- Spectral resolution is the sensitivity of a sensor to respond to a specific frequency range (mostly for satellite and airborne sensors). The frequency ranges covered often include not only visible light but also non-visible light and other electromagnetic radiation such as Radar and microwaves.
- Radiometric resolution is often called contrast. It describes the ability of the sensor to measure the signal strength of a reflected beam or brightness of objects.
"Accuracy" refers to the spatial positioning of the remotely sensed data and how close objects in the imagery are in respect to their actual ground position as it would be surveyed in whatever co-ordinate system is being used.
A common misconception is that Remotely Sensed Data (RSD) of high resolution is also highly accurate. Unfortunately while this is generally the case, in some instances RSD can have high resolution but have poor positional accuracy.
What is Ground Sample Distance (GSD)?
For our imagery it is used to describe the smallest sized object the imagery has the ability to separate from adjacent objects in the data and it is expressed in terms of the imagery pixel size (in metres) at ground level or commonly referred to as the Ground Sample Distance.
Elevation data is contours isn't it?
Elevation data is any data set or information that describes the Earth's surface in terms of position and height. The data can be vectors (contour lines), points (xyz text files) or a raster depiction of the ground (tiff image). A common used form of elevation data are ground surface models such as DEMs. These are generally a regularly gridded arrangement of raster points that have a ground height attached to each raster grid point.
I have heard the terms DSM, DEM and DTM. What do these mean?
The terms DSM, DEM and DTM are sometimes used interchangeably for earth surface data models but it is generally recognised that there are distinct differences, especially between DSM and DTM.
Most agencies adopt the following guidelines:
- DSM - Digital Surface Model: Data that depicts the surface of the earth and includes vegetation, buildings, artificial works and any other above ground features.
- DTM - Digital Terrain Model: Data that depicts the earth surface without vegetation, man-made features and any other feature that is not the natural ground level.
- DEM - Digital Elevation Model: A generic term for DSMs and DTMs
What does the term Format mean in relation to remotely sensed data?
This refers to the type of file that the data is stored in. Data can be delivered in a number of file types and these vary depending on the actual data type (raster, vector or point). Modern software can have either a specific file format requirement or the capability of reading a variety of file types and it is important to know this when requesting remotely sensed data.
What does LiDAR stand for?
LiDAR (Light Detection And Ranging) remote sensing is a system that uses laser light technology to measure distance. In the mapping sense the light beam is passed over terrain and the reflected light is analysed both in its intensity and the time it takes to return to the instrument. From the reflected light, precise three-dimensional information can be obtained and DEMs can be produced even in heavily vegetated areas. Additional information can be obtained from the reflected beam including vegetation height and density as well as cultural structures. There are also Bathymetric LiDAR systems used for penetration of water for measuring coastal areas. LiDAR cannot be used in clouded areas as the beam is absorbed by the water.
What are Imagery Point Clouds?
Imagery Point Cloud DSM data is 3D data generated from overlapping digital aerial imagery using photo correlation method. The resolution is equivalent to the imagery used and therefore the data set can be very dense. It is based on the first reflected surface and produces DSMs and therefore it can be problematic in dense vegetation areas if the actual ground surface is the required output.
Where can I obtain Satellite Data?
Remotely sensed data that has been captured by satellite and covers a wide range of the electromagnetic spectrum is generally sold and distributed by commercial agents. While much of the data is purely scientific in its use, a variety of imagery and elevation data sets are available. NRE does not distribute satellite data.