Nowadays when remote sensing is mentioned most people immediately think of Unmanned Aerial Vehicles (UAVs) or drones. And while these are very useful for undertaking monitoring and surveys for a wide variety of purposes, the science of remote sensing had existed since the 1970s utilising satellite and airborne platforms.
There are a variety of remote sensing platforms and data available (some at little or even no cost) which can be used for a variety of applications. Further, data that has been captured for one purpose may be equally as usable for multiple other applications. That is part of the beauty of remote sensing – once the data is captured it can be assessed and utilised for many different purposes.
So how do you know which remote sensing platform to use?
Choosing the most appropriate platform to utilise will usually depend on both the application of the data and the scale of the area over which you wish to capture the data. In this article we discuss the main features and some of the potential uses of the following popular remote sensing platforms:
- Satellite
- Manned Aerial
- Unmanned Aerial (UAV)
Satellite
Earth observation satellites have been around since the 1970s and there are now literally hundreds of satellites orbiting the earth from which a variety of data can be sourced. Sensors are either active, such as radar, or passive, meaning that they rely on reflected electromagnetic radiation originating (usually) from the sun, or on emissions directly from the earth itself.
The type of data to choose depends very much on the intended purpose. Optical data can come in broad bands, such as:
• Red, Green and Blue for visual inspection
• Many narrow bands across the spectrum, as with multispectral or hyperspectral data, which confines independent values to very narrow sections of the spectrum, in order to distinguish features on Earth.
Wavelengths range from ultraviolet to thermal infrared and on into the microwave region. Observations target anything from the atmosphere, to land cover, to ocean salinity or wave height.
Coverage, capture frequency, spatial resolution and cost are all interdependent:
• Multispectral imagery for almost every 16 days over the last 30 years is available at a 30 metre resolution from Landsat at no cost.
• MODIS provides daily coverage at a 250 metre resolution, also at no cost.
• Worldview-3 provides multispectral imagery, including a panchromatic band with a 30 centimetre resolution, but costs between $50 and $140 per square kilometre (depending on spatial and spectral resolution). However, at this resolution, our ability to classify vegetation, for example, at a species level becomes a reality.
Satellite imagery is best suited for projects which require a large spatial area (> 100 km2) to be monitored and assessed from a consistent position and time of day. It is not suitable for applications that require very high resolution imagery as the best resolution currently available is around 30 centimetres for panchromatic values, and around one metre for multispectral data. Examples of applications for satellite imagery with different spatial resolutions are provided in the table below.
Astron has a longstanding relationship with leading satellite imagery provider, Geoimage, which allows us to work closely with our clients to source highly accurate and consistent satellite imagery to support a range of environmental applications.
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Satellite imagery courtesy of Geoimage.
Manned Aerial
Aerial imagery is obtained from sensors on manned aircraft. Without the payload weight restriction of most UAVs, manned flights can incorporate very sophisticated sensors, including hyperspectral cameras capable of recording hundreds of very narrow wavebands, or lidar, which uses laser technology to map surface height to millimetre precision. This allows a high sensitivity to different spectral signatures which enable us to classify features. The proximity to the ground not only means a higher spatial resolution, but also less atmospheric effects (such as clouds!).
Aerial imagery has a typical spatial resolution greater than five centimetres and suits survey areas from tens of square kilometres to hundreds of square kilometres. Manned flights allow great flexibility in survey area shape and are generally the most cost effective method of capturing long, linear survey areas. The aerial imagery from reputable vendors is typically of very high quality due to the sophisticated sensors and can be analysed for most of the applications listed above for satellite imagery. Due to the higher spatial resolution, aerial imagery is also suitable for species differentiation, feature extraction, precision agriculture and topographical surveys.
We have utilised airborne imagery from most Australian vendors, but wherever possible we choose to work with Aerometrex, because they consistently deliver cost effective data of exceptional quality. Our experience means we can easily coordinate airborne data capture for our clients, specifically tailored to the exact requirements of the project.
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Images courtesy of Aerometrex and Landgate - © Western Australian Land Information Authority (Landgate) (2015).
Unmanned Aerial (UAV)
UAV imagery is obtained from compact sensors mounted on unmanned aircraft. Most UAVs have significant payload restrictions both from an operational limits and licensing perspective. As a result there are limitations in the types of sensors available. However there is a rush of manufacturers developing small lightweight sensors for the burgeoning UAV market.
Imagery from UAV mounted sensors typically has a spatial resolution between one and eight centimetres and currently suits survey areas less than 20 km2. UAVs have advantages over manned flights including safety, lead time, simplicity of mobilisation, spatial resolution and cost. However, these must be considered in light of disadvantages such as payload limitations, sensor availability, flight endurance and stability in windy conditions. For some applications UAVs have unique capabilities, and Astron is excitedly pursuing the innovations that UAV data are enabling.
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A Note on the Third Dimension
All of the three remote sensing platforms discussed are capable of providing a Digital Surface Model (DSM), which is a data set in which each pixel has a Z value for height, as well as X and Y (horizontal) values. Satellites can use radar altimetry, or traditional stereoscopy, whereas UAVs use multiple image overlap and pattern-matching to build geometry in a process called “bundle adjustment”. Aerial surveys can use any of these approaches. A DSM is a necessary part of the processing of any such data.
Advantages and Disadvantages
The key advantages and disadvantages of the three platforms are summarised in the table below.
Recognising the rise of the global market for remote sensing technologies, Astron has been growing the remote sensing capacity of its Geospatial team for a number of years. We can assist clients with a broad range of remote sensing requirements including:
- UAV Data Acquisition - capturing data using our high endurance Bramor rTK or smaller Sensefly ebee UAV platforms
- Satellite & Aerial Data Acquisition – through our industry partnerships we have fast access to multispectral, hyperspectral, lidar and radar imagery from satellite and aerial platorms
- Image Processing - including QA checking, georectification, orthorectification, radiometric correction, photogrammetry, point cloud densification, mosaicking, balancing, complex and multilook radar data preprocessing, speckle filtering, antenna pattern removal and topographic correction.
- Data Analysis – including image classification at multiple spatial scales; feature extraction; canopy height models; biodiversity and landscape metrics; vegetation health analysis; habitat prediction; soil classification; weed detection; radar polarimetry for biomass estimation; sampling optimisation.
- Consultation – including fit for purpose platform and sensor selection and evaluation of existing data sets
- Presentation Solutions - formatting and presentation of results and spatial derivatives in the best way to suit the target audience.
To discuss your project with one of our remote sensing specialists, please call us on (08) 9421 9600 or fill out our contact form and we'll be in touch.